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Gluconeogenesis

Gluconeogenesis: contents in brief

What is gluconeogenesis?

Gluconeogenesis is a metabolic pathway that leads to the synthesis of glucose from pyruvate and other non-carbohydrate precursors, even in non-photosynthetic organisms.
It occurs in all microorganisms, fungi, plants and animals, and the reactions are essentially the same, leading to the synthesis of one glucose molecule from two pyruvate molecules. Therefore, it is in essence glycolysis in reverse, which instead goes from glucose to pyruvate, and shares seven enzymes with it.

Gluconeogenesis
Fig. 1 – Gluconeogenesis and Glycolysis

Glycogenolysis is quite distinct from gluconeogenesis: it does not lead to de novo production of glucose from non-carbohydrate precursors, as shown by its overall reaction:

Glycogen or (glucose)n → n glucose molecules

The following discussion will focus on gluconeogenesis that occurs in higher animals, and in particular in the liver of mammals.

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Why is gluconeogenesis important?

Gluconeogenesis is an essential metabolic pathway for at least two reasons.

  • It ensures the maintenance of appropriate blood glucose levels when the liver glycogen is almost depleted and no carbohydrates are ingested.
    Maintaining blood glucose within the normal range, 3.3 to 5.5 mmol/L (60 and 99 mg/dL), is essential because many cells and tissues depend, largely or entirely, on glucose to meet their ATP demands; examples are red blood cells, neurons, skeletal muscle working under low oxygen conditions, the medulla of the kidney, the testes, the lens and the cornea of the eye, and embryonic tissues. For example, glucose requirement of the brain is about 120 g/die that is equal to:

over 50% of the total body stores of the monosaccharide, about 210 g, of which 190 g are stored as muscle and liver glycogen, and 20 g are found in free form in body fluids;
about 75% of the daily glucose requirement, about 160 g.

During fasting, as in between meals or overnight, the blood glucose levels are maintained within the normal range due to hepatic glycogenolysis, and to the release of fatty acids from adipose tissue and ketone bodies by the liver. Fatty acids and ketone bodies are preferably used by skeletal muscle, thus sparing glucose for cells and tissues that depend on it, primarily red blood cells and neurons. However, after about 18 hours of fasting or during intense and prolonged exercise, glycogen stores are depleted and may become insufficient. At that point, if no carbohydrates are ingested, gluconeogenesis becomes important.
And, the importance of gluconeogenesis is further emphasized by the fact that if the blood glucose levels fall below 2 mmol/L, unconsciousness occurs.

  • The excretion of pyruvate would lead to the loss of the ability to produce ATP through aerobic respiration, i.e. more than 10 molecules of ATP for each molecule of pyruvate oxidized.

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Where does gluconeogenesis occurs?

In higher animals, gluconeogenesis occurs in the liver, kidney cortex and epithelial cells of the small intestine, that is, the enterocytes.
Quantitatively, the liver is the major site of gluconeogenesis, accounting for about 90% of the synthesized glucose, followed by kidney cortex, with about 10%. The key role of the liver is due to its size; in fact, on a wet weight basis, the kidney cortex produces more glucose than the liver.
In the kidney cortex, gluconeogenesis occurs in the cells of the proximal tubule, the part of the nephron immediately following the glomerulus. Much of the glucose produced in the kidney is used by the renal medulla, while the role of the kidney in maintaining blood glucose levels becomes more important during prolonged fasting and liver failure. It should, however, be emphasized that the kidney has no significant glycogen stores, unlike the liver, and contributes to maintaining blood glucose homeostasis only through gluconeogenesis and not through glycogenolysis.
Part of the gluconeogenesis pathway also occurs in the skeletal muscle, cardiac muscle, and brain, although at very low rate. In adults, muscle is about 18 the weight of the liver; therefore, its de novo synthesis of glucose might have quantitative importance. However, the release of glucose into the circulation does not occur because these tissues, unlike liver, kidney cortex, and enterocytes, lack glucose 6-phosphatase (EC 3.1.3.9), the enzyme that catalyzes the last step of gluconeogenesis (see below).
Therefore, the production of glucose 6-phosphate, including that from glycogenolysis, does not contribute to the maintenance of blood glucose levels, and only helps to restore glycogen stores, in the brain small and limited mostly to astrocytes. For these tissues, in particular for skeletal muscle due to its large mass, the contribution to blood glucose homeostasis results only from the small amount of glucose released in the reaction catalyzed by enzyme debranching (EC 3.2.1.33) of glycogenolysis.
With regard to the cellular localization, most of the reactions occur in the cytosol, some in the mitochondria, and the final step (see above) within the endoplasmic reticulum cisternae.

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Irreversible steps of gluconeogenesis

As previously said, gluconeogenesis is in essence glycolysis in reverse. And, of the ten reactions that constitute gluconeogenesis, seven are shared with glycolysis; these reactions have a ΔG close to zero, therefore easily reversible. However, under intracellular conditions, the overall ΔG of glycolysis is about -63 kJ/mol (-15 kcal/mol) and of gluconeogenesis about -16 kJ/mol (-3.83 kcal/mol), namely, both the pathways are irreversible.
The irreversibility of the glycolytic pathway is due to three strongly exergonic reactions, that cannot be used in gluconeogenesis, and listed below.

  • The phosphorylation of glucose to glucose 6-phosphate, catalyzed by hexokinase (EC 2.7.1.1) or glucokinase (EC 2.7.1.2).
    ΔG = -33.4 kJ/mol (-8 kcal/mol)
    ΔG’° = -16.7 kJ/mol (-4 kcal/mol)
  • The phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate, catalyzed by phosphofructokinase-1 or PFK-1 (EC 2.7.1.11)
    ΔG = -22.2 kJ/mol (-5.3 kcal/mol)
    ΔG’° = -14.2 kJ/mol (-3.4 kcal/mol)
  •  The conversion of phosphoenolpyruvate or PEP to pyruvate, catalyzed by pyruvate kinase (EC 2.7.1.40)
    ΔG = -16.7 kJ/mol (-4.0 kcal/mol)
    ΔG’° = -31.4 kJ/mole (-7.5 kcal/mol)

In gluconeogenesis, these three steps are bypassed by enzymes that catalyze irreversible steps in the direction of glucose synthesis: this ensures the irreversibility of the metabolic pathway.
Below, such reactions are analyzed.

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From pyruvate to phosphoenolpyruvate

The first step of gluconeogenesis that bypasses an irreversible step of glycolysis, namely the reaction catalyzed by pyruvate kinase, is the conversion of pyruvate to phosphoenolpyruvate.
Phosphoenolpyruvate is synthesized through two reactions catalyzed, in order, by the enzymes:

  • pyruvate carboxylase (EC 6.4.1.1);
  • phosphoenolpyruvate carboxykinase or PEP carboxykinase (EC 4.1.1.32).

Pyruvate → Oxaloacetate → Phosphoenolpyruvate

Gluconeogenesis
Fig. 2 – Phosphoenolpyruvate

Pyruvate carboxylase catalyzes the carboxylation of pyruvate to oxaloacetate, with the consumption of one ATP. The enzyme requires the presence of magnesium or manganese ions

Pyruvate + HCO3+ ATP → Oxaloacetate + ADP + Pi

The enzyme, discovered in 1960 by Merton Utter, is a mitochondrial protein composed of four identical subunits, each with catalytic activity. The subunits contain a biotin prosthetic group, covalently linked by amide bond to the ε-amino group of a lysine residue, that acts as a carrier of activated CO2 during the reaction. An allosteric binding site for acetyl-CoA is also present in each subunit.
It should be noted that the reaction catalyzed by pyruvate carboxylase, leading to the production of oxaloacetate, also provides intermediates for the citric acid cycle or Krebs cycle.
Phosphoenolpyruvate carboxykinase is present, approximately in the same amount, in mitochondria and cytosol of hepatocytes. The isoenzymes are encoded by separate nuclear genes.
The enzyme catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate, in a reaction in which GTP acts as a donor of high-energy phosphate. PEP carboxykinase requires the presence of both magnesium and manganese ions. The reaction is reversible under normal cellular conditions.

Oxaloacetate + GTP ⇄ PEP + CO2 + GDP

During this reaction, a CO2 molecule, the same molecule that is added to pyruvate in the reaction catalyzed by pyruvate carboxylase, is removed. Carboxylation-decarboxylation sequence is used to activate pyruvate, since decarboxylation of oxaloacetate facilitates, makes thermodynamically feasible, the formation of phosphoenolpyruvate.
More generally, carboxylation-decarboxylation sequence promotes reactions that would otherwise be strongly endergonic, and also occurs in the citric acid cycle, in the pentose phosphate pathway, also called the hexose monophosphate pathway, and in the synthesis of fatty acids.
The levels of PEP carboxykinase before birth are very low, while its activity increases several fold a few hours after delivery. This is the reason why gluconeogenesis is activated after birth.
The sum of the reactions catalyzed by pyruvate carboxylase and phosphoenolpyruvate carboxykinase is:

Pyruvate + ATP + GTP + HCO3 → PEP + ADP + GDP + Pi + CO2

ΔG’° of the reaction is equal to 0.9 kJ/mol (0.2 kcal/mol), while standard free energy change associated with the formation of pyruvate from phosphoenolpyruvate by reversal of the pyruvate kinase reaction is + 31.4 kJ/mol (7.5 kcal/mol).
Although the ΔG’° of the two steps leading to the formation of PEP from pyruvate is slightly positive, the actual free-energy change (ΔG), calculated from intracellular concentrations of the intermediates, is very negative, -25 kJ/mol (-6 kcal/mol). This is due to the fast consumption of phosphoenolpyruvate in other reactions, that maintains its concentration at very low levels. Therefore, under cellular conditions, the synthesis of PEP from pyruvate is irreversible.
It is noteworthy that the metabolic pathway for the formation of phosphoenolpyruvate from pyruvate depends on the precursor: pyruvate or alanine, or lactate.

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Phosphoenolpyruvate precursor: pyruvate or alanine

Gluconeogenesis
Fig. 3 – Conversion of Pyruvate to PEP

The bypass reactions described below predominate when alanine or pyruvate is the glucogenic precursor.
Pyruvate carboxylase is a mitochondrial enzyme, therefore pyruvate must be transported from the cytosol into the mitochondrial matrix. This is mediated by transporters located in the inner mitochondrial membrane, referred to as MPC1 and MPC2. These proteins, associating, form a hetero-oligomer that facilitates pyruvate transport.
Pyruvate can also be produced from alanine in the mitochondrial matrix by transamination, in the reaction catalyzed by alanine aminotransferase (EC 2.6.1.2).
Since the enzymes involved in the later steps of gluconeogenesis, except glucose-6-phosphatase, are cytosolic, the oxaloacetate produced in the mitochondrial matrix is transported into the cytosol. However, there are no oxaloacetate transporters in the inner mitochondrial membrane. The transfer to the cytosol occurs as a result of its reduction to malate, that, on the contrary, can cross the inner mitochondrial membrane. The reaction is catalyzed by mitochondrial malate dehydrogenase (EC 1.1.1.37), an enzyme also involved in the citric acid cycle, where the reaction proceeds in the reverse direction. In the reaction NADH is oxidized to NAD+.

Oxaloacetate + NADH + H+ ⇄ Malate + NAD+

Although ΔG’° of the reaction is highly positive, under physiological conditions, ΔG is close to zero, and the reaction is easily reversible.
Malate crosses the inner mitochondrial membrane through a component of the malate-aspartate shuttle, the malate-α-ketoglutarate transporter. Once in the cytosol, the malate is re-oxidized to oxaloacetate in the reaction catalyzed by cytosolic malate dehydrogenase. In this reaction NAD+ is reduced to NADH.

Malate + NAD+ → Oxaloacetate + NADH + H+

Note: malate-aspartate shuttle is the most active shuttle for the transport of NADH-reducing equivalents from the cytosol into the mitochondria. It is found in mitochondria of liver, kidney, and heart.
The reaction enables the transport into the cytosol of mitochondrial reducing equivalents in the form of NADH. This transfer is needed for gluconeogenesis to proceed, as in the cytosolic the NADH, oxidized in the  reaction catalyzed by glyceraldehydes 3-phosphate dehydrogenase, is present in very low concentration, with a [NADH]/[NAD+] ratio equal to 8×10-4, about 100,000 times lower than that observed in the mitochondria.
Finally, the oxaloacetate is converted to phosphoenolpyruvate in the reaction catalyzed by PEP carboxykinase.

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Phosphoenolpyruvate precursor: lactate

Lactate is one of the major gluconeogenic precursors. It is produced for example by:

  • red blood cells, that are completely dependent on anaerobic glycolysis for ATP production;
  • skeletal muscle during intense exercise, that is, under low oxygen condition, when the rate of glycolysis exceeds the rate of the citric acid cycle and oxidative phosphorylation.

When lactate is the gluconeogenic precursor, PEP synthesis occurs through a different pathway than that previously seen. In the hepatocyte cytosol NAD+ concentration is high and the lactate is oxidized to pyruvate in the reaction catalyzed by the liver isoenzyme of lactate dehydrogenase (EC 1.1.1.27). In the reaction NAD+ is reduced to NADH.

Lactate + NAD+ → Pyruvate + NADH + H+

The production of cytosolic NADH makes unnecessary the export of reducing equivalents from the mitochondria (see above).
Pyruvate enters the mitochondrial matrix to be converted to oxaloacetate in the reaction catalyzed by pyruvate carboxylase. In the mitochondria, oxaloacetate is converted to phosphoenolpyruvate in the reaction catalyzed by mitochondrial pyruvate carboxylase. Phosphoenolpyruvate exits the mitochondria through an anion transporter located in the inner mitochondrial membrane, and, once in the cytosol, continues in the gluconeogenesis pathway.
Note: the synthesis of glucose from lactate may be considered as the part of  the Cori cycle that takes place in the liver.

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From fructose 1,6-bisphosphate to fructose 6-phosphate

The second step of gluconeogenesis that bypasses an irreversible step of the glycolytic pathway, namely the reaction catalyzed by PFK-1, is the dephosphorylation of fructose 1,6-bisphosphate to fructose 6-phosphate.
This reaction, catalyzed by fructose 1,6-bisphosphatase or FBPasi-1 (EC 3.1.3.11), a Mg2+ dependent enzyme located in the cytosol, leads to the hydrolysis of the C-1 phosphate of fructose 1,6-bisphosphate, without production of ATP.

Fructose 1,6-bisphosphate + H2O → Fructose 6-phosphate + Pi

The ΔG°’ of the reaction is -16.3 kJ/mol (-3.9 kcal/mol), therefore an irreversible reaction.

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From glucose 6-phosphate to glucose

The third step of gluconeogenesis that bypasses an irreversible step of the glycolytic pathway, namely the reaction catalyzed by hexokinase (EC 2.7.1.1) or glucokinase (EC 2.7.1.2), is the dephosphorylation of glucose 6-phosphate to glucose.
This reaction is catalyzed by the catalytic subunit of glucose 6-phosphatase, a protein complex located in the membrane of the endoplasmic reticulum of hepatocytes, enterocytes and cells of the proximal tubule of the kidney. Glucose 6-phosphatase complex is composed of a glucose 6-phosphatase catalytic subunit and a glucose 6-phosphate transporter called glucose 6-phosphate translocase or T1.
Glucose 6-phosphatase catalytic subunit has the active site on the luminal side of the organelle. This means that the enzyme catalyzes the release of glucose not in the cytosol but in the lumen of the endoplasmic reticulum.
Glucose 6-phosphate, both resulting from gluconeogenesis, produced in the reaction catalyzed by glucose 6-phosphate isomerase or phosphoglucose isomerase (EC 5.3.1.9), and glycogenolysis, produced in the reaction catalyzed by phosphoglucomutase (EC 5.4.2.2), is located in the cytosol, and must enter the lumen of the endoplasmic reticulum to be dephosphorylated. Its transport is mediated by glucose-6-phosphate translocase.

The catalytic subunit of glucose 6-phosphatase, a Mg2+-dependent enzyme, catalyzes the last step of both gluconeogenesis and glycogenolysis. And, like the reaction catalyzed by fructose 1,6-bisphosphatase, this reaction leads to the hydrolysis of a phosphate ester.

Glucose 6-phosphate + H2O → Glucose + Pi

It should also be underlined that, due to orientation of the active site, the cell separates this enzymatic activity from the cytosol, thus avoiding that glycolysis, that occurs in the cytosol, is aborted by enzyme action on glucose 6-phosphate.
The ΔG°’ of the reaction is -13.8 kJ/mol (-3.3 kcal/mol), therefore it is an irreversible reaction. If instead the reaction were that catalyzed by hexokinase/glucokinase in reverse, it would require the transfer of a phosphate group from glucose 6-phosphate to ADP. Such a reaction would have a ΔG equal to +33.4 kJ/mol (+8 kcal/mol), and then strongly endergonic. Similar considerations can be made for the reaction catalyzed by FBPase-1.
Glucose and Pi group seem to be transported into the cytosol via different transporters, referred to as T2 and T3, the last one an anion transporter.
Finally, glucose leaves the hepatocyte via the membrane transporter GLUT2, enters the bloodstream and is transported to tissues that require it. Conversely, under physiological conditions, as previously said, glucose produced by the kidney is mainly used by the medulla of the kidney itself.

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Gluconeogenesis: energetically expensive

Like glycolysis, much of the energy consumed is used in the irreversible steps of the process.
Six high-energy phosphate bonds are consumed: two from GTP and four from ATP. Furthermore, two molecules of NADH are required for the reduction of two molecules of 1,3-bisphosphoglycerate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase. The oxidation of NADH causes  the lack of production of 5 molecules of ATP that are synthesized when the electrons of the reduced coenzyme are used in oxidative phosphorylation.
Also these energetic considerations show that gluconeogenesis is not simply glycolysis in reverse, in which case it would require the consumption of two molecules of ATP, as shown by the overall glycolytic equation.

Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Below, the overall equation for gluconeogenesis:

2 Pyruvate + 4 ATP + 2 GTP + 2 NADH+ + 2 H+ + 4 H2O → Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+

At least in the liver, ATP needed for gluconeogenesis derives mostly from the oxidation of fatty acids or of the carbon skeletons of the amino acids, depending on the available “fuel”.

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Coordinated regulation of gluconeogenesis and glycolysis

If glycolysis and gluconeogenesis were active simultaneously at a high rate in the same cell, the only products would be ATP consumption and heat production, in particular at the irreversible steps of the two pathways, and nothing more.
For example, considering PFK-1 and FBPasi-1:

ATP + Fructose 6-phosphate → ADP + Fructose 1,6-bisphosphate

Fructose 1,6-bisphosphate + H2O → Fructose 6-phosphate + Pi

The sum of the two reactions is:

ATP + H2O → ADP + Pi + Heat

Two reactions that run simultaneously in opposite directions result in a futile cycle or substrate cycle . These apparently uneconomical cycles allow to regulate opposite metabolic pathways. In fact, a substrate cycle involves different enzymes, at least two, whose activity can be regulated separately. A such regulation would not be possible if a single enzyme would operate in both directions. The modulation of the activity of involved enzymes occurs through:

  • allosterical mechanisms;
  • covalent modifications, such as phosphorylation and dephosphorylation;
  • changes in the concentration of the involved enzymes, due to changes in their synthesis/degradation ratio.

Allosteric mechanisms are very rapid and instantly reversible, taking place in milliseconds. The others, triggered by signals from outside the cell, such as hormones, like insulin, glucagon, or epinephrine, take place on a time scale of seconds or minutes, and, for changes in enzyme concentration, hours.
This allows a coordinated regulation of the two pathways, ensuring that when pyruvate enters gluconeogenesis, the flux of glucose through the glycolytic pathway slows down, and vice versa.

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Regulation of gluconeogenesis

The regulation of gluconeogenesis and glycolysis involves the enzymes unique to each pathway, and not the common ones.
While the major control points of glycolysis are the reactions catalyzed by PFK-1 and pyruvate kinase, the major control points of gluconeogenesis are the reactions catalyzed by fructose 1,6-bisphosphatase and pyruvate carboxylase.
The other two enzymes unique to gluconeogenesis, glucose-6-phosphatase and PEP carboxykinase, are regulated at transcriptional level.

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Pyruvate carboxylase

Gluconeogenesis
Fig. 4 – Two Alternative Fates for Pyruvate

In the mitochondrion, pyruvate can be converted to:

        • acetyl-CoA, in the reaction catalyzed by pyruvate dehydrogenase complex, reaction that connects glycolysis to the Krebs cycle;
        • oxaloacetate, in the reaction catalyzed by private carboxylase, to continue in the gluconeogenesis pathway.

The metabolic fate of pyruvate depends on the availability of acetyl-CoA, that is, by the availability of fatty acids in the mitochondrion.
When fatty acids are available, their β-oxidation leads to the production of acetyl-CoA, that enters the Krebs cycle and leads to the production of GTP and NADH. When the energy needs of the cell are met, oxidative phosphorylation slows down, the [NADH]/[NAD+] ratio increases, NADH inhibits the citric acid cycle, and acetyl-CoA accumulates in the mitochondrial matrix. Acetyl-CoA is a positive allosteric effector of pyruvate carboxylase, and a negative allosteric effector of pyruvate kinase. Moreover, it inhibits pyruvate dehydrogenase both through end-product inhibition and phosphorylation through the activation of a specific kinase.
This means that when the energy charge of the cell is high, the formation of acetyl-CoA from pyruvate slows down, while the conversion of pyruvate to glucose is stimulated. Therefore acetyl-CoA is a molecule that signals that additional glucose oxidation for energy is not required and that glucogenic precursors can be used for the synthesis and storage of glucose.
Conversely, when acetyl-CoA levels decrease, the activity of pyruvate kinase and pyruvate dehydrogenase increases, and therefore also the flow of metabolites through the citric acid cycle. This supplies energy to the cell.
Summarizing, when the energy charge of the cell is high pyruvate carboxylase is active, and that the first control point of gluconeogenesis determines what will be the fate of pyruvate in the mitochondria.

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Fructose 1,6-bisphosphatase

The second major control point in gluconeogenesis is the reaction catalyzed by fructose 1,6-bisphosphatase. The enzyme is allosterically inhibited by AMP. Therefore, when AMP levels are high, and consequently ATP levels are low, gluconeogenesis slows down. This means that, as previously seen, FBPase-1 is active when the energy charge of the cell is sufficiently high to support de novo synthesis of glucose.
Conversely, PFK-1, the corresponding glycolytic enzyme, is allosterically activated by AMP and ADP and allosterically inhibited by ATP and citrate, the latter resulting from the condensation of acetyl-CoA and oxaloacetate.

Gluconeogenesis
Fig. 5 – Regulation of FBPase-1 and PFK-1

Therefore:

  • when AMP levels are high, gluconeogenesis slows down, and glycolysis accelerates;
  • when ATP levels are high or when acetyl-CoA or citrate are present in adequate concentrations, gluconeogenesis is promoted, while glycolysis slows down.
    The increase in citrate levels indicates that the activity of the citric acid cycle can slow down; in this way,  pyruvate can be used in glucose synthesis.

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PFK-1, FBPase-1 and fructose 2,6-bisphosphate
Gluconeogenesis
Fig. 6 – Fructose 2,6-bisphosphate

The liver plays a key role in maintaining blood glucose homeostasis: this requires regulatory mechanisms that coordinate glucose consumption and production. Two hormones are mainly involved: glucagon and insulin. They act intracellularly through fructose 2,6-bisphosphate or F26BP, an allosteric effector of PFK-1 and FBPase-1. This molecule is structurally related to fructose 1,6-bisphosphate, but is not an intermediate in glycolysis or gluconeogenesis. It was discovered in 1980 by Emile Van Schaftingen and Henri-Gery Hers, as a potent activator of PFK-1. In the subsequent year, the same researchers showed that it is also a potent inhibitor of FBPase-1.
Fructose 2,6-bisphosphate, by binding to the allosteric site on PFK-1, reduces the affinity of the enzyme for ATP and citrate, allosteric inhibitors, and at the same time increases the affinity of the enzyme for fructose 6-phosphate, its substrate. PFK-1, in the absence of fructose 2,6-bisphosphate, and in the presence of physiological concentrations of ATP, fructose 6-phosphate, and of allosteric effectors AMP, ATP and citrate, is practically inactive. Conversely, the presence of fructose 2,6-bisphosphate activates PFK-1, thus stimulating glycolysis in the hepatocytes. At the same time fructose 2,6-bisphosphate slows down gluconeogenesis by inhibiting fructose 1,6-bisphosphatase, even in the absence of AMP. However, the effects of fructose-2,6-bisphosphate and AMP on FBPase-1 activity  are synergistic.

Gluconeogenesis
Fig. 7 – Role of F26BP in the Regulation of Gluconeogenesis and Glycolysis

Fructose-2,6-bisphosphate concentration is regulated by the relative rates of synthesis and degradation. It is synthesized from fructose 6-phosphate in the reaction catalyzed by phosphofructokinase-2 or PFK-2 (EC 2.7.1.105), and is hydrolyzed to fructose 6-phosphate in the reaction catalyzed by fructose 2,6-bisphosphatase or FBPasi-2 (EC 3.1.3.46). These two enzymatic activities are located on a single bifunctional enzyme or tandem enzyme. In the liver, the balance of these two enzymatic activities is regulated by insulin and glucagon, as described below.

  • Glucagon
    It is released into the circulation when blood glucose levels drop, signaling the liver to reduce glucose consumption for its own needs and to increase de novo synthesis of glucose and its release from glycogen stores.
    After binding to specific membrane receptors, glucagon stimulates hepatic adenylate cyclase (EC 4.6.1.1) to synthesize 3′,5′-cyclic AMP or cAMP, that activates cAMP-dependent protein kinase or protein kinase A or PKA (EC 2.7.11.11). The kinase catalyzes the phosphorylation, at the expense of one molecule of ATP, of a specific serine residue (Ser32) of PFK-2/FBPase-2. As a result of the phosphorylation, phosphatase activity increases while kinase activity decreases. Such reduction, due to the increase in the Km for fructose 6-phosphate, causes a decrease in the levels of fructose 2,6-bisphosphate, that, in turn, inhibits glycolysis and stimulates gluconeogenesis. Therefore, in response to glucagon, hepatic production of glucose increases, enabling the organ to counteract the fall in blood glucose levels.
    Note: glucagon, like adrenaline, stimulates gluconeogenesis also by increasing the availability of substrates such as glycerol and amino acids.
  • Insulin
    After binding to specific membrane receptors, insulin activates a protein phosphatase, the phosphoprotein phosphatase 2A or PP2A, that catalyzes the removal of the phosphate group from PFK-2/FBPase-2, thus increasing PFK-2 activity and decreasing FBPase-2 activity. (At the same time, insulin also stimulates a cAMP phosphodiesterase that hydrolyzes cAMP to AMP). This increases the level of fructose 2,6-bisphosphate, that, in turn, inhibits gluconeogenesis and stimulates glycolysis.
    In addition, fructose 6-phosphate allosterically inhibits FBPase-2, and activates PFK-2. It should be noted that the activities of PFK-2 and FBPase-2 are inhibited by their reaction products. However, the main effectors are the level of fructose 6-phosphate and the phosphorylation state of the enzyme.

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Glucose 6-phosphatase

Unlike pyruvate carboxylase and fructose-1,6-bisphosphatase, the catalytic subunit of glucose-6-phosphatase is not subject to allosteric or covalent regulation. The modulation of its activity occurs at the transcriptional level. Low blood glucose levels and glucagon, namely, factors that lead to increased glucose production, and glucocorticoids stimulate its synthesis, that, conversely, is inhibited by insulin.
Also, the Km for glucose 6-phosphate is significantly higher than the range of physiological concentrations of glucose 6-phosphate itself. This is why it is said that the activity of the enzyme is almost linearly dependent on the concentration of the substrate, that is, enzyme is controlled by the level of substrate.

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PEP carboxykinase

The enzyme is regulated mainly at the level of synthesis and degradation. For example, high levels of glucagon or fasting increase protein production through the stabilization of its mRNA and the increase in its transcription rate. High blood glucose levels or insulin have opposite effects.

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Xylulose 5-phosphate

Gluconeogenesis
Fig. 8 – Xylulose 5-phosphate

Xylulose 5-phosphate, a product of the hexose monophosphate shunt, is a recently discovered regulatory molecule. It stimulates glycolysis and inhibits gluconeogenesis by controlling the levels of fructose 2,6-bisphosphate in the liver.
When blood glucose levels increase, e.g. after a meal high in carbohydrates, the activation of glycolysis and hexose monophosphate pathway occurs in the liver. Xylulose 5-phosphate produced activates protein phosphatase 2A, that, as previously said, dephosphorylates PFK-2/FBPase-2, thus inhibiting FBPase-2 and stimulating PFK-2. This leads to an increase in the concentration of fructose 2,6-bisphosphate, and then to the inhibition of gluconeogenesis and stimulation of glycolysis, resulting in increased production of acetyl-CoA, the main substrate for lipid synthesis. At the same time, an increase in flow through the hexose monophosphate shunt occurs, leading to the production of NADPH, a source of electrons for lipid synthesis. Finally, PP2A also dephosphorylates carbohydrate-responsive element-binding protein or ChREBP, a transcription factor that activates the expression of hepatic genes for lipid synthesis. Therefore, in response to an increase in blood glucose levels, lipid synthesis is stimulated.
It is therefore evident that xylulose 5-phosphate is a key regulator of carbohydrate and fat metabolism.

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Precursors of gluconeogenesis

Besides the aforementioned pyruvate, the major gluconeogenic precursors are lactate (see above), glycerol, the majority of the amino acids, and, more generally, any compound that can be converted to pyruvate or oxaloacetate.

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Glycerol

Glycerol is released by hydrolysis of triglycerides in adipose tissue, and of glycerophospholipids. With the exception of propionyl-CoA (see below), it is the only part of the lipid molecule that can be used for de novo synthesis of glucose in animals.
Glycerol enters gluconeogenesis, or glycolysis, depending on the cellular energy charge, as dihydroxyacetone phosphate or DHAP, whose synthesis occurs in two steps.

Gluconeogenesis
Fig. 9 – Conversion of Glycerol to DHAP

In the first step, glycerol is phosphorylated to glycerol 3-phosphate, in the reaction catalyzed by glycerol kinase (EC 2.7.1.30), with the consumption of one ATP. The enzyme is absent in adipocytes but present in the liver; this means that glycerol needs to reach the liver to be further metabolized.
Glycerol 3-phosphate is then oxidized to dihydroxyacetone phosphate, in the reaction catalyzed by glycerol 3-phosphate dehydrogenase (EC 1.1.1.8). In this reaction NAD+ is reduced to NADH.
During prolonged fasting, glycerol is the major gluconeogenic precursor, accounting for about 20% of glucose production.

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Glucogenic amino acids

Pyruvate and oxaloacetate are the entry points for the glucogenic amino acids, i.e. those whose carbon skeleton or part of it can be used for de novo synthesis of glucose.
Amino acids result from the catabolism of proteins, both food and endogenous proteins, like those of skeletal muscle during the fasting state or during intense and prolonged exercise.
The catabolic processes of each of the twenty amino acids that made up the proteins converge to form seven major products, acetyl-CoA, acetoacetyl-CoA, α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate, and pyruvate.
Except acetyl-CoA, acetoacetyl-CoA , the other five molecules can be used for gluconeogenesis. This means that gluconeogenic amino acids may also be defined as those whose carbon skeleton or part of it can be converted to one or more of the above molecules.
Below, the entry points of the gluconeogenic amino acids are shown.

  • Pyruvate: alanine, cysteine, glycine, serine, threonine and tryptophan.
  • Oxaloacetate: aspartate and asparagine.
  • α-Ketoglutarate: glutamate, arginine, glutamine, histidine and proline.
  • Succinyl-CoA: isoleucine, methionine, threonine and valine.
  • Fumarate: phenylalanine and tyrosine.
Gluconeogenesis
Fig. 10 – Glucogenic and Ketogenic Amino Acids

α-Ketoglutarate, succinyl-CoA and fumarate, intermediates of the citric acid cycle, enter the gluconeogenic pathway after conversion to oxaloacetate.
The utilization of the carbon skeletons of the amino acids requires the removal of the amino group. Alanine and glutamate, the key molecules in the transport of amino groups from extrahepatic tissues to the liver, are major glucogenic amino acids in mammals. Alanine is the main gluconeogenic substrate for the liver; this amino acid is shuttled to the liver from muscle and other peripheral tissues through the glucose-alanine cycle.

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Ketogenic amino acids

Acetyl-CoA and acetoacetyl-CoA cannot be used for gluconeogenesis and are precursors of fatty acids and ketone bodies. The stoichiometry of the citric acid cycle elucidates why they cannot be used for de novo synthesis of glucose.
Acetyl-CoA, in the reaction catalyzed by citrate synthase, condenses with oxaloacetate to form citrate, a molecule with 6 carbon atoms instead of 4 as oxaloacetate. However, although the two carbon atoms from acetyl-CoA become part of the oxaloacetate molecule, two carbon atoms are oxidized and removed  as CO2, in the reactions catalyzed by isocitrate dehydrogenase (EC 1.1.1.42) and α-ketoglutarate dehydrogenase complex. Therefore, acetyl-CoA does not yield any net carbon gain for the citric acid cycle.
Furthermore, the reaction leading to the formation of acetyl-CoA from pyruvate, catalyzed by the pyruvate dehydrogenase complex, that is the bridge between glycolysis and the Krebs cycle, is irreversible, and there is no other pathway to convert acetyl-CoA to pyruvate.

Pyruvate + NAD+ + CoASH → Acetyl-CoA + NADH + H+ + C02

For this reason, amino acids whose catabolism produces acetyl-CoA and/or acetoacetyl-CoA, are termed ketogenic.
Only leucine and lysine are exclusively ketogenic.

Note: plants, yeasts, and many bacteria can use acetyl-CoA for de novo synthesis of glucose as they do have the glyoxylate cycle. This cycle has four reactions in common with the citric acid cycle, two unique enzymes, isocitrate lyase (EC 4.1.3.1)  and malate synthase (EC 2.3.3.9), but lacks the decarboxylation reactions. Therefore, organisms that have such pathway are able to use fatty acids for gluconeogenesis.

Five amino acids, isoleucine, phenylalanine, tyrosine, threonine and tryptophan, are both glucogenic and ketogenic, because part of their carbon backbone can be used for gluconeogenesis, while the other gives rise to ketone bodies.

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Propionate

Propionate, a three carbon fatty acid, is a gluconeogenic precursor because, as propionyl-CoA, the active molecule, can be converted to succinyl-CoA.
Below, the different sources of propionate are analyzed.

  • It may arise from β-oxidation of odd-chain fatty acids such as margaric acid, a saturated fatty acid with 17 carbon atoms. Such fatty acids are rare compared to even-chain fatty acids, but present in significant amounts in the lipids of some marine organisms, ruminants, and plants. In the last pass through the β-oxidation sequence, the substrate is a five carbon fatty acid. This means that, once oxidized and cleaved to two fragments, it produces an acetyl-CoA and propionyl-CoA.
  • Another source is the oxidation of branched-chain fatty acids, with alkyl branches with an odd number of carbon atoms. An example is phytanic acid, produced in ruminants by oxidation of phytol, a breakdown product of chlorophyll.
  • In ruminants, propionate is also produced from glucose. Glucose is released from breakdown of cellulose by bacterial cellulase (EC 3.2.1.4) in the rumen, one of the four chambers that make up the stomach of these animals. These microorganisms then convert, through fermentation, glucose to propionate, which, once absorbed, may be used for gluconeogenesis, synthesis of fatty acids, or be oxidized for energy.
    In ruminants, in which gluconeogenesis tends to be a continuous process, propionate is the major gluconeogenic precursor.
  • Propionate may also result from the catabolism of valine, leucine, and isoleucine (see above).

The oxidation of propionyl-CoA to succinyl-CoA involves three reactions that occur in the liver and other tissues.

Gluconeogenesis
Fig. 11 – Conversion of Propionyl-CoA to Succinyl-CoA

In the first reaction, propionyl-CoA is carboxylated to D-methylmalonyl-CoA in the reaction catalyzed by propionyl-CoA carboxylase (EC 6.4.1.3), a biotin-requiring enzyme. This reaction consumes one ATP. In the subsequent reaction, catalyzed by methylmalonyl-CoA epimerase (EC 5.1.99.1), D-methylmalonyl-CoA is epimerized to its L-stereoisomer. Finally, L-methylmalonyl-CoA undergoes an intramolecular rearrangement to succinyl-CoA, in the reaction catalyzed by methylmalonyl-CoA mutase (EC 5.4.99.2). This enzyme requires 5-deoxyadenosylcobalamin or coenzyme B12, a derivative of cobalamin or vitamin B12, as a coenzyme.

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References

Bender D.A. Introduction to nutrition and metabolism. 3rd Edition. Taylor & Francis, 2004

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

Kabashima T., Kawaguchi T., Wadzinski B.E., Uyeda K. Xylulose 5-phosphate mediates glucose-induced lipogenesis by xylulose 5-phosphate-activated protein phosphatase in rat liver. Proc Natl Acad Sci USA 2003;100:5107-12. doi:10.1073/pnas.0730817100

Kuriyama H. et all. Coordinated regulation of fat-specific and liver-specific glycerol channels, aquaporin adipose and aquaporin 9. Diabetes 2002;51(10):2915-21. doi:10.2337/diabetes.51.10.2915

McCommis K.S. and Finck B.N. Mitochondrial pyruvate transport: a historical perspective and future research directions. Biochem J 2015;466(3):443-54. doi:10.1042/BJ20141171

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Rosenthal M.D., Glew R.H. Medical biochemistry – Human metabolism in health and disease. John Wiley J. & Sons, Inc., Publication, 2009

Soty M., Chilloux J., Delalande F., Zitoun C., Bertile F., Mithieux G., and Gautier-Stein A. Post-Translational regulation of the glucose-6-phosphatase complex by cyclic adenosine monophosphate is a crucial determinant of endogenous glucose production and is controlled by the glucose-6-phosphate transporter. J Proteome Res  2016;15(4):1342-49. doi:10.1021/acs.jproteome.6b00110

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2013 [Google eBooks]

Van Schaftingen, E., and Hers, H-G. Inhibition of fructose-1,6-bisphosphatase by fructose-2,6-bisphosphate. Proc Natl Acad Sci USA 1981;78(5):2861-63 [PDF]

Van Schaftingen E., Jett M-F., Hue L., and Hers, H-G. Control of liver 6-phosphofructokinase by fructose 2,6-bisphosphate and other effectors. Proc Natl Acad Sci USA 1981;78(6):3483-86 [PDF]


Cori cycle: definition, function, biochemistry, involved tissues

Cori cycle: contents in brief

What is the Cori cycle?

The Cori cycle, or glucose-lactate cycle, was discovered by Carl Ferdinand Cori and Gerty Theresa Radnitz, a husband-and-wife team, in the ‘30s and ‘40s of the last century . They demonstrated the existence of a metabolic cooperation between the skeletal muscle working under low oxygen conditions and the liver. This cycle can be summarized as follows:

  • the conversion of glucose to lactic acid, or lactate, by anaerobic glycolysis in skeletal muscle cells;
  • the diffusion of lactate from muscle cells into the bloodstream, by which it is transported to the liver;
  • the conversion of lactate to glucose by hepatic gluconeogenesis;
  • the diffusion of glucose from the hepatocytes into the bloodstream, by which it is transported back to the skeletal muscle cells, thereby closing the cycle.

Summarizing, we have: part of the lactate produced in skeletal muscle is converted to glucose in the liver, and transported back to skeletal muscle, thus closing the cycle.

Glucose → Lactate →Glucose

The importance of this cycle is demonstrated by the fact that it may account for about 40% of plasma glucose turnover.

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Where does the Cori cycle occur?

In addition to skeletal muscle, this metabolic cooperation was also demonstrated between other extrahepatic tissues and liver.  Indeed, like the glucose-alanine cycle, the glucose-lactate cycle is active between the liver and all those tissues that do not completely oxidize glucose to CO2 and H2O, in which case pyruvate for conversion to lactate or, by transamination, to alanine would lack (see below).
In addition to skeletal muscle cells, examples of cells that continually produce lactic acid are red blood cells, immune cells in the lymph nodules,  proliferating cells in the bone marrow, and epithelial cells in the skin.
Note: skeletal muscle produces lactate even at rest, although at low rate.

Cori Cycle
Fig. 1 – The Cori Cycle

From a biochemical point of view, the Cori cycle links gluconeogenesis with anaerobic glycolysis, using different tissues to compartmentalize opposing metabolic pathways. In fact, in the same cell, regardless of the cell type, these metabolic pathways are not very active simultaneously. Glycolysis is more active when the cell requires ATP; by contrast, when the demand for ATP is low, gluconeogenesis, in those cells where it occurs, is more active.
And it is noteworthy that, although traditionally the metabolic pathways, such as glycolysis, citric acid cycle, or gluconeogenesis, are considered to be confined within individual cells, the Cori cycle, as well as the glucose-alanine cycle, occurs between different cell types.
Finally, it should be underscored that the Cori cycle also involves the renal cortex, particularly the proximal tubules, another site where gluconeogenesis occurs.

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The steps of the Cori cycle

The analysis of the steps of the Cori cycle is made considering the lactate produced by red blood cells and skeletal muscle cells.
Mature red blood cells are devoid of mitochondria, nucleus and ribosomes, and obtain the necessary energy only by glycolysis. The availability of NAD+ is essential for glycolysis to proceed as well as for its rate: the oxidized form of the coenzyme is required for the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12).

Glyceraldehyde 3-phosphate + NAD+ → 1,3-Bisphosphoglycerate + NADH + H+

The accumulation of NADH is avoided by the reduction of pyruvate to lactate, in the reaction catalyzed by lactate dehydrogenase (EC 1.1.1.27), where NADH acts as reducing agent.

Pyruvate + NADH + H+ → Lactate + NAD+

The skeletal muscle, particularly fast-twitch fibers which contain a reduced number of mitochondria, under low oxygen condition, such as during intense exercise, produces significant amounts of lactate. In fact, in such conditions:

  • the rate of pyruvate production by glycolysis  exceeds the rate of its oxidation by the citric acid cycle, so that less than 10% of the pyruvate enters the citric acid cycle;
  • the rate at which oxygen is taken up by the cells is not sufficient to allow aerobic oxidation of all the NADH  produced.

And, like in red blood cells, the reaction catalyzed by lactate dehydrogenase, regenerating NAD+, allows glycolysis to proceed.
However, lactate is an end product of metabolism that must be converted back into pyruvate to be used.
The plasma membrane of most cells is freely permeable to both pyruvate and lactate that can thus reach the bloodstream. And, regarding for example the skeletal muscle, the amount of lactate that leaves the cell is greater than that of pyruvate due to the high NADH/NAD+ ratio in the cytosol and to the catalytic properties of the skeletal muscle isoenzyme of LDH.
Once into the bloodstream, lactate reaches the liver, which is its major user, where it is oxidized to pyruvate in the reaction catalyzed by the liver isoenzyme of lactate dehydrogenase (see below).

Lactate + NAD+ → Pyruvate + NADH + H+

In the hepatocyte, this oxidation is favored by the low NADH/NAD+ ratio in the cytosol.
Then, pyruvate enters the gluconeogenesis pathway to be converted into glucose.
Glucose leaves the liver, enters into the bloodstream and is delivered to the muscle, as well as to other tissues and cells that require it, such as red blood cells and neurons, thus closing the cycle.

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Lactate dehydrogenase

The enzyme is a tetramer composed of two different types of subunits, designed as:

  • H subunit (heart) or B chain;
  • M subunit (muscle) or A chain.

The H subunit predominates in the heart, whereas the M subunit predominates in the  skeletal muscle and liver. Typically, tissues in which a predominantly or exclusively aerobic metabolism occurs, such as the heart, synthesize H subunits to a greater extent than M subunits, whereas tissues in which anaerobic metabolism is important, such as skeletal muscle, synthesize M subunits to a greater extent than H subunits.
The two subunits associate in 5 different ways to form homopolymers, that is, macromolecules formed by repeated, identical subunits, or heteropolymers, that is, macromolecules formed by different subunits. Different LDH  isoenzymes have different catalytic properties, as well as different distribution in various tissues, as indicated below:

  • H4, also called type 1, LDH1, or A4, a homopolymer of H subunits, is found in cardiac muscle, kidney, and red blood cells;
  • H3M1, also called type 2, LDH2, or A3B, has a tissue distribution similar to that of LDH1;
  • H2M2, also called type 3, LDH3, or A2B2, is found in the spleen, brain, white cells, kidney, and lung;
  • H1M3, also called type 4, LDH4, or AB3, is found in the spleen, lung, skeletal muscle, lung, red blood cells, and kidney;
  • M4, also called type 5, LDH5, or B4, a homopolymer of M subunits, is found in the liver, skeletal muscle, and spleen.

The H4 isoenzyme has a higher substrate affinity than the M4 isoenzyme.
The H4 isoenzyme is allosterically inhibited by high levels of pyruvate (its product), whereas the M4 isoenzyme is not.
The other LDH isoenzymes have intermediate properties, depending on the ratio between the two types of subunits.
It is thought that the H4 isoenzyme is the most suitable for catalyzing the oxidation of lactate to pyruvate that, in the heart, due to its exclusively aerobic metabolism, is then completely oxidized to CO2 and H2O. Instead, the M4 isoenzyme is the main isoenzyme found in skeletal muscle, most suitable for catalyzing the reduction of pyruvate to lactate, thus allowing glycolysis to proceed in anaerobic conditions.

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Other metabolic fates of lactate

From the above, it is clear that lactate is not a metabolic dead end, a waste product of glucose metabolism.
And it may have a different fate from that entering the Cori cycle.
For example, in skeletal muscle during recovery following an exhaustive exercise, that is, when oxygen is again available, or if the exercise is of low intensity, lactate is re-oxidized to pyruvate, due to NAD+ availability, and then completely oxidized to CO2 and H20, with a greater production of ATP than in anaerobic condition. In such conditions, the energy stored in NADH will be released, yielding on average 2.5 ATP per molecule of NADH.
In addition, lactate can be taken up by exclusively aerobic tissues, such as heart, to be oxidized to CO2 and H20.

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Energy cost of the Cori cycle

The Cori cycle results in a net consumption of 4 ATP.
The gluconeogenic leg of the cycle consumes 2 GTP and 4 ATP per molecule of glucose synthesized, that is, 6 ATP.
The ATP-consuming reactions are catalyzed by:

  • pyruvate carboxylase (EC 6.4.1.1): an ATP;
  • phosphoenolpyruvate carboxykinase (EC 4.1.1.32): a GTP;
  • glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12): an ATP.

Since two molecules of lactate are required for the synthesis of one molecule of glucose, the net cost is 2×3=6 high energy bonds per molecule of glucose.
Conversely, the glycolytic leg of the cycle produces only 2 ATP per molecule of glucose.
Therefore, more energy is required to produce glucose from lactate than that obtained by anaerobic glycolysis in extrahepatic tissues. This explains why the Cori cycle cannot be sustained indefinitely.

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Is the Cori cycle a futile cycle?

The continuous breakdown and resynthesis of glucose, feature of the Cori cycle, might seem like a waste of energy. Indeed, this cycle allows the effective functioning of many extrahepatic cells at the expense of the liver and partly of the renal cortex. Below, two examples.

  • Red blood cells
    These cells, lacking a nucleus, ribosomes, and mitochondria, are smaller than most other cells. Their small size allows them to pass through tiny capillaries. However, the lack of mitochondria makes them completely dependent on anaerobic glycolysis for ATP production. Then, the lactate is partly disposed of by the liver and renal cortex.
  • Skeletal muscle
    Its cells, and particularly fast-twitch fibers contracting under low oxygen conditions, such as during intense exercise, produce much lactate.
    In such conditions, anaerobic glycolysis leads to the production of 2 ATP per molecule of glucose, 3 if the glucose comes from muscle glycogen, therefore, much lower than the 29-30 ATP produced by the complete oxidation of the monosaccharide. However, the rate of ATP production by anaerobic glycolysis is greater than that produced by the complete oxidation of glucose. Therefore, to meet the energy requirements of contracting muscle, anaerobic glycolysis is an effective means of ATP production. But this could lead to an intracellular accumulation of lactate, and a consequent reduction in intracellular pH. Obviously, such accumulation does not occur, due also to the Cori cycle, in which the liver pays the cost of the disposal of a large part of the muscle lactate, thereby allowing the muscle to use ATP for the contraction.
    And the oxygen debt, which always occurs after a strenuous exercise, is largely due to the increased oxygen demand of the hepatocytes, in which the oxidation of fatty acids, their main fuel, provides the ATP required for gluconeogenesis from lactate.
  • During trauma, sepsis, burns, or after major surgery, an intense cell proliferation occurs in the wound, that is a hypoxic tissue, and in bone marrow. This in turn results in greater production of lactate, an increase in the flux through the Cori cycle and an increase in ATP consumption in the liver, which, as previously said, is supported by an increase in fatty acid oxidation. Hence, the nutrition plan provided to these patients must be taken into account this increase in energy consumption.
  • A similar condition seems to occur also in cancer patients with progressive weight loss.
  • The Cori cycle is also important during overnight fasting and starvation.

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The Cori cycle and glucose-alanine cycle

These cycles are metabolic pathways that contribute to ensure a continuous delivery of glucose to tissues for which the monosaccharide is  the primary source of energy.
The main difference between the two cycles consists in the three carbon intermediate which is recycled: in the Cori cycle, carbon returns to the liver in the form of pyruvate, whereas in the glucose-alanine cycle in the form of alanine.
For more information, see: glucose-alanine cycle.

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References

Bender D.A. Introduction to nutrition and metabolism. 3rd Edition. Taylor & Francis, 2004

Berg J.M., Tymoczko J.L., and Stryer L. Biochemistry. 5th Edition. W. H. Freeman and Company, 2002

Iqbal S.A., Mido Y. Biochemistry. Discovery Publishing House, 2005 [Google eBook]

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Newsholme E.A., Leech T.R. Functional biochemistry in health and disease. John Wiley J. & Sons, Inc., Publication, 2010 [Google eBook]

Rawn J.D. Biochimica. Mc Graw-Hill, Neil Patterson Publishers, 1990

Rosenthal M.D., Glew R.H. Medical biochemistry – Human metabolism in health and disease. John Wiley J. & Sons, Inc., Publication, 2009

Shils M.E., Olson J.A., Shike M., Ross A.C. Modern nutrition in health and disease. 9th Ed., by Lippincott, Williams & Wilkins, 1999

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2013 [Google eBooks]


Gluten: definition, structure, properties, wheat, cereal list

Gluten: contents in brief

What is gluten?

Gluten
Fig. 1 – Wheat

Gluten is not a single protein but a mixture of cereal proteins, about 80% of its dry weight (for example gliadins and glutenins in wheat grains), lipids, 5-7%, starch, 5-10%, water, 5-8%, and mineral substances, <2%.
It forms when components naturally present in the grain of cereals, the caryopsis, and in their flours, are joined together by means of mechanical stress in aqueous environment, i.e. during the formation of the dough.
The term is also related to the family of proteins that cause problems for celiac patients (see below).
Isolated for the first time in 1745 from wheat flour by the Italian chemist Jacopo Bartolomeo Beccari, it can be extracted from the dough by washing it gently under running water: starch, albumins and globulins, that are water-soluble, are washed out, and a sticky and elastic mass remains, precisely the gluten (it means glue in Latin).

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Cereals containing gluten

It is present in:

  • wheat, such as:

durum wheat (Triticum durum); groats and semolina for dry pasta making are obtained from it;
common wheat or bread wheat (Triticum aestivum), so called because it is used in bread and fresh pasta making, and in bakery products;

  • rye (Secale cereale);
  • barley (Hordeum vulgare);
  • spelt, in the three species:

einkorn (Triticun monococcum);
emmer (Triticum dicoccum Schrank);
spelta (Triticum spelta);

  • khorasan wheat (Triticum turanicum); a variety of it is Kamut®;
  • triticale (× Triticosecale Wittmack), which is a hybrid of rye and common wheat;
  • bulgur, which is whole durum wheat, sprouted and then processed;
  • seitan, which is not a cereal, but a wheat derivative, also defined by some as “gluten steak”.

Given that most of the dietary intake of gluten comes from wheat flour, of which about 700 million tons per year are harvested, representing about 30% of the global cereal production, the following discussion will focus on wheat gluten, and mainly on its proteins.

Note: the term gluten is also used to indicate the protein fraction that remains after removal of starch and soluble proteins from the dough obtained with corn flour: however, this “corn gluten” is “functionally” different from that obtained from wheat flour.

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Cereal grain proteins

Gluten
Fig. 2 – Cereal Grain Proteins

The study of cereal grain proteins, as seen, began with the work of Beccari. 150 years later, in 1924, the English chemist Osborne T.B., which can rightly be considered the father of plant protein chemistry, developed a classification based on their solubility in various solvents.
The classification, still in use today, divides plant proteins into 4 families.

  • Albumins, soluble in water.
  • Globulins, soluble in saline solutions; for example avenalin of oat.
  • Prolamins, soluble in 70% alcohol solution, but not in water or absolute alcohol.
    They include:

gliadins of wheat;
zein of corn;
avenin of oats;
hordein of barley;
secalin of rye.

They are the toxic fraction of gluten for celiac patients.

  • Glutelins, insoluble in water and neutral salt solutions, but soluble in acidic and basic solutions.
    They include glutenins of wheat.

Albumins and globulins are cytoplasmic proteins, often enzymes, rich in essential amino acids, such as lysine, tryptophan and methionine. They are found in the aleurone layer and embryo of the caryopsis.
Prolamins and glutelins are the storage proteins of cereal grains. They are rich in glutamine and proline, but very low in lysine, tryptophan and methionine. They are found in the endosperm, and are the vast majority of the proteins in the grains of wheat, corn, barley, oat, and rye.
Although Osborne classification is still widely used, it would be more appropriate to divide cereal grain proteins into three groups: structural and metabolic proteins, storage proteins, and defense proteins.

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Wheat gluten proteins

Proteins represent 10-14% of the weight of the wheat caryopsis (about 80% of its weight consists of carbohydrates).
According to the Osborne classification, albumins and globulins represent 15-20% of the proteins, while prolamins and glutelins are the remaining 80-85%, composed respectively of gliadins, 30-40%, and glutenins, 40-50%. Therefore, and unlike prolamins and glutelins in the grains of other cereals, gliadins and glutenins are present in similar amounts, about 40%.
Technologically, gliadins and glutenins are very important. Why?
These proteins are insoluble in water, and in the dough, that contains water, they bind to each other through a combination of intermolecular bonds, such as:

  • covalent bonds, i.e. disulfide bridges;
  • noncovalent bonds, such as hydrophobic interactions, van der Waals forces, hydrogen bonds, and ionic bonds.

Thanks to the formation of these intermolecular bonds, a three-dimensional lattice is formed. This structure entraps starch granules and carbon dioxide bubbles produced during leavening, and gives strength and elasticity to the dough, two properties of gluten widely exploited industrially.
In the usual diet of the European adult population, and in particular in Italian diet that is very rich in derivatives of wheat flour, gliadin and glutenin are the most abundant proteins, about 15 g per day. What does this mean? It means that gluten-free diet engages celiac patients both from a psychological and social point of view.

Note: the lipids of the gluten are strongly associated with the hydrophobic regions of gliadins and glutenins and, unlike what you can do with the flour, they are extracted with more difficulty (the lipid content of the gluten depends on the lipid content of the flour from which it was obtained).

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Gliadins: extensibility and viscosity

Gluten
Fig. 3 – Wheat Grain Proteins

Gliadins are hydrophobic monomeric prolamins, of globular nature and with low molecular weight. On the basis of electrophoretic mobility in low pH conditions, they are separated into the following types:

  • alpha/beta, and gamma, rich in sulfur, containing cysteines, that are involved in the formation of intramolecular disulfide bonds, and methionines;
  • omega, low in sulfur, given the almost total absence of cysteine and methionine.

They have a low nutritional value and are toxic to celiac patients because of the presence of particular amino acid sequences in the primary structure, such as proline-serine-glutamine-glutamine and glutamine-glutamine-glutamine-proline.
Gliadins are associated with each other and with glutenins through noncovalent interactions; thanks to that, they act as “plasticizers” in dough making. Indeed, they are responsible for viscosity and extensibility of gluten, whose three-dimensional lattice can deform, allowing the increase in volume of the dough as a result of gas production during leavening. This property is important in bread-making.
Their excess leads to the formation of a very extensible dough.

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Glutenins: elasticity and toughness

Glutenins are polymeric proteins, that is, formed of multiple subunits, of fibrous nature, linked together by intermolecular disulfide bonds. The reduction of these bonds allows to divide them, by SDS-PAGE, into two groups.

  • High molecular weight (HMW) subunits, low in sulfur, that account for about 12% of total gluten proteins. The noncovalent bonds between them are responsible for the elasticity and tenacity of the gluten protein network, that is, of the viscoelastic properties of gluten, and so of the dough.
  • Low molecular weight (LMW) subunits, rich in sulfur (cysteine residues).
    These proteins form intermolecular disulfide bridges to each other and with HMW subunits, leading to the formation of a glutenin macropolymer.

Glutenins allow dough to hold its shape during mechanical (kneading) and not mechanical stresses (increase in volume due to both the leavening and the heat of cooking that increases the volume occupied by gases present) which is submitted. This property is important in pasta making.
If in excess, glutenins lead to the formation of a strong and rigid dough.

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Properties of wheat gluten

From the nutritional point of view, gluten proteins do not have a high biological value, being low in lysine, an essential amino acid. Therefore, a gluten-free diet does not cause any important nutritional deficiencies.
On the other hand, it is of great importance in food industry: the combination, in aqueous solution, of gliadins and glutenins to form a three-dimensional lattice, provides viscoelastic properties, that is, extensibility-viscosity and elasticity-tenacity, to the dough, and then, a good structure to bread, pasta, and in general, to all foods made with wheat flour.
It has a high degree of palatability.
It has a high fermenting power in the small intestine.
It is an exorphin: some peptides produced from intestinal digestion of gluten proteins may have an effect in central nervous system.

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Gluten-free cereals

The following is a list of gluten-free cereals, minor cereals, and pseudocereals used as foods.

  • Cereals

corn or maize (Zea mays)
rice (Oryza sativa)

  • Minor cereals
    They are defined “minor” not because they have a low nutritional value, but because they are grown in small areas and in lower quantities than wheat, rice and maize.

Fonio (Digitaria exilis)
Millet (Panicum miliaceum)
Panic (Panicum italicum)
Sorghum (Sorghum vulgare)
Teff (Eragrostis tef)
Teosinte; it is a group of four species of the genus Zea. They are plants that grow in Mexico (Sierra Madre), Guatemala and Venezuela.

  • Pseudocereals.
    They are so called because they combine in their botany and nutritional properties characteristics of cereals and legumes, therefore of another plant family.

Amaranth; the most common species are:

Amaranthus caudatus;
Amaranthus cruentus;
Amarantus hypochondriacus.

Buckwheat (Fagopyrum esculentum)
Quinoa (Chenopodium quinoa), a pseudocereal with excellent nutritional properties, containing fibers, iron, zinc and magnesium. It belongs to Chenopodiaceae family, such as beets.

  • Cassava, also known as tapioca, manioc, or yuca (Manihot useful). It is grown mainly in the south of the Sahara and South America. It is an edible root tuber from which tapioca starch is extracted.

It should be noted that naturally gluten-free foods may not be truly gluten-free after processing. Indeed, the use of derivatives of gliadins in processed foods, or contamination in the production chain may occur, and this is obviously important because even traces of gluten are harmful for celiac patients.

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Oats and gluten

Oats (Avena sativa) is among the cereals that celiac patients can eat. Recent studies have shown that it is tolerated by celiac patients, adult and child, even in subjects with dermatitis herpetiformis. Obviously, oats must be certified as gluten-free (from contamination).

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References

Beccari J.B. De Frumento. De bononiensi scientiarum et artium instituto atque Academia Commentarii, II. 1745:Part I.,122-127

Bender D.A. “Benders’ dictionary of nutrition and food technology”. 8th Edition. Woodhead Publishing. Oxford, 2006

Berdanier C.D., Dwyer J., Feldman E.B. Handbook of nutrition and food. 2th Edition. CRC Press. Taylor & Francis Group, 2007

Phillips G.O., Williams P.A. Handbook of food proteins. 1th Edition. Woodhead Publishing, 2011

Shewry P.R. and Halford N.G. Cereal seed storage proteins: structures, properties and role in grain utilization. J Exp Bot 2002:53(370);947-958. doi:10.1093/jexbot/53.370.947

Yildiz F. Advances in food biochemistry. CRC Press, 2009


Digestion of starch and alpha-amylase

Factors affecting relationship between starch and alpha-amylase

alpha-amylase
Fig. 1 – Spaghetti

Amylose and amylopectin, the two families of homopolysaccharides constituting starch, during their biosynthesis within vegetable cells, are deposited in highly organized particles called granules.
Granules have a partially crystalline structure and diameter ranging from 3 to 300 µm.
The access of the alpha-amylase, the enzyme that catalyzes the breakdown of amylose and amylopectin into maltose, maltotriose, and alpha-dextrins or alpha-limit dextrins, to carbohydrates making up granules varies as a function of:

  • amylose-amylopectin ratio;
  • temperature and packaging of amylose and amylopectin;
  • granules-associated proteins;
  • presence of fibers.

Amylose-amylopectin ratio

Starch for foodstuff use is obtained from various sources, the most important of which are corn (normal, waxy or high amylose content), potatoes, rice, tapioca and wheat.
Depending on botanical origin, molecular weight, degree of branching, and amylose-amylopectin ratio will vary.
Generally, there is 20-30% amylose and 70-80% amylopectin, even if there are starches with high amylose or amylopectin content (e.g. waxy corn). These differences justify the existence of starches with different chemical-physical characteristics and, to a certain extent, different digestibility.

  • corn: 24% amylose, 76% amylopectin;
  • waxy corn: 0,8% amylose, 99.2% amylopectin;
  • Hylon VII corn: 70% amylose, 30% amylopectin;
  • potatoes: 20% amylose, 80% amylopectin;
  • rice: 18.5 amylose, 81.5% amylopectin;
  • tapioca: 16.7% amylose, 83.3% amylopectin;
  • wheat: 25% amylose, 75% amylopectin.

Temperature and packaging of amylose and amylopectin

The chains of amylose, and to a lesser extent ramifications of amylopectin, thanks to the formation of hydrogen bonds with neighboring molecules and within the same molecules, have the tendency to aggregate. For this reason, pure amylose and amylopectin are poorly soluble in water at below 55 °C (131°F), and are more resistant to alpha-amylase action (resistant starch).
However, in aqueous solution, these granules hydrate increasing in volume of about 10%.
Above 55°C (131°F), the partially crystalline structure is lost, granules absorb further water, swell and pass to a disorganized structure, that is, starch gelatinization occurs, by which starch assumes an amorphous structure more easily attachable by alpha-amylase.

Granules-associated proteins

In granules, starch is present in association with proteins, many of which are hydrophobic, that means with low affinity for water. This association have the effect to hinder the interaction, in the intestinal lumen, between alpha-amylase, a polar protein, and the polysaccharides making up starch granules.
The physical processes to which cereals undergo before being eaten, such as milling or heating for several minutes, change the relationship between starch and the associated proteins, making it more available to α-amylase action.

Fibers

Alpha-amylase activity may also be hindered by the presence of nondigestible polysaccharides, the fibers: cellulose, hemicellulose and pectin.

Conclusions

The presence of inhibitors, of both chemical and physical type, hinders starch digestion, even when pancreatic α-amylase secretion is normal. This means that a part of starch, ranging from 1% to 10%, may escape the action of the enzyme, being then metabolized by colonic bacteria.
Refined starch is instead hydrolyzed efficiently, even when there is an exocrine pancreatic insufficiency (EPI), condition in which alpha-amylase concentration in gut lumen may be reduced to 10% of the normal.

References

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

Belitz .H.-D., Grosch W., Schieberle P. “Food Chemistry” 4th ed. Springer, 2009

Bender D.A. “Benders’ Dictionary of Nutrition and Food Technology”. 8th Edition. Woodhead Publishing. Oxford, 2006

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

Osorio-Dıaz P., Bello-Perez L.A., Agama-Acevedo E., Vargas-Torres A., Tovar J., Paredes-Lopez O. In vitro digestibility and resistant starch content of some industrialized commercial beans (Phaseolus vulgaris L.). Food Chem 2002;78:333-7 [Abstract]

Shils M.E., Olson J.A., Shike M., Ross A.C. “Modern nutrition in health and disease” 9th ed., by Lippincott, Williams & Wilkins, 1999

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000

Maltodextrin, fructose and endurance sports

Carbohydrate ingestion can improve endurance capacity and performance.
The ingestion of different types of carbohydrates, which use different intestinal transporters, can:

  • increase total carbohydrate absorption;
  • increase exogenous carbohydrate oxidation;
  • and therefore improve performance.

Glucose and fructose

When a mixture of glucose and fructose is ingested (in the analyzed literature, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min), there is less competition for intestinal absorption compared with the ingestion of an iso-energetic amount of glucose or fructose,  two different intestinal transporters being involved. Furthermore, fructose absorption is stimulated by the presence of glucose.

This can:

The combined ingestion of glucose and fructose allows to obtain exogenous carbohydrate oxidation rate around 1,26 g/min, therefore, higher than the rate reported with glucose alone (1g/min), also in high concentration.
The observed difference (+0,26 g/min) can be fully attributed to the oxidation of ingested fructose.

Sucrose and glucose

The ingestion of sucrose and glucose, in the same conditions of the ingestion of glucose and fructose (therefore, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min), gives similar results.

Glucose, sucrose and fructose

Very high oxidation rates are found with a mixture of glucose, sucrose, and fructose (in the analyzed literature, respectively 1.2, 0.6 and 0.6 g/min, ratio 2:1:1, for total carbohydrate intake rate to 2.4 g/min; however, note the higher amounts of ingested carbohydrates).

Maltodextrin and fructose

Maltodextrin and Fructose: Oxidation of Ingested Carbohydrates
Fig. 1 – Oxidation of Ingested Carbohydrates

High oxidation rates are also observed with combinations of maltodextrin and fructose, in the same conditions of the ingestion of glucose plus fructose (therefore, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min).

Such high oxidation rates can be achieved with carbohydrates ingested in a beverage, in a gel or in a low-fat, low protein, low-fiber energy bar.

The best combination of carbohydrates ingested during exercise seems to be the mixture of maltodextrin and fructose in a 2:1 ratio, in a 5% solution, and in a dose around 80-90 g/h.
Why?

  • This mixture has the best ratio between amount of ingested carbohydrates and their oxidation rate and it means that smaller amounts of carbohydrates remain in the stomach or gut reducing the risk of gastrointestinal complication/discomfort during prolonged exercise (see brackets grafa in the figure).
  • A solution containing a combination of multiple transportable carbohydrates and a carbohydrate content not exceeding 5% optimizes gastric emptying rate and improves fluid delivery.

Example of a 5% carbohydrate solution containing around 80-90 g of maltodextrin and fructose in a 2:1 rate; ingestion time around 1 h.

Conclusion

During prolonged exercise, when high exogenous carbohydrate oxidation rates are needed, the ingestion of multiple transportable carbohydrates is preferred above that of large amounts of a single carbohydrate.
The best mixture seems to be maltodextrin and fructose, in a 2:1 ratio, in a 5% concentration solution, and at ingestion rate of around 80-90 g/h.

References

Prolonged exercise and carbohydrate ingestion

Prolonged Exercise: Open Water Swimming
Fig. 1 – Open Water Swimming

During prolonged exercise (>90 min), like marathon, Ironman, cross-country skiing, road cycling or open water swimming, the effects of supplementary carbohydrates on performance are mainly metabolic rather than central and include:

  • the provision of an additional muscle fuel source when glycogen stores become depleted;
  • muscle glycogen sparing;
  • the prevention of low blood glucose concentrations.

How many carbohydrates should an athlete take?

The optimal amount of ingested carbohydrate is that which results in the maximal rate of exogenous carbohydrate oxidation without causing gastrointestinal discomfort”. (Jeukendrup A.E., 2008).

Prolonged exercise: which carbohydrates should an athlete take?

Until 2004 it was believed that carbohydrates ingested during exercise (also prolonged exercise) could be oxidized at a rate no higher than 1 g/min, that is, 60 g/h, independent of the type of carbohydrate.
Exogenous carbohydrate oxidation is limited by their intestinal absorption and the ingestion of more than around 60 g/min of a single type of carbohydrate will not increase carbohydrate oxidation rate but it is likely to be associated with gastrointestinal discomfort (see later).
Why?
At intestinal level, the passage of glucose (and galactose) is mediated by a sodium dependent transporter called SGLT1. This transporter becomes saturated at a carbohydrate intake about 60 g/h and this (and/or glucose disposal by the liver that regulates its transport into the bloodstream) limits the oxidation rate to 1g/min or 60 g/h. For this reason, also when glucose is ingested at very high rate (>60 g/h), exogenous carbohydrate oxidation rates higher 1.0-1.1 g/min are not observed.

The rate of oxidation of ingested maltose, sucrose, maltodextrins and glucose polymer is fairly similar to that of ingested glucose.

Fructose uses a different sodium independent transporter called GLUT5. Compared with glucose, fructose has, like galactose, a lower oxidation rate, probably due to its lower rate of intestinal absorption and the need to be converted into glucose in the liver, again like galactose, before it can be oxidized.

Prolonged Exercise: Maltodextrin and Fructose: Oxidation of Ingested Carbohydrates
Fig. 1 – Oxidation of Ingested Carbohydrates

However, if the athlete ingests different types of carbohydrates, which use different intestinal transporters, exogenous carbohydrate oxidation rate can increase significantly.
It seems that the best mixture is maltodextrins and fructose.

Note: the high rates of carbohydrate ingestion may be associated with delayed gastric emptying and fluid absorption; this can be minimized by ingesting combinations of multiple transportable carbohydrates that enhance fluid delivery compared with a single carbohydrate. This also causes relatively little gastrointestinal distress.

Conclusion

The ingestion of different types of carbohydrates that use different intestinal transporters can:

References

Carbohydrate ingestion during exercise of relatively short duration and high intensity

Intermittent high intensity exercise and carbohydrate ingestion

High Intensity: During-Exercise Nutrition
Fig. 1- During-Exercise Nutrition

Carbohydrate ingestion during intermittent high intensity or prolonged (>90 min) sub-maximal exercise can:

  • increase exercise capacity;
  • improve exercise performance;
  • postpone fatigue.

The intake of very small amounts of carbohydrates or carbohydrate mouth rinsing (for example with a 6% maltodextrin solution) may improve exercise performance by 2-3% when the exercise is of relatively short duration (<1 h) and high intensity (>75% VO2max), that is, an exercise not limited by the availability of muscle glycogen stores, given adequate diet.
The underlying mechanisms for the ergogenic effect of carbohydrates during this type of activity are not metabolic but may reside in the central nervous system: it seems that carbohydrates are detected in the oral cavity by unidentified receptors, promoting an enhanced sense of well-being and improving pacing.
These effects are independent of taste or sweet and non-sweet of carbohydrates but are specific to carbohydrates.

It should be noted that performance effects with drink ingestion are similar to the mouth rinse; therefore athletes, when they don’t complain of gastrointestinal distress when ingesting too much fluid, may have an advantage taking the drink (in endurance sports, dehydration and carbohydrate depletion are the most likely contributors to fatigue).

Conclusion

It seems that during exercise of relatively short duration (<1 h) and high intensity (>75% VO2max) it is not necessary to ingest large amounts of carbohydrates: a carbohydrate mouth rinsing or the intake of very small amounts of carbohydrates may be sufficient to obtain a performance benefit.

References

Hydration before endurance sports

Dehydration and endurance sports

Pre-hydration
Fig. 1 – Pre-hydration

In endurance sports, like Ironman, open water swimming, road cycling, marathon, or cross-country skiing, the most likely contributors to fatigue are dehydration and carbohydrate (especially liver and muscle glycogen) depletion.

Pre-hydration

Due to sweat loss needed to dissipate the heat generated during exercise, dehydration can compromise exercise performance.
It is important to start exercising in a euhydrated state, with normal plasma electrolyte levels, and attempt to maintain this state during any activity.
When an adequate amount of beverages with meals are consumed and a protracted recovery period (8-12 hours) has elapsed since the last exercise, the athlete should be euhydrated.
However, if s/he has not had adequate time or fluids/electrolytes volume to re-establish euhydration, a pre-hydration program may be useful to correct any previously incurred fluid-electrolyte deficit prior to initiating the next exercise.

Pre-hydration program

If during exercise the nutritional target is to reduce sweat loss to less than 2–3% of body weight, prior to exercise the athlete should drink beverages at least 4 hours before the start of the activity, for example, about 5-7 mL/kg body weight.
But if the urine is still dark (highly concentrated) and/or is minimal, s/he should slowly drink more beverages, for example, another 3-5 mL/kg body weight, about 2 hours before the start of activity so that urine output normalizes before starting the event.

It is advisable to consume small amounts of sodium-containing foods or salted snacks and/or beverages with sodium that help to stimulate thirst and retain the consumed fluids.
Moreover, palatability of the ingested beverages is important to promote fluid consumption before, during, and after exercise. Fluid palatability is influenced by several factors, such as:

  • temperature, often between 15 and 21 °C;
  • sodium content;
  • flavoring.

And hyper-hydration?

Hyper-hydration, especially in the heat, could improve thermoregulation and exercise performance, therefore, it might be useful for those who lose body water at high rates, as during exercise in hot conditions or who have difficulty drinking sufficient amounts of fluid during exercise.
However there are several risks:

  • fluids that expand the intra- and extra-cellular spaces (e.g. glycerol solutions plus water) greatly increase the risk of having to void during exercise;
  • hyper-hydration may dilute and lower plasma sodium which increases the risk of dilutional hyponatraemia, if during exercise, fluids are replaced aggressively.

Finally, it must be noted that plasma expanders or hyper-hydrating agents are banned by the World Anti-Doping Agency (WADA).

Conclusion

“Pre-hydrating with beverages, if needed, should be initiated at least several hours before the exercise task to enable fluid absorption and allow urine output to return toward normal levels. Consuming beverages with sodium and/or salted snacks or small meals with beverages can help stimulate thirst and retain needed fluids” (Sawka et al., 2007).

References

Ingestion of solid, liquid or gel carbohydrates 60 min before exercise

Liquid Carbohydrate Ingestion
Fig. 1 – Liquid Carbohydrate Ingestion

The form of carbohydrates ingested before exercise may have different effects on both metabolism and performance. Moreover, the ingestion of solid foods slows gastric empty, digestion and absorption rates compared with liquid foods and this has a different impact on glycemia.
For these reasons, several studies have investigated the effects of the form of carbohydrates on glycemic responses, oxidation rates and performance.

  • Studies comparing solid versus liquid carbohydrates and solid versus gel carbohydrates have found no difference in glycemic responses between groups.
  • Studies that have investigated difference in performance effects have found no differences.
  • Furthermore, no differences are found in carbohydrate oxidation rates between the carbohydrate ingestion in the three forms during exercise.

Therefore, it seems not to be the form of ingested carbohydrates that can enhance or reduce performance (in addition, even glycogen synthesis doesn’t vary; study conducted with liquid or solid carbohydrates).

Conclusion

It is advisable that athlete ingests whichever form of carbohydrates best suits, based on his experience and cost-effectiveness of the product.

References

Hypoglycemia and carbohydrate ingestion 60 min before exercise

Hypoglycemia: strategies to limit it in susceptible subjects

Hypoglycemia: Fatigue
Fig. 1 – Fatigue

From several studies it appears that the risk of developing hypoglycemia (blood glucose < 3.5 mmol /l or < 63 mg/l) is highly individual: some athletes are very prone to develop it and others are much more resistant.
A strategy to minimize glycemic and insulinemic responses during exercise is to delay carbohydrate ingestion just prior to exercise: in the last 5-15 min before exercise or during warm-up (even though followed by a short break).
Why?

  • Warm-up and then exercise increase catecholamine concentrations blunting insulin response.
  • Moreover, it has been shown that ingestion of carbohydrate-containing beverages during a warm-up (even if followed by a short break) does not lead to rebound hypoglycemia, independent of the amount of carbohydrates, but instead increases glycemia. When carbohydrates are ingested within 10 min before the onset of the exercise, exercise will start before the increase of insulin concentration.

Therefore, this timing strategy would provide carbohydrates minimizing the risk of a possible reactive hypoglycaemia.
In addition, it is possible to choose low glycemic index carbohydrates that lead to more stable glycemic and insulinemic responses during subsequent exercise.

Example: a 5-6% carbohydrate solution, often maltodextrin (i.e. 50-60 g maltodextrin in 1000 ml) or maltodextrin plus fructose (e.g. respectively 33 g plus 17 g in 1000 ml).

An intriguing observation is the lack of a clear relation between hypoglycaemia and its symptoms (likely related to a reduced delivery of glucose to the brain). In fact, symptoms are often reported in the absence of true hypoglycemia and hypoglycemia is not always associated with symptoms. Though the cause of the symptoms is still unknown, it is clearly not related to a glycemic threshold.

Conclusion

Some athletes develop symptoms similar to those of hypoglycemia, even though they aren’t always linked to actual low glycemia. To minimize these symptoms, for these subjects an individual approach is advisable. It may include:

  • carbohydrate ingestion just before the onset of exercise or during warm-up;
  • choose low-to-moderate GI carbohydrates that result in more stable glycemic and insulinemic responses;
  • or avoid carbohydrates 90 min before the onset of exercise.

References