Category Archives: Carbohydrates

Glucose-alanine cycle

What is the glucose-alanine cycle?

The glucose-alanine cycle, or Cahill cycle, proposed for the first time by Mallette, Exton and Park, and Felig et al. between 1969 and 1970, consists of a series of steps through which extrahepatic tissues, for example the skeletal muscle, export pyruvate and amino groups as alanine to the liver, and  receive glucose from the liver via the bloodstream.
The main steps of the glucose-alanine cycle are summarized below.

  • When in extrahepatic tissues amino acids are used for energy, pyruvate, derived from the glycolytic pathway, is used as amino group acceptor, forming alanine, a nonessential amino acid.
  • Alanine diffuses into the bloodstream and reaches the liver.
  • In the liver, the amino group of alanine is transferred to α-ketoglutarate to form pyruvate and glutamate, respectively.
  • The amino group of glutamate mostly enters the urea cycle, and in part acts as a nitrogen donor in many biosynthetic pathways.
    Pyruvate enters the gluconeogenesis pathway and is used for glucose synthesis.
  • The newly formed glucose diffuses into the bloodstream and reaches the peripheral tissues where, due to glycolysis, is converted into pyruvate that can accept amino groups from the free amino acids, thus closing the cycle.

Therefore, the glucose-alanine cycle provides a link between carbohydrate and amino acid metabolism, as schematically described below.

Glucose → Pyruvate → Alanine → Pyruvate → Glucose

Glucose-Alanine Cycle
Fig. 1 – Glucose-Alanine Cycle

The glucose-alanine cycle occurs not only between the skeletal muscle, the first tissue in which it was observed, and the liver, but involves other cells and extrahepatic tissues including cells of the immune system, such as lymphoid organs.

The steps of the glucose-alanine cycle

The analysis of the steps of the glucose-alanine cycle is made considering the cycle between skeletal muscle and the liver.
Both intracellular and extracellular proteins are continuously hydrolyzed to the constituent amino acids and resynthesized, and the rate at which these processes occur is balanced precisely, thereby preventing loss of fat free mass.
However, under catabolic conditions, such as intense and prolonged exercise or fasting, the rate of muscle protein breakdown exceeds synthesis. This leads to the liberation of amino acids, some of which are used for energy and others for gluconeogenesis. And the oxidation of the carbon skeletons of amino acids, in particular branched chain amino acids or BCAA (leucine, isoleucine  and valine), may be a significant source of energy for the muscle. For example, after about 90 minutes of strenuous exercise, amino acid oxidation in muscle provides 10-15% of the energy needed for contraction.
The utilization of the carbon skeletons of amino acids for energy involves the removal of the amino group, and then the excretion of amino nitrogen in a non-toxic form.
The removal of the α-amino group occurs by transamination, that can be summarized as follows:

α-Keto acid + Amino acid ⇄ New amino acid + New α-keto acid

Such reactions, catalyzed by enzymes called aminotransferases or transaminases (EC 2.6.1) are freely reversible (see below).
Branched chain amino acids, for example, transfer the amino group to α-ketoglutarate or 2-oxoglutaric acid, to form glutamate and the α-keto acid derived from the original amino acid, in a reaction catalyzed by branched chain aminotransferase or BCAT (EC 2.6 .1.42).

The glucose-alanine cycle in skeletal muscle

In skeletal muscle, the newly formed glutamate may react with ammonia to form glutamine, for many tissues and organs, such as the brain, the major vehicle for interorgan transport of nitrogen. The reaction is catalyzed by the cytosolic enzyme glutamine synthetase (EC 6.3.1.2), and consumes an ATP.

Glutamate + NH4+ + ATP → Glutamine + ADP + Pi

In this case, glutamate leaves the Cahill cycle.
Alternatively, and in contrast to what happens in most of the other tissues, the newly formed glutamate may transfer the amino group to pyruvate, derived from glycolysis, to form alanine and α-ketoglutarate. This transamination is catalyzed by alanine aminotransferase or ALT (EC 2.6.1.2), an enzyme found in most animal and plant tissues.

Glutamate + Pyruvate ⇄ Alanine + α-Ketoglutarate

The alanine produced and that derived directly from protein breakdown, and muscle proteins are rich in alanine, can leave the cell and be carried by the bloodstream to the liver; in this way the amino group reaches the liver. And the rate at which alanine formed by transamination of pyruvate is transferred into the circulation is proportional to the intracellular pyruvate production.
Note: alanine and glutamine are the major sources of nitrogen and carbon in interorgan amino acid metabolism.

The glucose-alanine cycle in the liver

Once in the liver, a hepatic alanine aminotransferase catalyzes a transamination in which alanine, the major gluconeogenic amino acid, acts as an amino group donor and α-ketoglutarate as an α-keto acid acceptor. The products of the reaction are pyruvate, i.e. the carbon skeleton of alanine, and glutamate.

Alanine + α-Ketoglutarate ⇄ Glutamate + Pyruvate

Glutamate, in the reaction catalyzed by glutamate dehydrogenase (EC 1.4.1.2), an enzyme present in the mitochondrial matrix, forms ammonium ion, which enters the urea cycle, and α-ketoglutarate, which can enter the Krebs cycle. This reaction is an anaplerotic reaction that links amino acid metabolism with the Krebs cycle.

Glucose-Alanine Cycle

However, glutamate can also react  with oxaloacetate to form aspartate and α-ketoglutarate, in a reaction catalyzed by aspartate aminotransferase (EC 2.6.1.1). Aspartate is involved in the formation of urea as well as in the synthesis of purines and pyrimidines.

Glutamate + Oxaloacetate ⇄ Aspartate + α-Ketoglutarate

Also the pyruvate produced may have different metabolic fates: it can be oxidized for ATP production, and then leave the glucose-alanine cycle, or enter the gluconeogenesis pathway, and thus continue in the cycle.
The glucose produced is released from the liver into the bloodstream and delivered to various tissues that require it, as the skeletal muscle, in which it is used for pyruvate synthesis. In turn, the newly formed pyruvate may react with glutamate, thus closing the cycle.

Transaminases

As previously mentioned, the removal of the amino group from amino acids occurs through transamination (see above for the general reaction). These reactions are catalyzed by enzymes called aminotransferases or transaminases.
They are cytosolic enzymes, present in all cells and particularly abundant in the liver, kidney, intestine and muscle; they require pyridoxal phosphate or PLP, the active form of vitamin B6 or pyridoxine, as a coenzyme, which is tightly bound to the active site.
In transamination reactions, the amino group of free amino acids, except of threonine and lysine, is channeled towards a small number of α-keto acids, notably pyruvate, oxaloacetate and α-ketoglutarate.
Cells contain different types of aminotransferases: many are specific for α-ketoglutarate as α-keto acid acceptor, but differ in specificity for the amino acid, from which they are named. Examples are the aforementioned alanine aminotransferase, also called alanine transaminase and glutamic pyruvic transferase or GPT, and aspartate aminotransferase or AST, also called glutamic-oxaloacetic transaminase or GOT.
It should be underlined that there is no net deamination in these reactions, no loss of amino groups, as the α-keto acid acceptor is aminated and the amino acid deaminated.

Functions of the glucose-alanine cycle

This cycle has various functions.

  • It transports nitrogen in a non-toxic form from peripheral tissues to the liver.
  • It transports pyruvate, a gluconeogenic substrate, to the liver.
  • It removes pyruvate from peripheral tissues.  This leads to a higher production of ATP from glucose in these tissues. In fact, the NADH produced during glycolysis can enter the mitochondria and be oxidized through oxidative phosphorylation.
  • It allows to maintain a relatively high concentration of alanine in hepatocytes, sufficient to inhibit protein degradation.
  • It may play a role in host defense against infectious diseases.

Finally, it is important to underline that there is no net synthesis of glucose in the glucose-alanine cycle.

Energy cost of the glucose-alanine cycle

Like the Cori cycle, also the glucose-alanine cycle has an energy cost, equal to 3-5 ATP.
The part of the cycle that takes place in peripheral tissues involves the production of 5-7 ATP per molecule of glucose:

  • 2 ATP are produced by glycolysis;
  • 3-5 ATP derive from NADH/FADH2 (see below).

Instead in the liver, gluconeogenesis and the urea cycle cost 10 ATP:

  • 6 ATP are consumed in the during gluconeogenesis per molecule of glucose synthesized;
  • 4 ATP are consumed in the urea cycle per molecule of urea synthesized.

The glucose-alanine cycle, like the Cori cycle, shifts part of the metabolic burden from extrahepatic tissues to the liver. However, the energy cost paid by the liver is justified by the advantages that the cycle brings to the whole body, as it allows, in particular conditions, an efficient breakdown of proteins in extrahepatic tissues (especially skeletal muscle), which in turn allows to obtain gluconeogenic substrates as well as the use of amino acids for energy in extrahepatic tissues.

Similarities and differences between glucose-alanine cycle and Cori cycle?

There are some analogies between the two cycles, which are listed below.

  • The Cahill cycle partially overlaps the Cori cycle when pyruvate is converted to glucose and the monosaccharide is transported to extrahepatic tissues, in which it is converted again to pyruvate via the glycolytic pathway.
  • The entry into gluconeogenesis pathway is similar for the two cycles: both alanine and lactate are converted to pyruvate.
  • Like the Cori cycle, the glucose-alanine cycle occurs between different cell types, unlike metabolic pathways such as glycolysis, Krebs cycle or gluconeogenesis that occur within individual cells
Glucose-Alanine Cycle
Fig. 2 – Glucose-Alanine Cycle and Cori Cycle

Below, some differences between the two cycles.

  • The main difference concerns the three carbon intermediate that from peripheral tissues reach the liver: lactate in the Cori cycle, and alanine in the glucose-alanine cycle.
  • Another difference concerns the fate of the NADH produced by glycolysis in peripheral tissues.
    In the Cori cycle, the coenzyme acts as reducing agent to reduce pyruvate to lactate, in the reaction catalyzed by lactate dehydrogenase (EC 1.1.1.27).
    In the glucose-alanine cycle, this reduction does not occur and the electrons of NADH can be transported into the mitochondria via the malate-aspartate and glycerol 3-phosphate shuttles, generating NADH, the first shuttle, and FADH2, the other shuttle. And the yield of ATP from NADH and FADH2 is 2.5 and 1.5, respectively.
  • Finally, from the previous point, it is clear that, unlike the Cori cycle, the Cahill cycle requires the presence of oxygen and mitochondria in the peripheral tissues.
References

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

Felig P., Pozefsk T., Marlis E., Cahill G.F. Alanine: key role in gluconeogenesis. Science 1970;167(3920):1003-4 [Abstract]

Gropper S.S., Smith J.L., Groff J.L. Advanced nutrition and human metabolism. Cengage Learning, 2009 [Google eBooks]

Lecker S.H., Goldberg A.L. and Mitch W.E. Protein degradation by the ubiquitin–proteasome pathway in normal and disease states. J Am Soc Nephrol 2006;17(7):1807-19 [PDF]

Mallette L. E., Exton J. H., and Park C. R. Control of gluconeogenesis from amino acids in the perfused  rat liver. J Biol Chem 1969;244(20):5713-23 [PDF]

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

Raju S.M., Madala B. Illustrated medical biochemistry. Jaypee Brothers Publishers, 2005 [Google eBooks]

Wu G. Amino acids: biochemistry and nutrition. CRC Press, 2013 [Google eBooks]

Cori cycle: definition, function, biochemistry, involved tissues

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.

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 lactic acid 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.

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.

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.

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.

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.

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 lactic acid, 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.

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.

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. 4th Edition. W.H. Freeman and Company, 2004

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]

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

Glycogen: an efficient storage form of energy in aerobic conditions

What is the net energy yield for the oxidation of a glucose unit from glycogen in aerobic conditions?

Aerobic Conditions: Glycogen Structure
Fig. 1 – Glycogen Structure

In aerobic conditions, the oxidation of a free glucose to CO2 and H2O (glycolysis, Krebs cycle and oxidative phosphorylation) leads to the net production of about 30 molecules of ATP.

Glucose from the action of glycogen phosphorylase: glucose-1-phosphate release (about 90% of the removed units).

Glycogen synthesis from free glucose costs two ATP units for each molecule; a glucose-1-phosphate is released by the action of glycogen phosphorylase with recovering/saving one of the two previous ATP molecules.
Therefore in aerobic condition, the oxidation of glucose starting from glucose-6-phosphate and not from free glucose yields 31 ATP molecules and not 30 (one ATP instead of two is expended in the activation phase, 30 ATP are produced during Krebs cycle and oxidative phosphorylation: 31 ATP gained).
The net rate between cost and yield is 1/31 (an energy conservation of about 97%).
The overall reaction is:

glycogen(n glucose residues) + 31 ADP + 31 Pi → glycogen(n-1 glucose residues) + 31 ATP + 6 CO2 + 6 H2O

If we combine glycogen synthesis, glycogen breakdown and finally the oxidation of glucose to CO2 and H2O we obtain 30 molecules of ATP per stored glucose unit, that is the overall reaction is:

glucose + 29 ADP + 30 Pi → 29 ATP + 6 CO2 + 6 H2O

Glucose from the action of debranching enzyme: free glucose release (about 10% of the removed units).

The net yield in ATP between glycogen synthesis and breakdown is two ATP molecules expended because of free glucose is released.
In this case the oxidation of glucose starts from the not-prephosphorylated molecule so we obtain 30 ATP molecules.
The net rate between cost and yield is 2/30 (a energy conservation of about 93,3%).
Considering the oxidation of the glucose units from glycogen to CO2 and H2O we have an energy conservation of:

1-(((1/31)*0,9)+((2/30)*0,1))=0,9643

Conclusion

In aerobic conditions, there is the conservation of about 97% of energy into the glycogen molecule, an extremely efficient storage form of energy.

References

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

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

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

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

Glycogen: an efficient storage form of energy in anaerobic conditions

What is the net energy yield for the oxidation of a glucose unit from glycogen in anaerobic conditions?

In anaerobic conditions, the oxidation of a free glucose to lactate leads to the net production of two molecules of ATP.

Anaerobic Conditions: Glycolysis to Lactate
Fig. 1 – Glycolysis to Lactate

Glucose from the action of glycogen phosphorylase: glucose-1-phosphate release (about 90% of the removed units).

Glycogen synthesis from free glucose costs two ATP units for each molecule; a glucose-1-phosphate is released by the action of glycogen phosphorylase, with recovering/saving of one of the two previous ATP molecules.
Therefore the oxidation of glucose to lactate starting from glucose-6-phosphate and not from free glucose yields three ATP molecules and not two (one ATP is expended in the activation stage instead of two, 4 ATP are produced in the third stage: three ATP gained).
The net rate between cost and yield is 1/3 (an energy conservation of about 66,7%).
The overall reaction is:

glycogen(n glucose residues) + 3 ADP + 3 Pi → glycogen(n-1 glucose residues) + 2 lactate + 3 ATP

If we combine glycogen synthesis, glycogen breakdown and finally glycolysis to lactate we obtain only one ATP molecule per stored glucose unit, that is the overall sum is:

glucose + ADP + Pi → 2 lactate + ATP

Glucose from the action of debranching enzyme: free glucose release (about 10% of the removed units).

The net yield in ATP between glycogen synthesis and breakdown is two ATP molecules expended because of free glucose is released.
In this case the oxidation of glucose starts from the not-prephosphorylated molecule and it yields two ATP molecules.
Therefore the net yield in ATP is zero.
Considering the oxidation of the glucose units from glycogen to lactate we have an energy conservation of:

1-(((1/3)*0,9)+((2/2)*0,1))=0,60

Conclusion

In anaerobic conditions, there is the conservation of about 60% of energy into the glycogen molecule, a good storage form of energy.

References

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

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

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

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

Carbohydrate mouth rinse and endurance exercise performance

Carbohydrate mouth rinse and performance responses

The importance of carbohydrates as an energy source for exercise is well known: one of the first study to hypothesize and recognize their importance was the study of Krogh and Lindhardt at the beginning of the 20th century (1920); later, in the mid ‘60’s, Bergstrom and Hultman discovered the crucial role of muscle glycogen on endurance capacity.
Nowdays, the ergogenic effects of carbohydrate supplementation on endurance performance are well known; they are mediated by mechanisms such as:

  • a sparing effect on liver glycogen;
  • the maintenance of glycemia and rates of carbohydrate oxidation;
  • the stimulation of glycogen synthesis during low-intensity exercise ;
  • a possible stimulatory effect on the central nervous system.

However, their supplementation, immediately before and during exercise, has an improving effect also during exercise (running or cycling) of a shorter and more intense nature: >75% VO2max (maximal oxygen consumption) and ≤1 hour, during which euglycaemia is rarely challenged and adequate muscle glycogen store remains at the cessation of the exercise.

Hypothesis for carbohydrate mouth rinse

In the absence of a clear metabolic explanation it was speculated that ingesting carbohydrate solutions may have a ‘non-metabolic’ or ‘central effect’ on endurance performance. To explore this hypothesis many studies have investigated the performance responses of subjects when carbohydrate solutions (about 6% carbohydrate, often maltodextrins) are mouth rinsed during exercise, expectorating the solution before ingestion.
By functional magnetic resonance imaging and transcranial stimulation it was shown that carbohydrates in the mouth stimulate reward centers in the brain and increases corticomotor excitability, through oropharyngeal receptors which signal their presence to the brain.
Probably salivary amylase releases very few glucose units from maltodextrins which is probably what is needed in order to activate the purported carbohydrate receptors in the oropharynx (no glucose transporters in the oropharynx are known).
However, the performance response appears to be dependent upon the pre-exercise nutritional status of the subject: most part of the studies showing an improving effect on performance was conducted in a fasted states (3- to 15-h fasting).
Only one study has shown improvements of endurance capacity   in both fed and fasted states by carbohydrate mouth rinse, but in non-athletic subjects.

References

Beelen M., Berghuis J., Bonaparte B., Ballak S.B., Jeukendrup A.E., van Loon J. Carbohydrate mouth rinsing in the fed state: lack of enhancement of time-trial performance. Int J Sport Nutr Exerc Metab 2009;19(4):400-9 [Abstract]

Bergstrom J., Hultman E. A study of glycogen metabolism during exercise in man. Scand J Clin Invest 1967;19:218-28 [Abstract]

Bergstrom J., Hultman E. Muscle glycogen synthesis after exercise: an enhancing factor localized in muscle cells in man. Nature 1966;210:309-10 [Abstract]

Painelli V.S., Nicastro H., Lancha A. H.. Carbohydrate mouth rinse: does it improve endurance exercise performance? Nutrition Journal 2010;9:33 [Abstract]

Fares E.J., Kayser B. Carbohydrate mouth rinse effects on exercise capacity in pre- and postprandial States. J Nutr Metab 2011;385962. doi: 10.1155/2011/385962. Epub 2011 Jul 27 [Abstract]

Krogh A., Lindhard J. The relative value of fat and carbohydrate as sources of muscular energy. Biochem J 1920;14:290-363 [PDF]

Rollo I. Williams C. Effect of mouth-rinsing carbohydrate solutions on endurance performance. Sports Med. 2011;41(6):449-61 [Abstract]

Blood glucose levels and liver

Blood glucose levels and hepatic glycogen

One of the main functions of the liver is to participate in the maintaining of blood glucose levels within well defined range (in the healthy state before meals 60-100 mg/dL or 3.33-5.56 mmol/L). To do it the liver releases glucose into the bloodstream in:

  • fasting state;
  • between meals;
  • during physical activity.

Blood glucose levels and hepatic glucose-6-phosphatase

In the liver, glycogen is the storage form of glucose which is released from the molecule not as such, but in the phosphorylated form i.e. with charge, the glucose-1-phosphate (this process is called glycogenolysis). The phosphorylated molecule can’t freely diffuse from the cell, but in the liver it is present the enzyme glucose-6-phosphatase that hydrolyzes glucose-6-phosphate, produced from glucose-1-phosphate in the reaction catalyzed by phosphoglucomutase, to glucose (an irreversible dephosphorylation).

glycogen(n glucose residues) + Pi → glucose-1-phosphate + glycogen(n-1 glucose residues)

glucose-1-phosphate ↔ glucose-6-phosphate

glucose-6-phosphate + H2O → glucose + Pi

Then, glucose can diffuse from the hepatocyte, via a transporter into the plasma membrane called GLUT2, into the bloodstream to be delivered to extra-hepatic cells, in primis neurons and red blood cells for which it is the main, and for red blood cells the only energy source (neurons, with the exception of those in some brain areas that can use only glucose as energy source, can derive energy from another source, the ketone bodies, which becomes predominant during periods of prolonged fasting).

Note: the liver obtains most of the energy required from the oxidation of fatty acids, not from glucose.

Glucose-6-phosphatase is present also in the kidney and gut but not in the muscle and brain; therefore in these tissues glucose-6-phosphate can’t be released from the cell.
Glucose-6-phosphatase plays an important role also in gluconeogenesis.

Glucose-6-phosphatase is present into the membrane of endoplasmic reticulum and the hydrolysis of glucose-6-phosphate occurs into its lumen (therefore this reaction is separated from the process of glycolysis). The presence of a specific transporter, the glucose-6-phosphate translocase, is required to transport the phosphorylated molecule from citosol into the lumen of endoplasmic reticulum. Although a glucose transporter is present into the membrane of endoplasmic reticulum, most of the released glucose is not transported back into the cytosol of the cell but is secreted into the bloodstream. Finally, an ion transporter transports back into the cytosol the inorganic phosphate released into the endoplasmic reticulum.

References

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

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

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

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

Glycogen: definition, structure and functions

What is glycogen?

Glycogen Structure
Fig. 1 – Glycogen Structure

Glycogen is an homopolysaccharide formed by units of glucose. Chemically similar to amylopectin, and therefore sometimes referred to as animal starch, compared to the latter it is more compact, extensively branched and larger, reaching a molecular weight up to 108 Da corresponding to about 600000 glucose molecules.
As in the amylopectin, glucose units in the main chain and in the lateral chains are linked by α-(1→4) glycosidic bonds. Lateral chains are joined to the main chain by an α-(1→6) glycosidic bond; unlike amylopectin branches are more frequent, approximately every 10 glucose units (rather than every 25-30 as in amylopectin) and are formed by a smaller numbers of glucose units.
Glycogen is located in the cytosol of the cell in the form of hydrated granules of diameter between 1 to 4 µm and forms complexes with regulatory proteins and enzymes responsible for its synthesis and degradation.

Functions of glycogen

Glycogen, discovered in 1857 by French physiologist Claude Bernard, is the storage form of glucose, and therefore of energy, in animals in which it is present in the liver, muscle (skeletal and heart muscle) and in lower amounts in nearly all the other tissues and organs.
In humans it represents less than 1% of the body’s caloric stores (the other form of caloric reserve, much more abundant, is triacylglycerols stored in adipose tissue) and is essential for maintaining normal glycemia too.
It represents about 10% of liver weight and 1% of muscle weight; although it is present in a higher concentration in the liver, the total stores in muscle are much higher thanks to its greater mass (in a non-fasting 70 kg adult male there are about 100 g of glycogen in the liver and 250 g in the muscle).

  • Liver glycogen stores is a glucose reserve that hepatocyte releases when needed to maintain a normal blood sugar levels: if you consider glucose availability (in a non-fasting 70 kg adult male) there is about 10 grams or 40 kcal in body fluids while hepatic glycogen can supply, also after a fasting night, about 600 kcal.
  • In skeletal and cardiac muscle, glucose from glycogen stores remains within the cell and is used as an energy source for muscle work.
  • The brain contains a small amount of glycogen, primarily in astrocytes. It accumulates during sleep and is mobilized upon waking, therefore suggesting its functional role in the conscious brain. These glycogen reserves also provide a moderate degree of protection against hypoglycemia.
  • It has a specialized role in fetal lung type II pulmonary cells. At about 26 weeks of gestation these cells start to accumulate glycogen and then to synthesize pulmonary surfactant, using it as a major substrate for the synthesis of surfactant lipids, of which dipalmitoylphosphatidylcholine is the major component.
Glycogen: Dipalmitoylphosphatidylcholine
Fig. 1 – Dipalmitoylphosphatidylcholine

Glycogen and foods

It is absent from almost all foods because after an animal is killed it is rapidly broken down to glucose and then to lactic acid; it should be noted that the acidity consequently to lactic acid production gradually improves the texture and keeping qualities of the meat. The only dietary sources are oysters and other shellfish that are eaten virtually alive: they contain about 5% glycogen.

In humans, accumulation of glycogen is associated with weight gain due to water retention: for each gram of stored glycogen 3 grams of water are retained.

References

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

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

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

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

Strategies to maximize muscle glycogen resynthesis after exercise

Post-exercise muscle glycogen synthesis

An important energy source for working muscle is its glycogen store, whose level is correlated with the onset of fatigue.
The highly trained athlete not only has glycogen stores potentially higher but he is also able to synthesize it faster thanks to more efficient enzymes.
To synthesize glycogen it is necessary to ingest carbohydrates; but how many, which, when, and how often?

The two phases of muscle glycogen synthesis after exercise

In order to restore as quickly as possible muscle glycogen depots, it is useful to know that, as a result of training sessions that deplete muscle glycogen to values below 75% those at rest and not fasting, glycogen synthesis occurs in two phases.
To know and therefore take advantage of the biphasicity is important for those athletes who are engaged in more daily training sessions, or who otherwise have little time for recovery between a high intensity exercise and the subsequent one (less than 8 hours), in order to maximize glycogen synthesis and achieve the optimal performance during a second close exercise session.
The two phases are characterized by:

  • a different sensitivity to circulating insulin levels;
  • a different velocity.

Muscle glycogen synthesis after exercise: the first phase

Muscle Glycogen
Fig. 1 – Glycogen Structure

The first phase, immediately following the end of an activity and lasting 30-60 minutes, is insulin-independent, i.e. glucose uptake by muscle cell as glycogen synthesis are independent from hormone action.
This phase is characterized by an elevated rate of synthesis that however decreases rapidly if you do not take in carbohydrates: the maximum rate is in the first 30 minutes, then declines to about one fifth in 60 minutes, and to about one ninth in 120 minutes from the end of exercise.
How is it possible to take advantage of this first phase to replenish muscle glycogen stores as much as possible? By making sure that the greatest possible amount of glucose arrives to muscle in the phase immediately following to the end of exercise, best if done within the first 30 minutes.

  • What to ingest?
    High glycemic index, but easy to digest and absorb, carbohydrates.
    Therefore, it is advisable to replace foods, even though of high glycemic index, that need some time for digestion and the subsequent absorption, with solutions/gel containing for example glucose and/or sucrose. These solutions ensure the maximal possible absorption rate and resupply of glucose to muscle because of they contain only glucose and are without fiber or anything else that could slow their digestion and the following absorption of the monosaccharide, that is, they are capable of producing high blood glucose levels in a relatively short time.
    It is also possible to play on temperature and concentration of the solution to accelerate the gastric transit.
    It should be further underlined that the use of these carbohydrate solutions is recommended only when the recovery time from a training/competition session causing significant depletion of muscle glycogen and the following one is short, less than 8 hours.
  • How many carbohydrates do you need?
    Many studies has been conducted to find the ideal amount of carbohydrates to ingest.
    If in post-exercise the athlete does not eat, glycogen synthesis rate is very low, while if he ingests adequate amounts of carbohydrates immediately after cessation of exercise, synthesis rate can reach a value over 20 times higher.
    From the analysis of scientific literature it seems reasonable to state that, as a result of training sessions that deplete muscle glycogen stores as seen above (<75% of those at rest and not fasting), the maximum synthesis rate is obtained by carbohydrate intake, with high glycemic index and high digestion and absorption rates, equal to about 1.2 g/kg of body weight/h for the next 4-5 hours from the end of exercise.
    In this way, the amount of glycogen produced is higher than 150% compared to the ingestion of 0.8 g/kg/h.
    Because further increases, up to 1.6 g/kg/h, do not lead to further rise in glycogen synthesis rate, the carbohydrate amount equal to 1.2 g/kg/h can be considered optimum to maximize the resynthesis rate of muscle glycogen stores during post-exercise.
  • And the frequency of carbohydrate ingestion?
    It was observed that if carbohydrates are ingested frequently, every 15-30 minutes, it seems there is a further stimulation of muscle glucose uptake as of muscle glycogen replenishment compared with ingestion at 2-hours intervals. Particularly, ingestions in the first post-exercise hours seem to optimize glycogen levels.

Muscle glycogen synthesis after exercise: the second phase

The second phase begins from the end of the first, lasts until the start of the last meal before the next exercise (hence, from several hours to days), and is insulin-dependent i.e. muscle glucose uptake and glycogen synthesis are sensitive to circulating hormone levels.
Moreover, you observe a significant reduction in muscle glycogen synthesis rate: with adequate carbohydrate intake the synthesis rate is at a value of about 10-30% lower than that observed during the first phase.
This phase can last for several hours, but tends to be shorter if:

In order to optimize the resynthesis rate of glycogen, experimental data indicate that meals with high glycemic index carbohydrates are more effective than those with low glycemic index carbohydrates; but if between a training/competition session and the subsequent one days and not hours spend, the evidences do not favor high glycemic index carbohydrates as compared to low glycemic index ones as long as an adequate amount is taken in.

Muscle glycogen synthesis rate and ingestion of carbohydrates and proteins

The combined ingestion of carbohydrates and proteins (or free insulinotropic amino acids) allows to obtain post-exercise glycogen synthesis rate that does not significantly differ from that obtained with larger amounts of carbohydrates alone. This could be an advantage for the athlete who may ingest smaller amount of carbohydrates, therefore reducing possible gastrointestinal complications commons during training/competition afterward to their great consumption.
From the analysis of scientific literature it seems reasonable to affirm that, after an exercise that depletes at least 75% of muscle glycogen stores, you can obtain a glycogen synthesis rate similar to that reached with 1.2 g/kg/h of carbohydrates alone (the maximum obtainable) with the coingestion of 0.8 g/kg/h of carbohydrates and 0.4 g/kg /h of proteins, maintaining the same frequency of ingestion, therefore every 15-30 minutes during the first 4-5 hours of post-exercise.

The two phases of muscle glycogen synthesis: molecular mechanisms

The biphasicity is consequence, in both phases, of an increase in:

  • glucose transport rate into cell;
  • the activity of glycogen synthase, the enzyme that catalyzes glycogen synthesis.

However, the molecular mechanisms underlying these changes are different.
In the first phase, the increase in glucose transport rate, independent from insulin presence, is mediated by the translocation, induced by the contraction, of glucose transporters, called GLUT4, on the cytoplasmatic membrane of the muscle cell.
In addition, the low glycogen levels also stimulate glucose transport as it is believed that a large portion of transporter-containing vesicles are bound to glycogen, and therefore they may become available when its levels are depleted.
Finally, the low muscle glycogen levels stimulate glycogen synthase activity too: it has been demonstrated that these levels are a regulator of enzyme activity far more potent than insulin.
In the second phase, the increase in muscle glycogen synthesis is due to insulin action on glucose transporters and on glycogen synthase activity of muscle cell. This sensibility to the action of circulating insulin, that can persist up to 48 hours, depending on carbohydrate intake and the amount of resynthesized muscle glycogen, has attracted much attention: it is in fact possible, through appropriate nutritional intervention, to increase the secretion in order to improve glycogen synthesis itself, but also protein anabolism, reducing at the same time the protein-breakdown rate.

Glycogen synthesis rate and insulin (H4)

The coingestion of carbohydrates and proteins (or free amino acids) increases postprandial insulin secretion compared to carbohydrates alone (in some studies there was an increase in hormone secretion 2-3 times higher compared to carbohydrates alone).
It was speculated that, thanks to the higher circulating insulin concentrations, further increases in post-exercise glycogen synthesis rate could be obtained compared to those observed with carbohydrates alone, but in reality it does not seem so. In fact, if carbohydrate intake is increased to 1.2 g/kg/h plus 0.4 g/kg/h of proteins no further increases in glycogen synthesis rate are observed if compared to those obtained with the ingestion of carbohydrates alone in the same amount (1,2 g/kg/h, that, as mentioned above, like the coingestion of 0,8 g/kg/h of carbohydrates and 0,4 g/kg/h of proteins, allows to attain the maximum achievable rate in post-exercise) or in isoenergetic quantities, that is, 1.6 g/kg (proteins and carbohydrates contain the same calorie/g)

Insulin and preferential carbohydrate storage

The greater circulating insulin levels reached with the coingestion of carbohydrates and proteins (or free amino acids) might stimulate the accumulation of ingested carbohydrates in tissues most sensitive to its action, such as liver and previously worked muscle, thus resulting in a more efficient storage, for the purposes of sport activity, of the same carbohydrates.

References

Beelen M., Burke L.M., Gibala M.J., van Loon J.C. Nutritional strategies to promote postexercise recovery. Int J Sport Nutr Exerc Metab 2010:20(6);515-32 [Abstract]

Berardi J.M., Noreen E.E., Lemon P.W.R. Recovery from a cycling time trial is enhanced with carbohydrate-protein supplementation vs. isoenergetic carbohydrate supplementation. J Intern Soc Sports Nutrition 2008;5:24 [PDF]

Betts J., Williams C., Duffy K., Gunner F. The influence of carbohydrate and protein ingestion during recovery from prolonged exercise on subsequent endurance performance. J Sports Sciences 2007;25(13):1449-60 [Abstract]

Howarth K.R., Moreau N.A., Phillips S.M., and Gibala M.J. Coingestion of protein with carbohydrate during recovery from endurance exercise stimulates skeletal muscle protein synthesis in humans. J Appl Physiol 2009:106;1394–1402  [Abstract]

Jentjens R., Jeukendrup A. E. Determinants of post-exercise glycogen synthesis during short-term recovery. Sports Medicine 2003:33(2);117-144 [Abstract]

Millard-Stafford M., Childers W.L., Conger S.A., Kampfer A.J., Rahnert J.A. Recovery nutrition: timing and composition after endurance exercise. Curr Sports Med Rep 2008;7(4):193-201 [Abstract]

Price T.B., Rothman D.L., Taylor R., Avison M.J., Shulman G.I., Shulman R.G. Human muscle glycogen resynthesis after exercise: insulin-dependent and –independent phases. J App Physiol 1994:76(1);104–111 [Abstract]

van Loon L.J.C., Saris W.H.M., Kruijshoop M., Wagenmakers A.J.M. Maximizing postexercise muscle glycogen synthesis: carbohydrate supplementation and the application of amino acid or protein hydrolysate mixtures. Am J Clin Nutr 2000;72: 106-111 [Abstract]

Skeletal muscle glycogen stores and sports

Functions of skeletal muscle glycogen

Muscle glycogen represents a source of glucose, therefore energy, that can be used by muscle during physical activity: it is an energy store where needed!
Furthermore a close relationship exists between the onset of fatigue and depletion of its muscle stores.

Glycogen as energy source

Carbohydrates and fatty acids (lipids) represent the main energy source for muscle during exercise and their relative contribution varies depending on:

  • the intensity and duration of exercise;
  • the level of training.

If for fatty acids there are no problems regarding body stores so it is not for carbohydrates whose stores, present in glycogen form principally in the liver and the muscle, are modest, less than 5% of total body energy stores: in a non-fasting 70 kg adult male there are about 250 g of glycogen in the muscle and 100 g in the liver, for a total energy of about 1400 kcal. In athletes the amount could be higher, for example in the best marathoners, again considering an adult male as above, you can reach up to 475 g in total, muscle plus liver, which corresponds to about 1900 kcal.
In spite of this, glycogen contribution to the total energy needed to sustain muscular workload rises with the increase of exercise intensity, whereas we reduce that in the form of fatty acids.
Furthermore, in the absence of replenishment with exogenous carbohydrates, performance is determined by the endogenous stores of liver and skeletal muscle glycogen, of which relative consumption is different: an increase of intensity increases that of the second (muscle) while remain more or less constant in that of the first (liver).

Skeletal muscle glycogen and intese exercises

In fact, skeletal muscle glycogen represents the most important energy reserve for prolonged moderate-high intensity exercise, an importance that increase in the case of high-intensity interval exercise (common in training session undertaken by swimmers runners, rowers or in team-sport players) or in resistance exercise, therefore both endurance and resistance exercises. If for example we consider the marathon about 80% of utilized energy derives from carbohydrate oxidation, for the most part skeletal muscle glycogen.
Finally, the replenishment rate of glycogen stores in post-exercise is one of the most important factors in establishing necessary recovery time.

Muscle glycogen and fatigue

Fatigue and low glycogen levels are closely correlate but it is not clear which mechanisms are at the basis of this relationship; one hypothesis is that there exists a minimum glycogen concentration that is “protected” and is resistant to being used during exercise, perhaps to ensure an energy reserve in case of extreme necessity.
Because of the closely relationship between skeletal muscle glycogen depletion and fatigue, its replenish rate in the post-exercise is one of the most important factors in determining necessary recovery time.

References

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

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

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

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