Tag Archives: blood glucose levels

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

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