Nutrition recommendations about eating carbohydrates food before physical effort, presented in scientific sources, suggest either eating or avoiding carbohydrates one hour before undertaking physical effort.
This contradictory information about eating or avoiding carbohydrates, appears frequently and shows that nutrition recommendation or sports dietetics are fields of science still in progress and requiring further researches.
Organism reaction after eating carbohydrates is immediate- at the beginning it is a higher level of insuline in blood (hyperinsulinemia) and a bigger level of glucose in blood (hyperglycemia). As a result, one can observe a lower level of glucose in blood and the increase of the glycogen decomposition level. This phenomenon is called “the effect of reflection” that means a reactive hypoglycaemia.
According to this fact, it is suggested that eating carbohydrates directly before physical effort may cause hypoglycaemia, suppress fat metabolism, accelerate glycogen decomposition and may lower a sportsperson’s efficiency while physical exercises.
The analysis of the article ‘’The myths surrounding pre-exercise carbohydrate feeling” published in Annals of Nutrition and Metabolism may be useful in understanding the topic of eating carbohydrates while practicing sports.
SUFFICIENT AMMOUNT OF CARBOHYDRATES
Short and co and Sherman and co’s researches show that easting a small amount of carbohydrates (according to the authors 22 g) or much bigger amounts (75 g, 158 g) one hour before a physical effort with an intensity of approximately 62- 72% VO2max results in lowering glucose concentration in blood up to the same or similar values. That means, carbohydrates eaten before effort cause similar metabolic reaction independently on the amount eaten.
TIME OF CARBOHYDRATES INTAKE
Researches confirm that eating 75 g of glucose 15 minutes before effort causes a bigger growth of glucose and insulin level in blood in comparison to eating the same dose of carbohydrates 45 or 75 minutes before exercises.
However, no difference in a glucose level in blood was observed after eating a nutritional sports snack (20 g of carbohydrates, 12 g of protein, 4.5 g of fat) 15 or 60 minutes before exercises. It may be caused by the fact that carbohydrates are absorbed in a different way in presence of other nutritional components.
To minimalize the risk of hyperglycaermia during effort, carbohydrates must be eaten directly before effort (eg. in the last 5 minutes before exercises) or during a warm-up.
Bronus and co. were giving competitors carbohydrate drinks (with saccharose, fructose, maltodextrines and dextrose) during a warm-up. The researches results confirmed that during warm-up and final exercises the level of cathelomines and slowing down of an insulin reaction appeared. Drinking carbohydrate drinks during a warm-up with a little break, does not cause hyperglycaemia with a “reflection effect”, while a growth of dextrose level is observed.
When carbohydrates are consuming directly before physical effort (about 10 minutes before), then a reactive hyperglycaemia does not occur because of an inhibiting physical exercise effort influence on insulin secretion.
Jeukendrup AE, Killer SC. The myths surrounding pre-exercise carbohydrate feeding. Ann Nutr Metab. 2010; 57 Suppl 2:18-25