Carbohydrates are utilized in the body primarily for the purpose of maintaining blood sugar levels between meals. We do this through the stored form of carbohydrate (glycogen) mostly in our liver and muscles- hubs of energy regulation and expenditure respectively. Glycogen is a large molecule however and isn’t really suitable for long-term storage. In fact, the average 150lb male can only store about an 18-hour fuel supply as glycogen and when the limits have been reached, the body transforms excess available carbohydrates (now in the form of glucose) into fat for longer-term energy storage where it could be utilized as energy for up to 2 months. Physical training can increase glycogen stores but utilization is dependent on the hormonal control in our liver (Alberts, 2002).
The rate at which we digest carbohydrates depends on 3 factors: 1.) The glycemic load or impact, 2.) Availability of enzymes for breakdown and 3.) The presence of other macronutrients such as protein and fat which tend to delay gastric emptying- thus increasing the time it takes to reach the small intestine where most of the carbohydrate metabolism takes place. In general, carbohydrate metabolism can begin as quickly as 15-20 minutes but may continue 3-4 hours or until all the food has passed through the small intestine. A healthy diet rich in foods such as vegetables, fruits, nuts and whole grains tend to slow down the rate of glucose absorption-the basic concept behind glycemic value.
There’s been a number of studies that examine glycemic load in relation to glycogen storage and most indicate that high glycemic meals, that is foods that rapidly break down to simple sugars can drive slightly greater glycogen storage, but that the body interestingly compensates by expending more glycogen during the next exercise session (Wee, 2005).
Alberts B et al: Molecular biology of the cell, ed. 4, New York, 2002, Garland Science Taylor and Francis Group.
Wee SL, et al: Ingestion of a high-glycemic index meal increases muscle glycogen storage at rest but augments its utilization during subsequent exercise, Journal of Applied Physiology 2005 Aug; 99(2): 707-14.