Foods and food supplements do not all have the same effect on insulin and rates of blood glucose appearance in the body.
The Glycemic Index
The Glycemic Index (GI) is a system for ranking carbohydrates according to their effects on postprandial glucose concentration1. It provides useful information and a means to measure the individual blood glucose response achieved by consuming various foods. The development of the GI was an extension of the original fiber hypothesis, suggesting that slow digestion and absorption of plant-based foods may have an impact on metabolic profiles and ultimately affect the risk of metabolic disease2.
Each unit of GI represents the equivalent of 1g of carbohydrate from glucose or white bread2. Several factors influence the GI of a food, including organic acids, tannins, fiber content, ratio of amylose to amylopectin, amount of processing, and physical nature of dietary starch3. The GI of a particular food is calculated by measuring the blood glucose response of that food over a period of 2 hours and then compared to the reference foods, glucose or white bread. This comparison provides a number that identifies on a scale from 0 to 100 how similar the metabolic effects of a certain food is to that of glucose or white bread. Foods labeled with a low GI are less than 60, moderate GI is between 60 and 85, and a high glycemic food has a value greater than 85.
Recent research has shown that the glycemic index perhaps should be used when determining the types of foods consumed prior to and after training or competition. Because foods with a higher glycemic index will cause a greater appearance of blood insulin, an increase in glucose uptake by biologically active tissues will occur (muscle). Studies have shown that if large enough, the levels of insulin will produce what’s known as transient hypoglycemia, or low blood glucose. If this occurs prior to exercise, an earlier-onset of fatigue is likely to occur. The resulting consequences would be a decreased ability to sustain higher work outputs, which would lead to a possible decrease in athletic performance4.
However, with regards to post-training meals, the increased secretion of insulin leading to a faster rate of fuel uptake by the muscles is of benefit to the athlete. This will increase the repletion of substrates, or energy, and allow for optimal recovery4. This is of particular interest during periods of multiple, daily training sessions, such as with pre-season training. In the two hours after training, foods with a moderate to high glycemic index should comprise the majority of the carbohydrates consumed. Relying perhaps mostly on liquid carbohydrates in the earlier minutes after training has ceased. However, it should be noted that it has been suggested that carbohydrate ingestion during endurance exercise negates the effects of the consumption of pre-exercise GI meals5.
The Glycemic Load
The Glycemic Load (GL) considers both the GI and the amount of carbohydrate in a food. GL is calculated by the following formula6:
(GI)/100 x (CHO in grams) = Glycemic Load
A GL of 20 or greater is high, 11 to 19 is moderate, and 10 or less is low. Carrots, for example, have a GI of 71, considered moderate, but carrots are low in carbohydrate so its GL is 6, considered low.
Research on the effect of the GL on exercise performance and capacity is still at an early stage, but recent studies have shown that the concept may have some merit as far as sports nutrition is concerned. It has been suggested that the GL may be a better predictor of glycemic responses than the GI alone5. Download Paper
By
Robert Rezende, MS
REFERENCES
- Miles, JM. A role of glycemic index in preventing or treating diabetes. American Journal of Clinical Nutrition. 2008;87(1):1-2.
- Roberts, CK, Liu, S. Effects of glycemic load on metabolic health and type 2 diabetes mellitus. Journal of Diabetes Science and Technology. 2009;3(4):697-704.
- Liu, S, Willett, WC. Dietary Glycemic load and atherothrombotic risk. Current Atherosclerosis Reports. 2002;4(6)454-461.
- Thomas, DE, Brotherhood, JR, Brand JC. Carbohydrate feeding before exercise: Effect of the glycemic index. International Journal of Sports Medicine. 1991;180-186.
- O’Reilly, J, Wong, SH, Chen, Y. Glycaemic index, glycaemic load and exercise performance. Sports Medicine. 2010;1:40(1):27-39.
- Brand-Miller, JC, Thomas, M, Swan, V, Ahmad, ZI, Petocz, P, Colagiuri, S. Physiological validation of the concept of glycemic load in lean young adults. The Journal of Nutrition. 2003;133(9):2728-32.
