What is the Glycemic Index?
When you eat something, it is processed by various enzymes (body chemicals) released from saliva, the stomach, the pancreas and the intestines.The role of these enzymes is to break down the food we eat so that the nutritional components can be absorbed and used by the body. Your body needs proteins, carbohydrates, fats, vitamins, minerals and various antioxidants to maintain good health.
Carbohydrate foods -- cereals, potatoes, fruits -- are broken down by enzymes to the basic nutrients glucose and fructose before they can be absorbed. The rate at which they are absorbed, fast or slow, can be measured. This is called the glycemic index or GI, a scale where glucose is ranked at 100 and carbohydrate foods mostly fall below this. A food with an index number below 50 -- a heavy wholegrain bread, for example -- would be ranked as low glycemic index or low GI.
The GI ranking indicates how quickly the nutrient glucose is absorbed from 50 grams of carbohydrate in any particular food over two hours compared to pure glucose. Don't worry if you can't grasp exactly how this works. Just think of it as how quickly any carbohydrate food you eat is absorbed into the bloodstream as glucose (blood sugar.)
There is growing evidence that the spikes in blood glucose levels associated with high-GI diets in excess contribute to weight gain and increased cardiovascular risk. Spikes in blood glucose levels cause the sudden release of the hormone insulin. Rapid fluctuations in insulin levels are also thought to contribute to the hunger sometimes experienced two to three hours after a high-GI meal, although there is no general agreement on this. However, low-GI meals cause carbohydrates to be released and absorbed gradually, thus avoiding spikes in glucose and insulin levels. As a result, appetite stimulation may be avoided. Because of these considerations, more and more nutrition experts are endorsing low-GI diets, yet others are skeptical.
Examples of High and Low GI Foods
Although critics point to inconsistencies in how the GI is measured and used, below are examples of low and high GI foods.- Higher GI
- White bread
- Potatoes
- Crackers and pastries
- White rice
- Lower GI
- Wholegrain bread
- Whole grains
- Pasta
- Milk and dairy
- Most fruits
David Mendosa has a large list of GI rankings of various carbohydrate foods.
How Can the GI Help with Fitness Training?
There's still much to be discovered about how different glycemic index foods affect sports and exercise performance. However, here are some general principles that may help you decide how and when you might use low or high glycemic index foods.Fat burning
In general, your body will burn more fat rather than carbohydrate in the following sequence during activity:
- When you have eaten no food: most fat burned
- When you have eaten low-GI food: more fat burned than when you've eaten high-GI food, but less than when you haven't eaten at all
- When you have eaten high-GI food: least fat burned
Low-GI foods may be most useful prior to beginning training or an activity because they release glucose slowly. One possible advantage of this, especially for endurance exercisers like marathoners, is that it allows you to use more fat early in the session and save the important muscle glucose for later, which may result in better performance. This is called "glucose sparing."
High-GI foods may be more suitable after training or competition because they release glucose rapidly, thus replenishing your depleted blood and muscle glucose quickly. Along with this rush of glucose is an insulin spike, which is important for muscle repair and development in this important post-exercise period for muscle builders and strength trainers.
Low-GI eating is successful as a weight-loss strategy, according to several studies. And for bodybuilders, this may be useful in the cutting phase in preparation for events where very low body fat levels are necessary for competition success.
High-GI eating may produce more fat conversion from carbohydrates. When blood glucose levels are high, and storage in muscle and liver is saturated, some carbohydrate can be converted to fat. In an energy-balanced diet, this would probably not result in weight gain, yet it could be important for bodybuilders trying to reach body fat levels of around 5 to 6%.
Summary of the Glycemic Index
If you are an active, lean athlete or trainer, and you already eat a healthy, balanced diet with plenty of fiber, fruit and vegetables, then the GI may not be of great importance to you as a healthy-eating measure. However, it may be worth experimenting with the GI to get the most from your sports nutrition until more definitive information is available. If you need to lose weight, then low-GI eating is indeed worth a try.Sources
Thomas D, Elliott E, Baur L. Low glycaemic index or low glycaemic load diets for overweight and obesity. Cochrane Database Syst Rev. 2007 Jul 18;3:CD005105.
Wu CL, Nicholas C, Williams C, Took A, Hardy L. The influence of high-carbohydrate meals with different glycaemic indices on substrate utilisation during subsequent exercise. Br J Nutr. 2003 Dec;90(6):1049-56.
Wu CL, Williams C. A low glycemic index meal before exercise improves endurance running capacity in men. Int J Sport Nutr Exerc Metab. 2006 Oct;16(5):510-27.
