bike75.gif (2872 bytes)


  Last updated: 12/06/2009

Glycemic Index

Carbohydrates are the backbone of the athlete's nutritional program. However, all carbohydrates are not created equal (and thus are not interchangeable) in their rate of digestion and absorbtion, and as a result their subsequent effect on the athlete's performance. Simple carbohydrates (single sugar molecules) are rapidly absorbed into the blood stream, and rapidly available as an enegy source for exercising muscle. But they also have the greatest potential to stimulate an insulin surge with reactive hypoglycemia in the non exercising individual. Although it has traditionally been taught that complex carbohydrates (molecules of linked simple sugar units) are digested and absored more slowly than single sugar molecules (producing a flatter, more sustained blood glucose level with a less intense insulin response i.e. minimizing reactive hypoglycemia) this is not always the case.

The glycemic index (GI) reflects how quickly an ingested carbohydrate will trigger a rise in the circulating blood glucose level, the higher the GI, the faster the blood sugar response. The GI of any carbohydrate ranks that food relative to pure glucose -- and runs from 0 to 100 (with 100 being equal to pure glucose). All else being equal (a liquid versus solid supplement, a low fat versus fat containing food), the glycemic index will identify the energy supplement which will provide the quickest blood sugar boost, again, the higher the GI, the faster the energy boost.

The concept of the glycemic index was developed by diabetes specialists who were amazed to find that simple carbohydrate foods did not always produce the high and short-lived blood glucose responses traditionally attributed to them. For example, fruit and sweetened dairy products produced a relatively flattened blood glucose curve, sugar (sucrose. a two sugar molecule) has a medium blood sugar profile, and some foods high in complex carbohydrates such as bread and potatoes actually produced a relatively rapid blood glucose response. Even dietary fiber does not necessarily delay absorption and flatten the blood glucose curve - blood glucose levels after eating whole-grain breads are similar to those after white bread. What is very clear is that there is no way to predict blood glucose responses (and the GI) of a specific food without actually measuring the response. Tables with specific GIs for different carbohydrates are available on the WWW.

A number of factors appear to determine a food's glycemic index (excerpted from the website of Harvard School of Public Health). One of the most important is how highly processed its carbohydrates are. In highly processed carbohydrates, the outer bran and inner germ layer are removed from the original kernel of grain, which causes bigger spikes in blood sugar levels than would occur with less-processed grains. Whole-grain foods tend to have a lower glycemic index than their more highly processed counterparts. For example, white rice, which is highly processed, has a higher glycemic index than brown rice, which is less highly processed.

A number other factors influence how quickly the carbohydrates in food raise blood sugar levels, including:

Understanding the variable effect of carbohydrates on blood glucose levels provides an approach to advise those who need to closely control their blood glucose profiles - diabetics being a classic example of a population that benefits from tight control of blood glucose levels. Patients with high blood lipid levels also benefit from a more even blood glucose profile during the day. And the glycemic index has been proposed as a useful tool in weight control based on the observation that low GI foods seem to produce a longer lasting, satiated feeling after meals.

Some athletes and coaches have speculated that altering the GI of the training diet or pre race meal might influence their performance with a low GI pre race meal conferring an advantage (less insulin surge and blood sugars remaining elevated over a longer period of time post meal). However, controlled studies have failed to demonstrate any advantages of a low compared to a high GI pre-race meal.

A recent study attempted to blend sports nutrition guidelines with the real-life practices of competitive athletes. Six well-trained cyclists (average maximum oxygen uptake of 68 ml/kg/min) performed three trials in which they consumed a different pre-race meal two hours before undertaking an exercise test. The three test meals consisted of a high GI carbohydrate meal (mashed potatoes topped with pasta sauce), a low GI carbohydrate meal (pasta topped with the pasta sauce), and a placebo or control meal (subjects ate low-calorie jelly, believing it to be a new "sports jelly"). The cyclists rode for two hours at 70% of their maximum oxygen uptake, equivalent to marathon pace or about 80% of maximum heart rate. During the ride, blood and breath samples were collected to determine which food groups they were burning. And at the end of the two hours, the cyclists did a time trial lasting approximately 15 minutes.

Fifteen minutes before starting their time trial, the cyclists consumed about 300 ml of a sports drink. Then, throughout two hours of steady riding, they continued to take regular drinks of the carbohydrate mixture. In total, they drank approximately 700 ml per hour of the sports drink, taking in the recommended carbohydrate intake of about 60 g each hour.

This study demonstrated that the regular intake of carbohydrate supplements, meeting the energy needs of athletes for prolonged, moderate intensity exercise, appeared to override any metabolic or performance effects of pre-event meals with varying GIs. These results suggest that in endurance events, athletes needn't worry about the glycemic index of their pre race diet, assuming they consume adequate amounts of carbohydrate (drinks or food) during the endurance events. Thus they can choose their pre-exercise menu based on personal preferences.

In Summary - using the Glycemic Index (GI) to develop a training or exercise diet plan:

1. There is insufficient evidence to support the concept that athletes will benefit from eating low GI carbohydrate meals prior to prolonged exercise, if they use carbohydrate supplements during the ride. They should let practical issues and individual experience guide their choice of the pre-event meal.

2. A limited number of individuals may benefit from a low GI pre-event meal. Those athletes that show an exaggerated (and negative) response to eating carbohydrates in the hour immediately before exercise (approximately 5% of the population that experience rebound hypoglycemia or blood sugar drop) may benefit from focusing on low GI foods. And during unusual endurance sessions, such as open water swimming, where practical difficulties prevent the athlete from consuming carbohydrate supplements during the session, the pre-event meal may have greater bearing with the slower absorption and release of glucose from a low GI carbohydrate meal theoretically sustaining blood glucose and thus enhancing performance.

3. Athletes involved in events lasting more than 2 hours should focus on maintaining adequate carbohydrate supplementation during the event. Which carbohydrate drink or food they choose shoould be determined by their previous experiences, the logistics of the event, gastrointestinal tolerance, and the requirements for fluid replacement. A glucose-based sports drink with a moderate to high GI would appear to make the most sense to quickly provide carbohydrate energy to the muscles .

4. Moderate and high GI carbohydrate foods are logical choices for glycogen repletion after exercise.

5. Other aspects (tasty, portable, cheap, easy to prepare and unlikely to cause stomach upsets) may outweigh the GI in making diet choices. These will be specific to the individual and the exercise situation.

Questions on content or suggestions to improve this page are appreciated.

Cycling Performance Tips
Home | Table of Contents | Local Services/Information