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CYCLING PERFORMANCE TIPS

  Last updated: 7/1/2015

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 absorption, 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 as a result are rapidly available as an energy source for exercising muscle. Thus they are an ideal alternative to muscle glycogen as an energy source. But rapid absorption has its risks as well. The surge in blood glucose in a non exercising individual stimulates an insulin surge with many potential side effects, such as reactive hypoglycemia. Another is an increased tendency to store the excess glucose as fat, and a third is an increase in the complications of diabetes. It is this third effect that has led to many articles and investigations. Although it has traditionally been taught that complex carbohydrates (molecules of linked simple sugar units) are digested and absorbed 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) is a measure of how quickly an ingested carbohydrate is absorbed (triggering a rise in the circulating blood glucose level) - the higher the GI, the faster the blood sugar rise. 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 (liquids compared to liquids, low fat snacks compared to equally low fat snacks - both of which can impact the rate of stomach emptying and thus the potential for absorption in the small intestine), the glycemic index will identify the energy supplement which will provide the quickest blood sugar boost. The higher the GI, the faster the energy boost.

The concept of the glycemic index was originally 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 glucose 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 some 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: 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 shouldn'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.

For this interested, this article from Medscape.com is a reprint of a recent interview on the glycemic index.

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 slower but prolonged blood glucose level and thus enhance 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 should 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 make the most sense as it would quickly provide carbohydrate energy to the muscles .

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

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

  6. Those using exercise to lose weight will benefit from using low GI foods for their daily dietary carbohydrates. Likewise, those who may have a borderline blood sugar and thus be considered as pre diabetics.


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