Nutritionists like me have succeeded in convincing athletes to think of carbohydrates as their best fuel source. What they may not recognize is that carbohydrates cannot be lumped into one category. Most athletes have even heard that there are simple and complex carbohydrates. However, pardon the pun, carbohydrates are more complex than that. When it comes to planning their training meals, athletes need to understand and use the glycemic index.
For years, carbohydrate sources have been labeled as simple (containing sugars like glucose and sucrose) or complex (containing fiber and starch) based on the structure of the main carbohydrate. Simple carbohydrate foods have been charged with causing large and rapid changes in blood glucose. They are alleged to cause a rapid rise followed by a rapid and often greater fall - this is known as rebound hypoglycemia or the "sugar blues". Simple carbohydrates have also been considered to be lacking in nutrient value. On the other hand, it has been believed that the digestion and absorption of complex carbohydrate foods is slower, producing a flatter and more sustained blood glucose and insulin response. Complex carbohydrate foods have also been regarded as being more "healthy" or "nutritious".
While this classification system may have been developed as a quick education tool for the lay person, it has become a major headache for nutritionists. Because we now know that the effect of specific carbohydrate foods on the blood glucose response is neither simple nor predictable.
During the 1970s, diabetes specialists 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 produce a flattened blood glucose curve when they are eaten. The old no-no, sugar (sucrose), has a medium blood sugar profile. Curiously, some foods high in complex carbohydrates (e.g. bread and potatoes) produce a rapid blood glucose response, similar to that following the ingestion of glucose itself. Even the presence of dietary fiber in foods does not always delay absorption and flatten the after-meal blood glucose curve. For example, blood glucose responses to whole-grain breads are similar to those after eating white bread. The glycemic index (GI) was introduced in the early 1980s to classify the real effects of carbohydrate-rich foods on blood glucose levels. The GI is a ranking of foods based on their measured blood glucose response compared to that following a standard food. In some laboratories the standard food is glucose, while other scientists prefer to use white bread.
Tables of the glycemic index of a large number of carbohydrate-rich foods have now been published internationally. The numbers vary according to who measured them and the exact type of food. For example, there are a lot of different types of "white bread" in the world. And even things like potatoes and rice come in a variety of plant types. Each has a slightly different GI. Generally, nutritionists now divide foods into those that have a high GI (bread, potatoes, breakfast cereal, glucose-based sports drinks), a moderate GI (sugar, soft drinks, tropical fruit) or a low GI (dairy foods, lentils, legumes, oats, cold climate fruits such as apples). Some foods sit on the borderline, but this is not really a problem. The real interest is in foods that are extremely different in their GI. And the real message is that there is no way to predict blood glucose responses to eating specific foods without these actual measures.
Now that we know the effect of specific food items on blood glucose responses, we can advise people who want to control their blood glucose profiles during the day or after meals. In other words, you can eat the same amount of carbohydrate, but manipulate whether you want blood glucose spikes during the day (eat high and moderate GI foods), or a more even level (low GI foods). Diabetics are a classic example of a population that benefits from tight control of blood glucose and low GI foods. People with high blood lipid levels may also benefit from being able to achieve a more even blood glucose profile that has smaller rises and falls during the day. The glycemic index may also be a useful tool in weight control, since low GI foods have recently been shown to produce a longer-lasting "satisfaction" after meals - you don't feel hungry quite so soon. A recently published book, The G.I. Factor, has made this information widely accessible.
Some people have quickly grabbed on to the idea that altering the GI of specific meals or the training diet may influence training and performance. The focus is on optimizing the muscle carbohydrate fuel sources, particularly for prolonged moderate-intensity exercise. Research at the Australian Institute of Sport, in conjunction with researchers at Deakin University and University of Melbourne, has examined the use of GI in sport. The following guidelines are drawn from this research.
1. The glycemic index may be useful in sport and deserves further attention. However, it is not intended to provide a single way to rank the virtues of carbohydrate foods. There are many other features of foods which may be of value to the athlete, such as nutritional value or practicality. Sometimes foods need to be chosen because they are tasty, portable, cheap, easy to prepare and unlikely to cause stomach upsets. These issue are specific to the individual and the exercise situation. In other words, foods must always be chosen to fit the "Big Picture" and not one single issue. In the case of food eaten before or during exercise, the athlete should practice any strategies in training so that they can be assessed and fine-tuned.
2. Despite early speculation, there is insufficient evidence to support the statement that all athletes will benefit from eating low GI carbohydrate meals prior to prolonged exercise. The idea is that a more sustained glucose response might sustain fuel and performance. In fact, in sports events where carbohydrate stores can become depleted, the typical way to sustain the carbohydrate supply during exercise is to consume carbohydrate during the event. The athlete should let practical issues and individual experience guide the choice of a pre-event meal. You may happen to like a carbohydrate food that is low GI (e.g. pasta), or you may find that your choices tend to foods with a high glycemic index such as rice, breakfast cereal, toast. Both choices can work.
3. For specific individuals or during unique training situations, a low GI pre-event meal may be of particular benefit. Some athletes show an exaggerated and negative response when they eat carbohydrate foods in the hour before exercise. About 5% of the population experience a rebound hypoglycemia or blood sugar drop - and they feel terrible. Why this response occurs in some people is unknown. During unusual endurance sessions such as open water swimming where practical difficulties prevent the athlete from consuming carbohydrate during the session, the pre-event meal may have greater bearing on metabolism and fuel availability during the event, and a low GI carbohydrate meal may sustain blood glucose, and performance.
4. Athletes performing prolonged exercise should consume carbohydrate during the event to supply additional fuel and thereby enhance their performance. Which carbohydrate drink or food to consume depends generally on their previous experience, the logistics of the event, gastrointestinal comfort and the need for fluid replacement. A carbohydrate source of moderate to high GI appears to be sensible - such as a glucose-based sports drink. However, practical issues and individual tastes are more important than GI when choosing a carbohydrate source for prolonged exercise situations
5. Moderate and high GI carbohydrate foods appear to enhance glycogen recovery after exercise compared with low GI foods. The reason for this is not clear. The most important point, however, in post exercise refueling is to eat enough total carbohydrate. We give recommendations to athletes about how much carbohydrate they should consume immediately after exercise and throughout the day to meet their refueling needs. Foods must be available and appetizing to the athlete so that these recommendations can be met. It is OK to let some favorite low GI carbohydrate foods contribute to total fuel intake - especially if these are foods that are handy and easy to eat. However, it makes sense to focus on carbohydrate foods and drinks with a moderate to high GI for glycogen recovery. The overall message: choose what is practical.
Brand Miller, J., Foster-Powell, K., & Colagiuri, S.(1996). The G.I. Factor: The Glycaemic Index Solution. Sydney, Australia: Hodder and Stoughton.