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CARBOHYDRATES? They aren't that simple!
Louise M Burke, Australian Institute of Sport, Canberra, Australia
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.
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