Download CARBOHYDRATES

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.
Mulligan, which shoes are you wearing? 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.