Simple carbohydrates (single sugar molecules) are rapidly absorbed into the blood stream and are 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 multiple linked simple sugar units) are digested and absored more slowly than single sugar molecules (producing a flatter, more sustained blood glucose level, a less intense insulin response, and less reactive hypoglycemia) this is not always the case.

The glycemic index (GI) reflects how quickly an ingested carbohydrate will raise the circulating blood glucose level - the higher the GI, the faster the blood sugar response. The GI of a 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.

The dabetes specialists who developed the concept of a GI were amazed to find that simple carbohydrate foods did not always produce the highest and shortest-lived blood glucose responses traditionally attributed to them. For example, fruit and sweetened dairy products (both containing relatively simple sugars) produced a relatively flattened blood glucose curve, sugar (sucrose. another two sugar molecule) a medium blood sugar profile, and some foods high in complex carbohydrates (such as bread and potatoes) actually produced a more rapid blood glucose rise. Even foods high in dietary fiber (traditionally thought to retard sugar digestion and absorption) do not necessarily have a low GI. Blood glucose levels after eating whole-grain breads are similar to those after white bread. There is no way to predict blood glucose responses (and GI) of a specific food without actually measuring the blood glucose response. Tables with specific GIs for different carbohydrates are available on the WWW.

In addition to the level of complexity of a starch or sugar, other factors that might impact a food's glycemic index are summarized on the website of the Harvard School of Public Health). One of the most important is the degree to which a carbohydrates has been processed. In highly processed carbohydrates, the outer bran and inner germ layers, both of which tend to inhibit digestion and absorption, are removed from the original kernel of grain. These processed carbohydrates have a higher GI than similar unprocessed whole-grain foods. As an example, white rice, which is highly processed has a higher glycemic index than less processed brown rice.

Factors which can influence how quickly the carbohydrates in a food raise blood sugar levels include:

• Fiber content. As discussed, fiber shields the carbohydrates in food from immediate digestion. Thus the sugars in fiber-rich foods are generally absorbed into the bloodstream more slowly.
• Ripeness. A ripe fruit or vegetable has higher sugar content and, therefore, has a higher glycemic index than a less ripe alternative.
• Form of the starch. The type of starch granules in a food influences how fast the carbohydrates are digested and absorbed into the bloodstream. The starch in potatoes, for example, is digested and absorbed into the bloodstream relatively quickly.
• Fat content and acid content. The higher a food's fat content or acid content, the slower it is emptied from the stomach into the small intestine, and inturn the rate at which its carbohydrates are digested into single molecules and absorbed into the bloodstream.
• Physical form. Finely ground flour has a higher glycemic index than more coarsely ground flour.

Some athletes and coaches have speculated that altering the GI of the training diet or pre race meal might influence 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").

Fifteen minutes before starting their ride, the cyclists consumed 300 ml of a sports drink. Then, throughout the two hours of steady riding, they continued the carbohydrate mixture, drinking approximately 700 ml per hour of the sports drink, equal to a carbohydrate intake of about 60 g per hour.

This study demonstrated that a regular intake of carbohydrates, adequate to meet the energy needs of athletes for prolonged, moderate intensity exercise, overrides any performance impact of pre-event meals of 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. This lets them choose their pre-exercise menu based on personal preferences.

My thoughts on the use of the Glycemic Index (GI) in planning a training or exercise diet plan:

1. There is insufficient evidence to support athletes benefiting from high or low GI carbohydrate meals prior to prolonged exercise, IF they use carbohydrate supplements during the ride. Instead 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 are the athletes that show an exaggerated (and negative) response to eating high GI carbohydrates in the hour immediately before exercise. (Approximately 5% of the population that experience rebound hypoglycemia with high GI foods in this scenario.)

Another group are those, such as open water swimmers, where practical difficulties prevent the athlete from consuming carbohydrate supplements during the session. For them a low GI pre-event meal may have greater benefit with the slower absorption and release of glucose theoretically sustaining their 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 a foods GI in making diet choices. These will be specific to the individual and the exercise situation.