The nutritional goals of athletes must include the use of carbohydrates to provide prolonged energy for exercise sessions, improve performance, and restore glycogen for optimal recovery. The attention paid to the type of carbohydrate, however, has only recently become an area of focus in sports nutrition, and has given rise to an effective classification system. Apparently, not all carbohydrates are created equal because the use of carbohydrates spans further than its function as an energy fuel (such as in its potentially large and varied influence on fat usage). Thus, both the relevance and guidelines of categorizing carbohydrates can be revealed via the body's ensuing glucose and insulin (a hormone released from the pancreas) responses following food ingestion.

All carbohydrates are eventually broken down into glucose and released to the bloodstream. The amount and speed of glucose entering the blood determines the corresponding insulin response. Naturally, a large and rapid glucose response will spark a considerable surge of insulin, which stimulates the transport of glucose, proteins, and fats out of the blood and into cells. Consequently, this creates a reduction of readily available energy as a result of the banishment of glucose; the condition has been termed rebound hypoglycemia. 2o Particularly before exercise, this outcome diminishes the energy potential that can be gleaned from glucose and fats. Furthermore, the resulting increased fat storage presents a profound disadvantage for those seeking weight loss. Ideally, to provide sufficient energy for training (and possibly the optimal environment for physique enhancement), the glucose response should be gradual and maintained. Not only does this ensure adequate energy for sustained exercise bouts, but it also attenuates the corresponding insulin response.

Traditionally, classifying carbohydrates into simple and complex was the norm in educating athletes and lay people about the alleged benefits and pitfalls of this macronutrient. According to accepted sports nutrition dogma, simple carbohydrates, such as those found in fruits and high-sugar foods, reportedly triggered a rapid and large rise in blood glucose levels, which was followed by a rapid and often greater fall. In contrast, complex carbohydrates, such as pasta and starchy vegetables, were labeled as nutrient rich and seemingly induced a more sustained blood glucose response.

Unfortunately, this system does not accurately account for the delicate diversity of carbohydrate-rich foods, nor does it provide meaningful information to direct food selection. In fact, the preceding characterizations are largely inaccurate and oversimplified. For example, although fruits contain primarily simple carbohydrates, there exists considerable disparity in the corresponding glucose responses following their ingestion. Interestingly, many kinds of fruit provide an ideal glucose response, whereas many forms of pasta (a complex carbohydrate) spawn an unfavorably rapid one. Confusing matters further is the fact that many foods contain both simple and complex carbohydrates. Moreover, numerous foods that contain simple sugars are often incorrectly labeled as unhealthy, and many complex carbohydrates are in fact nutrient poor.

As a successful alternative, the glycemic index (GI) has emerged as an accepted tool to guide nutritional selections. The GI is a ranking of foods based on their actual blood glucose response following consumption. The ranking of a particular food is determined by its blood glucose response relative to a reference food, either glucose or white bread. Therefore, the GI reflects the rate of digestion and absorption of carbohydrate-rich food and by using the GI, one can reliably form appropriate meals to potentially promote enhanced fitness.

The number assigned to each food represents the speed at which the food is digested and subsequently absorbed, with higher numbers reflecting faster introduction of glucose into the bloodstream. Generally, these values represent the average indices as ascertained from a number of studies and laboratories. The table also illustrates the obvious unpredictability of making nutritional choices based on traditional belief and/or subjective appeal. For instance, fructose possesses a surprisingly low index, while apparent powerhouse energy sources such as potatoes and bagels rank rather high. Because the GI also, albeit indirectly, indicates the ensuing insulin response of food consumption, high-GI foods eaten in isolation are not recommended before training. They are also considered disadvantageous for those who seek to lose weight and body fat, though these individuals do see the value of high-GI foods fulfilled in post-exercise meals.

Foods are usually eaten in combinations; therefore, it is important to note that the glycemic index of a meal is usually lower than the glycemic index of the highest constituent. For instance, if equal calories of a bagel and an apple are combined, the glycemic index of that meal becomes a more acceptable. Protein also helps because it efficiently decreases the total glycemic index by slowing the absorption rate of carbohydrates, concomitantly abating the insulin response. Because those seeking weight loss should strive for a reduced insulin response, this emphasizes the importance of combining protein and carbohydrates in each meal.

Finally, the GI provides clinical practicality in that a low-GI diet improves glucose control in diabetics and reduces hyperlipidemia low-GI foods are also indicated as increasing satiety furthering their benefit for those involved in weight control regimens. Overall, therefore, the GI has become an accepted nutritional tool not only for athletes, but also for anyone aspiring to improve his or her health.