Carbohydrate Requirements

The Institute of Medicine recommends 130 grams (520 kilocalories) of carbohydrate per day, which is the average minimal usage of glucose by the brain. The desirable range of carbohydrate intake is 45 to 65 percent of total caloric intake (also referred to as the Acceptable Macronutrient Distribution Range, or AMDR), and the Daily Value (DV) for carbohydrate on food labels is based on a recommended intake of 60 percent of total caloric consumption. These recommendations also generally advise that no more than 25 percent of carbohydrate intake be derived from sugars (mono- and disaccharides).

Dietary fiber consumption (from indigestible and partially digestible polysaccharides) should be at the level of 38 grams per day for adult men and 25 grams per day for adult women. Adequate fiber consumption aids in the maintenance of normal blood sugar, reduces heart disease risk, and lowers constipation risk. The difference between genders in recommended fiber consumption is based on the lower total food mass and calories typically consumed by women.

It has been suggested that our only true requirement for carbohydrate is for vitamin C, a six-carbon, glucose-like substance that most animals can derive through an enzyme conversion of glucose. There is some historical evidence that our human ancestors consumed very little carbohydrate and survived. However, when considering athletes and the mountain of research demonstrating that carbohydrate is the limiting substrate in athletic performance, it becomes clear that human survival and human performance are entirely different issues. Athletes need carbohydrate.

Athlete requirements for carbohydrate are based on several factors. Athletes must consume enough carbohydrate to

• provide energy to satisfy the majority of caloric needs;
• optimize glycogen stores;
• allow for muscle recovery after physical activity;
• provide a well-tolerated source of energy during practice and competition; and
• provide a quick and easy source of energy between meals to maintain blood sugar.

The traditional guideline for determining caloric intake has been to consider the amount of carbohydrate to be consumed as a proportion of total caloric intake. The recommendation for the general population is that carbohydrate should supply 50 to 55 percent of total calories, and the Dietary Reference Intake (DRI) is 130 grams per day (520 calories per day) for male and female adults. However, the amount typically recommended for athletes is between 55 and 65 percent of total calories, assuming an adequate total caloric intake. Another, and clearly better, way of determining carbohydrate requirement is by taking into consideration the amount of carbohydrate to be consumed (in grams) per kilogram of body mass. The carbohydrate intake recommendations for endurance-trained athletes range from between 7 and 8 grams of carbohydrate per kilogram of body weight per day. Studies of carbohydrate consumption of different athlete groups have found differences in carbohydrate intakes.

These data suggest that an athlete should consume between 5 and 10 grams of carbohydrate per kilogram of body mass, or between 20 and 40 kilocalories of carbohydrate, per kilogram of body mass. A hypothetical 155-pound (70 kilogram) athlete would take in between 1,400 and 2,800 calories from carbohydrate, which represents a far greater carbohydrate consumption than the DRI of 520 calories. Assuming this represents approximately 60 percent of total calories from carbohydrate, this athlete would consume a total of 2,300 to 4,700 kilocalories per day. Following the same logic, a 300-pound (136 kilogram) lineman on a football team would require 2,700 to 5,400 kilocalories just from carbohydrate each day, an amount that would be difficult to consume because carbohydrate has a relatively low energy density (i.e., only 4 kilocalories per gram). The generally recommended carbohydrate intake is based on the intensity and duration of exercise, with a higher requirement for greater duration and greater intensity.

Most carbohydrates are derived from cereals, legumes, fruits, and vegetables. There is no discernable amount of carbohydrate in meats, and only a small amount of carbohydrate in milk and cheese. A number of dairy products have sugars added (yogurt, ice cream) to make them more widely acceptable.

The glycemic index is a measure of how quickly consumed carbohydrates manifest themselves as blood glucose. Foods are compared with the ingestion of glucose, which enters the blood quickly because it requires no digestion and is readily absorbed. Glucose has a glycemic index score of 100, which is the basis of comparison for other foods. Foods are compared on an isocaloric basis (i.e., all providing the same number of calories), which is logical but is also the cause of some of the confusion associated with the glycemic index. For instance, carrots have a high glycemic index (>85) but the amount of carrots typically consumed is so low that the total calories of glucose from carrots entering the blood would be small.

Some foods have a surprisingly high glycemic index, while others are surprisingly low. Corn Flakes cereal, for instance, has a much higher glycemic index (84) than table sugar (65). When you assess the carbohydrate makeup of these two foods, however, it makes sense. The cereal grain in Corn Flakes is mainly composed of the disaccharide maltose, which is made of two glucose molecules. Table sugar, on the other hand, is made of sucrose (glucose plus fructose). The fructose must be converted by the liver to glucose, and this extra conversion slows the speed with which it manifests itself as blood glucose.

Because the volume of glucose and the speed at which it enters the blood can influence the amount of insulin produced, it is generally desirable for people (including athletes) to consume carbohydrates that have a medium to low glycemic index. However, there are times, such as during and immediately after exercise, when high-glycemic foods might be better for athletes.

Generally speaking, carbohydrate foods higher in fiber have a lower glycemic index, so they are good choices for athletes. However, dietary fiber may be a source of gas and distention, making them poor choices for consumption just before or during competition. Soluble fiber foods may create less of a problem, but athletes should experiment to determine which foods are most easily tolerated. Athletes often find that starchy carbohydrates with low fiber concentrations, such as pasta, are the most easily tolerated and deliver the high volume of carbohydrates athletes require.