These shifts in metabolism toward the use of carbohydrates, with either a switch to a high-carbohydrate diet or with overfeeding, appear to be the result of the ready availability of the carbohydrates as opposed to fats and proteins. Additionally, the relative biochemical simplicity of metabolizing carbohydrates, as opposed to fats and proteins, also enhances the use of carbohydrates.

Given the fact that when carbohydrate intake increases your body has a tendency to metabolize more carbohydrates and less fats, how does this affect the exercise clinician? These facts help the exercise clinician in at least two ways. First, a high-carbohydrate diet helps with client weight maintenance. Weight maintenance is "easier" because excess carbohydrates (less than 500 grams/day) are not stored to any significant level as fat. It has been found that even with a 500 gram/day overfeeding of carbohydrates, only about 4 grams are turned to fat.

Therefore, for clients that have a hectic schedule and poor exercise adherence, it is not as critical to match their daily caloric intake with their expenditure because the excess carbohydrates will not lead to as much fat accumulation as would a diet high in fat.

Secondly, the shift toward the use of carbohydrates with a high-carbohydrate diet may lead to a slowed reduction of body fat because the body is using less fat, especially during times when the client is not exercising. For example, an individual who weighs 200 pounds with 30 percent body fat that is expending 2,500 kcal with a normal diet of 14 percent protein, 48 percent carbohydrate and 38 percent fat, burns approximately 111 grams of fat per day. However, if this same individual increases the carbohydrate content of their diet to 62 percent (fat = 25 percent and protein = 13 percent), then at the same expenditure level (2,500 kcal), this individual is only metabolizing 64 grams of fat per day. This is a 42 percent decrease in fat metabolism, which means that fat loss is going to take 42 percent longer, unless other factors are changed (e.g., energy expended during exercise, etc.). Therefore, the exercise clinician must be aware that fat metabolism may decrease with a change to a high-carbohydrate diet and, thus, the client may see a slower weight loss.

The accumulation of fat and hunger

The finding that an increase in fat intake will increase carbohydrate use and decrease fat use has led to a revised theory as to how obesity arises. It has been proposed that hunger is stimulated when glycogen levels decrease. When an individual eats a high-fat diet, such as one that normal Americans eat, the metabolism tends to burn the ingested carbohydrates, and because the ingested carbohydrate level is low (high-fat diet), it begins to use the glycogen stores for its carbohydrate needs. This scenario leads to a depleted glycogen content which causes hunger to be stimulated. The person then eats more of the high-fat diet and the cycle is repeated.

This vicious cycle is not stopped until some intervention either increases fat metabolism or causes a change in the pathological condition of the individual (e.g., hyperinsulinemia or insulin resistance). Fat metabolism is increased naturally when the adipose tissue mass is expanded by fat accumulation and provides an increased amount of free fatty acids to stimulate fat metabolism. Unfortunately, this method of increasing fat metabolism actually compounds the underlying problem, due to the further accumulation of fat in the adipose tissue. However, low-intensity exercise over long durations (e.g. LSD exercise -- long, slow distance) can provide a stimulus to increase fat metabolism which can break the cycle.

Conclusion

Recent research in nutrition arenas has led to a better understanding of the fate of various nutrients, particularly carbohydrates and fats, and how these nutrients are used and stored. Fitness enthusiasts need not be concerned about the possibility of increasing body fat by increasing the intake of carbohydrates. However, exercise clinicians need to keep in mind the reduction of fat metabolized as a result of a high-carbohydrate diet and adjust their client's exercise prescriptions and expectations accordingly.

Recommended Reading

Jequier, E. Body weight regulation in humans: The importance of nutrient balance. News in Physiological Sciences 8 (Dec.): 273-276, 1993.

Ravussin, E., Y. Schutz, K.J. Acheson, M. Dusmet, L. Bourquin, E. Jequier. Short-term, mixed-diet overfeeding in man: No evidence for "luxuskonsumption." American Journal of Physiology 249: E470-E477, 1985.

Swinburn, B., E. Ravussin. Energy balance or fat balance? American Journal of Clinical Nutrition, 57(supplement): 766S-771S, 1993.

Thomas, C.D., J.C. Peters, G.W. Reed, N.N. Abumrad, M. Sun, J.O. Hill. Nutrient balance and energy expenditure during ad libitum feeding of high-fat and high-carbohydrate diets in humans. American Journal of Clinical Nutrition, 55: 934-942, 1992.