It is important to understand what role excess nutrients, in particular fat, play in the overall physiological process leading to obesity.

Many people who are health-conscious try to eat a high-carbohydrate, low-fat diet. One reason this type of diet has been recommended is that with less fat intake, there is less fat stored. Research from the past 15 years has convincingly shown that the only nutrient that is primarily stored as fat, is fat. However, with an increase in carbohydrate and protein intake, one must consider what happens to excess nutrients that are not used.Whereas it appears that neither excess carbohydrates nor proteins are stored as fat, an important question is “What happens to the excess carbohydrates or proteins?” It is important for the exercise clinician to understand what role excess nutrients, in particular fat, play in the overall physiological process leading to obesity.

Excess nutrients and change of diet

The fate of excess carbohydrates and proteins has been the focus of several recent investigations. E. Ravussin and colleagues, in one of the most cited studies, gave five young, healthy men a diet that had approximately 60 percent more calories than the subjects normally ate to maintain their weight. The increased food intake was of mixed composition. This overfeeding protocol continued for 9 days after a 13-day baseline period. Figure 1 shows that in these subjects, usage of carbohydrates gradually shifted to where they were using almost all of the carbohydrates they were consuming. As a consequence of this shift toward an increased use of carbohydrates, the use of fats as fuel decreased. Therefore, in these healthy young men, overfeeding them 60 percent of their normal caloric intake resulted in more carbohydrates being burned and more fats being stored.

While it appears that overfeeding will increase the amount of carbohydrates burned, what happens if an individual changes from a high-fat diet to a low-fat, high-carbohydrate diet? Cecilia Thomas and colleagues studied both lean and obese men and women to find whether a change in diet, while maintaining normal calorie levels, causes a change in human metabolism. When these researchers gave their subjects a diet consisting of 13 percent protein, 35 percent carbohydrates, and 62 percent fat (average kcal/day = 2,696), they found that protein use went up.

Carbohydrate use was not increased, primarily because of the lower carbohydrate level, and fat use was also not increased overall. However, when the subjects were given a high-carbohydrate diet (13 percent protein, 62 percent carbohydrates, 26 percent fat), the amount of carbohydrates used as fuel increased, and the amount of fat used went down. Further, these researchers found that obese subjects, even when given a high-fat diet, didn’t necessarily increase their usage of fat. The implications of this work are that when a high-carbohydrate diet is consumed, your body shifts toward more carbohydrate and less fat use.

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.


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.

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