Carbohydrates and Other Macronutrients: How Much?

by ADA

What fuel we burn and when...

Nutrition and Performance: Nutrition For Active Adults : Energy Needs : Body Composition : Vitamins, Minerals : References

MACRONUTRIENT REQUIREMENTS FOR EXERCISE

The fuel burned during exercise depends on the intensity and duration of the exercise performed, the sex of the athlete, and prior nutritional status. All other conditions being equal, an increase in the intensity of an exercise will increase the contribution of carbohydrate to the energy pool (43,44). As the length of the exercise continues, the source of this carbohydrate may shift from the muscle glycogen pool to circulating blood glucose, but in all circumstances, if blood glucose cannot be maintained, the intensity of the exercise performed will decrease (45). Fat contributes to the energy pool over a wide range of exercise intensities, being metabolized at somewhat the same absolute rate throughout the range; however, the proportion of energy contributed by fat decreases as exercise intensity increases because the contribution of carbohydrate increases (46). Protein contributes to the energy pool at rest and during exercise, but in fed individuals it probably provides less than 5% of the energy expended (47,48). As the duration of exercise increases, protein may contribute to the maintenance of blood glucose through gluconeogenesis in the liver. In experiments in which subjects are tested in a fasting state, the contribution of fat to the energy pool will be greater than in people who are tested postprandially when exercise performed is moderate (approximately 50% of maximal oxygen uptake [VO2max]) (49). With exercise of higher intensity (greater than 65% of VO2 max), neither prior feeding nor training markedly affects the fuel used (49).

Data are not presently available, however, to suggest that athletes need a diet substantially different from that recommended in the Dietary Guidelines for Americans(50) or the Nutrition Recommendations for Canadians (51) (55% to 58% of energy from carbohydrate, 12% to 15% of energy from protein and 25% to 30% of energy from fat). Although high-carbohydrate diets (more than 60% of energy intake) have been advocated in the past, the use of proportions in making dietary recommendations may actually be misleading in terms of providing optimum nutrition. When energy intake is 4,000 to 5,000 kcal per day, even a diet containing 50% of the energy from carbohydrate will provide 500 to 600 g of carbohydrate (or approximately 7 to 8 g/kg for a 70 kg athlete), which is sufficient to maintain muscle glycogen stores from day to day (52,53). Similarly, if protein intake in such a diet was even as low as 10% of energy intake, absolute protein intake (100 to 125 g per day) would exceed the recommendations for protein intake for athletes (1.2 to 1.7 g per day or 84 to 119 g in a 70 kg athlete, see the following discussion on nitrogen balance in men). Conversely, when energy intake is less than 2,000 kcal per day, even a diet providing 60% of the energy from carbohydrate may not provide sufficient carbohydrate to maintain optimal carbohydrate stores (4 to 5 g/kg in a 60 kg athlete). Typically, diets containing 20-25% energy from fat have been recommended to facilitate adequate carbohydrate intake and to assist in weight management where necessary. Thus, specific recommendations for individual energy components may be more useful when they are based on body size, weight and body composition goals, the sport being performed, and sex of the athlete.

Protein needs of athletes have received considerable investigation, not only in regard to whether athletesí protein requirements are increased, but also in relation to whether individual amino acids are a benefit to performance. Mechanisms suggested to increase athletesí protein requirements include the need to repair exercise-induced microdamage to muscle fibers, use of small amounts of protein as an energy source for exercise, and the need for additional protein to support gains in lean tissue mass (54,55). If protein needs are increased, the magnitude of the increase may depend on the type of exercise performed (endurance vs resistance), the intensity and duration of the activity, and possibly the sex of the participants.

For endurance athletes, nitrogen balance studies in men suggest a protein recommendation of 1.2 g/kg per day (56). Little information is available regarding requirements of endurance athletes who are women. Resistance exercise is thought to increase protein requirements even more than endurance exercise, and it has been recommended that experienced male bodybuilders and strength athletes consume 1.6 to 1.7 g/kg body weight per day to allow for the accumulation and maintenance of lean tissue (55,57). Again, data on female strength athletes are not available. Athletes should be aware that increasing protein intake beyond the recommended level is unlikely to result in additional increases in lean tissue because there is a limit to the rate at which protein tissue can be accrued (54), whereas other sources have suggested an intake of 1.2 to 1.4 g/kg per day (55). It must be ensured that energy intake is adequateóotherwise, protein will be used as an energy source, falsely elevating estimates of the requirements under conditions of energy balance. It is worth noting that the customary diets of most athletes provide sufficient protein to cover even the increased amounts that may be needed (7).

© ADAF 1999. Reproduction of this fact sheet is permitted for educational purposes.

 
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