Nutrition and Athletic Performance

Macronutrient Requirements for Exercise

Athletes do not need a diet substantially different from that recommended in the Dietary Guidelines for Americans^[16] and Eating Well with Canada's Food Guide.^[28] Although high-carbohydrate diets (more than 60% of energy intake) have been advocated in the past, caution is recommended in using specific proportions as a basis for meal plans for athletes. For example, when energy intake is 4000-5000 kcal·d⁻¹, even a diet containing 50% of the energy from carbohydrate will provide 500-600 g of carbohydrate (or approximately 7-8 g·kg⁻¹ (3.2-3.6 g·lb⁻¹) for a 70-kg (154 lb) athlete), an amount sufficient to maintain muscle glycogen stores from day to day.^[29] Similarly, if protein intake for this plan was 10% of energy intake, absolute protein intake (100-125 g·d⁻¹) could exceed the recommended protein intake for athletes (1.2-1.7 g·kg⁻¹·d⁻¹ or 84-119 g in a 70-kg athlete). Conversely, when energy intake is less than 2000 kcal·d⁻¹, a diet providing 60% of the energy from carbohydrate may not be sufficient to maintain optimal carbohydrate stores (4-5 g·kg⁻¹ or 1.8-2.3 g·lb⁻¹) in a 60-kg (132 lb) athlete.

Protein. Protein metabolism during and after exercise is affected by sex, age, intensity, duration, and type of exercise, energy intake, and carbohydrate availability. More detailed reviews of these factors and their relationship to protein metabolism and needs of active individuals can be found elsewhere.^[30,31] The current recommended dietary allowance (RDA) is 0.8 g·kg⁻¹ body weight and the acceptable macronutrient distribution range (AMDR) for protein intake for adults older than 18 yr is 10%-35% of total calories.^[15] Because there is not a strong body of evidence documenting that additional dietary protein is needed by healthy adults who undertake endurance or resistance exercise, the current DRI for protein and amino acids does not specifically recognize the unique needs of routinely active individuals and competitive athletes. However, recommending protein intakes in excess of the RDA to maintain optimum physical performance is commonly done in practice.

Endurance Athletes. An increase in protein oxidation during endurance exercise, coupled with nitrogen balance studies, provides the basis for recommending increased protein intakes for recovery from intense endurance training.^[32] Nitrogen balance studies suggest that dietary protein intake necessary to support nitrogen balance in endurance athletes ranges from 1.2 to 1.4 g·kg⁻¹·d⁻¹.^[29,30,31] These recommendations remain unchanged, although recent studies have shown that protein turnover may become more efficient in response to endurance exercise training.^[29,32] Ultra-endurance athletes who engage in continuous activity for several hours or consecutive days of intermittent exercise should also consume protein at or slightly above 1.2-1.4 g·kg⁻¹·d⁻¹.^[32] Energy balance, or the consumption of adequate calories, particularly carbohydrates, to meet those expended, is important to protein metabolism so that amino acids are spared for protein synthesis and not oxidized to assist in meeting energy needs.^[33,34] In addition, discussion continues as to whether sex differences in protein-related metabolic responses to exercise exist.^[35,36]

Strength Athletes. Resistance exercise may necessitate protein intake in excess of the RDA, as well as that needed for endurance exercise, because additional protein, essential amino acids in particular, is needed along with sufficient energy to support muscle growth.^[30,31] This is particularly true in the early phase of strength training when the most significant gains in muscle size occurs. The amount of protein needed to maintain muscle mass may be lower for individuals who routinely resistance train because of more efficient protein use.^[30,31] Recommended protein intakes for strength-trained athletes range from approximately 1.2 to 1.7 g·kg⁻¹·d⁻¹.^[30,32]

Protein and Amino Acid Supplements. High-protein diets have been popular throughout history. Although earlier investigations in this area involved supplementation with individual amino acids,^[37,38] more recent work has shown that intact high-quality proteins such as whey, casein, or soy are effectively used for the maintenance, repair, and synthesis of skeletal muscle proteins in response to training.^[39] Protein or amino acids consumed near strength and endurance exercise can enhance maintenance of, and net gains in, skeletal muscle.^[39,40] Because protein or amino acid supplementation has not been shown to positively impact athletic performance,^[41,42] recommendations regarding protein supplementation are conservative and directed primarily at optimizing the training response to and the recovery period after exercise. From a practical perspective, it is important to conduct a thorough nutrition assessment specific to the athlete's goals before recommending protein powders and amino acid supplements to athletes.

Fat. Fat is a necessary component of a normal diet, providing energy and essential elements of cell membranes and associated nutrients such as vitamins A, D, and E. The acceptable macronutrient distribution range (AMDR) for fat is 20%—35% of energy intake.^[17] The Dietary Guidelines for Americans^[16] and Eating Well with Canada's Food Guide^[28] make recommendations that the proportion of energy from fatty acids be 10% saturated, 10% polyunsaturated, 10% monounsaturated, and include sources of essential fatty acids. Athletes should follow these general recommendations. Careful evaluation of studies suggesting a positive effect of consuming diets for which fat provides ≥70% of energy intake on athletic performance^[43,44] does not support this concept.^[45]

Cite this: Nutrition and Athletic Performance - Medscape - Mar 01, 2009.