Because resistance training stimulates an increase in protein turnover and results in muscle damage, it is widely recommended that individuals who resistance train consume protein at intakes above those recommended for healthy, nonexercising individuals. Although determining precise requirements for protein is difficult, analysis of several nitrogen balance studies indicate that a protein intake of approximately 1.3 g/kg protein/day is sufficient for strength-trained individuals to achieve nitrogen balance when energy balance exists (8). For individuals just beginning a strength training program, protein needs may be higher initially, given the heightened effect of resistance exercise on increasing skeletal muscle protein turnover. However, given that typical protein intakes for those who strength train is usually in excess of current recommendations, consuming adequate protein is likely not a concern unless they are restricting calorie intake to lose weight.
Much research has been done on the timing, composition, and quantity of amino acids needed in relation to resistance exercise to optimize muscle protein synthesis. Studies have found no difference in protein synthesis when amino acids are consumed before versus 1 hour after exercise, and no difference when amino acids are consumed at 1 hour versus 3 hours after exercise, providing individuals with some flexibility with timing of protein intake (9). Interestingly, when amino acids are consumed at 1 hour and again at 3 hours after exercise (10), there is an increase in synthesis after consumption of amino acids at both time points, suggesting an added benefit to two small feedings after exercise. The amount of amino acids needed to elicit this anabolic effect is surprisingly small, with as little as 6 g of essential amino acids (in combination with 35 g of sucrose) eliciting an increase in synthesis and resulting in a positive protein balance in muscle (9). Quality, and not quantity, of protein is an important concept, with studies showing that nonessential amino acids are not necessary to increase protein synthesis (11); rather, provision of adequate amounts of essential amino acids is important to maximize protein utilization in response to routine resistance exercise.
The impact that endurance exercise has on protein turnover is far less studied than resistance exercise. Protein has typically not been considered to play a major role in the diets of endurance athletes because carbohydrates and fat are the primary fuel sources during endurance exercise and because muscle hypertrophy is not typically observed with endurance training. However, protein metabolism is affected by endurance exercise, especially when either energy intake or carbohydrate intake is inadequate (12, 13). During exercise, it is generally thought that protein synthesis is decreased and breakdown is either increased or not different from rest. After exercise, rates of protein synthesis are restored.
At rest, the use of amino acids for energy is very low. During aerobic exercise, however, amino acid oxidation for energy to support muscular work increases, especially with increasing intensity, duration, and low glycogen stores. Whether this increases the protein requirements for individuals who routinely participate in endurance exercise has been debated for several years. It is likely that significant increases in protein needs (up to 1.6 g/[kg · day]) apply only to "elite" endurance athletes exercising at high intensities for several hours a day (14). For people who engage in routine endurance exercise and have adequate energy intake, the recommendations for protein intake (1.0 g/[kg · day]) are not much greater than for nonexercisers (0.8 g/[kg · day]) (14). Although evidence exists that suggests people beginning an endurance exercise program may have increased protein needs during the initial few weeks (15), consumption of calories sufficient to maintain energy balance during this time will improve protein utilization in response to aerobic training (12, 16).
Habitual reduction in energy intake for weight loss has been shown to have a negative impact on protein utilization because of the simultaneous slowing down of metabolic processes (17). Typically, weight loss is associated with a reduction in both body fat and lean body (i.e., muscle) mass, although the intent of diet interventions remains the loss of fat and maintenance of muscle mass. Recent reports by Donald K. Layman, Ph.D., et al (18, 19) have demonstrated that consumption of reduced calorie diets for which protein provides a greater percentage of calorie intake than conventional weight loss diet plans (i.e., 30% vs. ~15%, respectively) is associated with better maintenance of muscle mass during weight loss. When exercise is added to a weight loss program that uses a higher-protein diet intervention, reductions in body fat and sparing of muscle mass is even greater (19). These studies encourage integration of exercise programs with nutrition plans that provide a higher amount of dietary energy in the form of protein for weight management.
Protein is essential to the diets of healthy, physically active men and women. The essential amino acids provided by high-quality proteins are important to maximize rates of protein synthesis and to optimize protein utilization in response to habitual participation in resistance training and endurance exercise. The role of energy balance should not be overlooked. Clearly, anabolic processes (i.e., tissue building) require energy. Therefore, attention should be given to sufficient consumption of calories for either weight maintenance or increases in muscle mass if amino acids are to be directed to synthetic (i.e., anabolic) rather than catabolic processes. When negative energy balance is intended for the purpose of weight loss, a small amount of additional protein in the diet is likely beneficial to maintenance of lean body mass.
Dietary protein remains a critical component of the diet for healthy, fit men and women. Attention should be given to adequate energy intake for optimal use of dietary protein by the body and by the muscle of active individuals.
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