Exercise—particularly resistance training—leads to increases in muscle mass and strength through the growth of muscle fibers. Muscle fibers are largely made of protein. To repair and build new muscle fibers, the body needs amino acids for the assembly of new protein. A large industry exists around the promotion of amino acid and protein powders marketed to athletes and body-builders, based on the notion that added protein in the diet will help to build additional muscle. This is false. Amino acids and protein powders alone do not cause muscle growth; protein intake has to be combined with exercise—particularly strength training—and sufficient calorie intake to increase muscle mass.

Most people, including athletes, get plenty of protein from food—adult men in the United States tend to consume about 99 grams of protein per day, about 75% more than the recommended daily allowance (RDA) for protein, whereas women eat around 68 grams, about 40% more than their RDA. Some strength athletes, such as competitive weight lifters, may benefit from protein intakes that are 50% to 100% over the recommended intake, but most individuals already consume this amount. (For more on the protein needs of athletes refer to Chapter 8.) Furthermore, consuming more protein than the body needs will cause an increase in urea production and increased amounts of urine as the kidneys excrete excess nitrogen, which may make it more difficult to stay well hydrated.

What about vitamins and minerals? As with protein powders, supplemental vitamins and minerals have few advantages. Athletes who eat an adequate, balanced, and varied diet are typically able to meet their vitamin and mineral needs. There is limited evidence that athletes may require slightly more riboflavin and vitamin B₆ than sedentary individuals. However, the increased energy expenditure that accompanies training generally results in an increase in food intake and an adequate intake of these vitamins when sound nutritional principles inform the athlete’s food choices. The exception to this may be in situations when athletes are restricting calories during training to reduce body fat. In this circumstance, a multivitamin supplement may be useful. There is more extensive evidence that some athletes—female endurance athletes, especially—are at risk for iron deficiencies. Iron is a component of hemoglobin, the protein that carries oxygen to muscles. Because of possible depletion of iron stores with intense training, loss of iron from menstruation, as well as inadequate intake of foods rich in iron, many female athletes may have suboptimal iron status and require supplementation. (Refer to Chapter 14 for more information.)