Fit people have more muscle than fat; exercise involves muscles; muscles are made largely of protein. It seems logical, then, that to become or stay fit, an athlete might need more protein. Although it’s true that fat and glucose are the primary fuels for working muscles, 5 to 10 percent of energy needs for weightlifters and endurance-sport athletes comes from muscle protein. So, do athletes need more protein?
The athlete’s body may use slightly more protein, especially during the early stages of training. Initial increases in muscle mass, numbers of red blood cells to carry oxygen, and amounts of aerobic enzymes in muscles to use fuel efficiently may elevate an athlete’s protein needs. In addition, hormonal changes during exercise can temporarily slow the amount of protein the muscle makes and can encourage the muscle to break down its protein stores.14 How much protein an athlete uses for fuel during hard exercise (endurance exercise and heavy weightlifting) depends on exercise intensity and duration, the athlete’s fitness level, and the glycogen stores in the athlete’s muscles. When glycogen stores are well stocked, however, protein contributes only 5 percent of fuel needs.
The important factor here is that, although muscle protein breakdown dominates during heavy exercise, muscle growth escalates after exercise. The muscles use the available amino acids to repair and build, and the net effect of these changes is muscle protein buildup. Consistent training enhances muscle protein buildup after exercise.
The American Dietetic Association recommends that endurance athletes consume 1.2 to 1.6 grams of protein per kilogram of desirable body weight.15 Athletes who are involved in prolonged heavy resistance training may need even more protein (see Table 10-3).
Many people wonder if eating even more protein will help build muscles. Unfortunately, muscles don’t respond to excess protein by simply accepting it. Instead, they respond to the hormones that regulate them and to the demands put upon them. Thus, the way to make muscle cells grow is to put a demand on them—that is, to make them work. They will respond by taking up nutrients—amino acids included—so that they can grow.