Role of Protein in Human Body

The RDA for protein in adults is 0.8 g/kg body weight per day 76 A margin of safety is also included in this recommendation to account for individual differences in protein metabolism, variations in the biological value of protein, and nitrogen loss via urine and feces. Intake beyond this amount is therefore unnecessary. However, as the demands of training increase, so do caloric needs and protein requirements. Furthermore, athletes and those who seek weight and muscular gains also demand increased energy and corresponding protein intake.

During exercise, protein synthesis is depressed and protein degradation may be increased. Because this may result in catabolism, training may increase the need for dietary protein. In fact, research has discovered that both intensive aerobic and anaerobic training may lead to a negative nitrogen balance, which could produce losses in lean body mass. This could also increase the chances of injury.

An additional factor influencing protein and nitrogen balance is pre-exercise muscle glycogen concentrations. If initial concentrations are low, the use of protein for energy takes on added importance. Specifically, during aerobic activity, 10% or more of total energy demands may be derived from protein in the face of inadequate muscle glycogen stores Therefore, the potential for lean tissue loss is increased in the case of over exercise and/or inadequate diet. Consequently, because carbohydrates typically represent 60% of an athlete's total caloric intake, protein (PRO) should make up approximately 15% of the daily total.

Studies have examined the protein needs of athletes undergoing periods of increased volume or intensity of training. Perhaps surprisingly in one study, protein intakes as high as 2 g PRO/kg body weight were not sufficient enough to maintain a positive nitrogen balance. Therefore, inadequate protein intake during heavy training could exacerbate the overstrained state. As a result, in the case of especially intense training, the percentage of protein making up total calories can be increased to 20% or more. This increase would also be suitable for athletes in a hypocaloric state or those involved in sports where weight loss is necessary (i.e., making weight in wrestling), thus offsetting the negative nitrogen balance and loss of lean tissue. This was confirmed in a study by Walberg et al.,90 who concluded that, during energy restriction, experienced weightlifters consuming a moderate-protein intake (0.8 g/kg) possessed a negative nitrogen balance, whereas those who consumed 1.6 g/kg maintained a positive balance.