There still exists considerable confusion and controversy regarding the protein requirements for athletes­particularly for those who are involved in high-intensity training. Even scientists have disagreed on exact protein requirements, and thus many strength and conditioning professionals who desire muscular enhancement in their athletes have increased their protein intake well beyond the recommended daily allowance (RDA). Much of the confusion arises from poorly designed research studies and a lack of understanding of protein metabolism, resulting in recommendations that are unnecessary and occasionally harmful. Proteins are complex molecules that have many enzymatic and structural functions related to the promotion of body growth, maintenance, and repair. The function of dietary protein in the athlete, fitness enthusiast, or sedentary individual is to contribute the amino acids necessary for the body to assimilate proteins that comprise skeletal structures (including muscle) and hormones, function as cell membrane receptors, and maintain fluid balance. Protein, which carries 4 kilocalories per gram, also makes a minor contribution (~5-10%) to energy production, but this offering may increase in episodes of decreased energy intake.

The basic building blocks of protein are amino acids, which contain nitrogen. The body's nitrogen status can be determined by measuring dietary nitrogen intake (via protein intake) and subtracting nitrogen loss (sweat, urine, and feces). Measuring the body's nitrogen balance provides a valuable estimate of a person's overall protein balance because muscle protein is in a constant state of turnover. For example, impaired protein synthesis allows catabolic (breakdown) effects to predominate, resulting in an increased excretion of nitrogen. If protein is ingested in excessive amounts, it can be oxidized for energy or converted to fat. Therefore, the amount of protein in muscle is determined in large part by the balance between a positive nitrogen balance, which promotes protein anabolism (growth or build-up), and a negative nitrogen balance, which promotes catabolism.

Protein quality is an important factor in establishing its daily requirement. Of the 22 distinct amino acids required for protein synthesis, 9 are essential and 13 are nonessential. Essential amino acids are those that the body cannot synthesize and thus they must be obtained from the diet. Nonessential amino acids are synthesized as long as there is an adequate source of nitrogen in the body, but can also be acquired from the diet. Individuals must obtain a sufficient amount of essential amino acids for the proper synthesis of human tissue protein. Dietary proteins that contain all the necessary amino acids are known as complete proteins, and are generally derived from meat, fish, and dairy products. Incomplete proteins lack one or more of the essential amino acids and are the constituents of such plant foods as nuts, grains, and legumes. Consequently, this signifies the need to eat a balanced diet containing a variety of protein-rich foods. Also, it is apparent that vegetarians may become deficient in one or more essential amino acids and thus may benefit from protein supplementation.