The human body is unable to store extra protein. Protein consumed in excess of the body's needs is not used to build muscle; rather, it is used for non-protein bodily functions.
If individuals consume protein in excess of their caloric and protein needs, the extra protein will not be stored as protein. Unfortunately such extra protein is converted to and stored as fat. As a result, if individuals consume large amounts of extra protein in addition to their regular dietary intake, any weight gain would very likely be in the form of fat.
Another important point to keep in mind is that the potential for harm exists if protein is consumed in excess. Such harm is most likely to occur in the individual who consumes protein or amino acid supplements.
For example, excess protein may lead to dehydration, because protein metabolism requires extra water for utilization and excretion (i.e., elimination) of its by-products. Since exercising individuals are already at an increased risk for dehydration, the additional strain of protein waste excretion may further promote dehydration.
Excess protein has also been shown to lead to an increase in the loss of urinary calcium. A chronic calcium loss, due to excess protein intake, is of particular concern because it may increase the risk of osteoporosis, especially in women.