Does protein supplementation prevent muscle disuse atrophy and loss of strength?

Recently there has been much interest in the use of dietary amino acids supplements to decrease the losses in muscle mass and strength observed after space flight or during aging using bed rest analogs. This interest persists even if the results have been mixed. Of the six published amino acid supplementation studies, three showed benefit, three did not. A recent study re-evaluating protein requirements in humans suggests that the official RDA is 41% underestimated. Interestingly, the three studies that showed benefits fed their test subjects a baseline protein level around the old RDA for protein. The three that did not show benefit gave the subjects a baseline protein intake higher than the new RDA. We suggest that the positive effects observed on protein metabolism might just reflect the benefits of adequate protein intake achieved during bed rest rather than a protective effect against bed-rest induced disuse. In conclusion, the efficiency of amino acid countermeasures for preventing the loss in protein mass during space flight or bed rest needs to be seriously questioned. These results extend to clinical situations such as serious illness and progress of aging in which physical inactivity is a significant component of the loss in muscle function.