This belief often depends on the idea that muscles are made of protein, so more protein (in the form of supplements) you take and the more you increase your muscles. False!!! The 70% of the muscle is water, the amount of protein required to replace those that are lost with physical activity is small and especially is equipped from food! The excess protein is eliminated from the body.
Furthermore the intake of high amounts of protein can cause metabolic imbalances and problems of absorption; increases the loss of calcium, and this effect is particularly relevant for women.
Hypothesis that the intake of amino acids (especially branched) enhance performance is not demonstrated by any clinical study; some studies have, in contrast, showed that intake of amino acids will not counteract the muscle fatigue and will not improve exercise performance. They may also have adverse reactions such as high doses of arginine resulted in headache, low blood pressure and nephrotoxicity.
Remember that individual amino acids as supplements do not build your muscles!!! In fact are needed all amino acids in the right proportions!