Skeletal muscle is stimulated to grow during the periods of recovery following exercise, most usually resistance training. However, for this to happen stimulation from the diet in the form of protein is required. In particular, it is known that certain amino acids within proteins are more able to facilitate this process compared to other amino acids. Although the actual amino acid composition of a protein that optimally stimulates skeletal muscle growth is not known, studies suggest that the branched chain amino acids, particularly valine, are the most important component. This may relate to the unique metabolic pathways that these amino acids enter following ingestion, and may also relate to their ability to stimulate the release of insulin. As the branched chain amino acids are all essential amino acids, it makes sense that a protein mixture of essential amino acids may be more effective at stimulating skeletal muscle growth compared to a mixture of non-essential amino acids.
Animal sources of protein such as meat, milk and eggs are higher in the essential amino acids compared to most plant proteins. This makes them better able to stimulate protein synthesis in skeletal muscle, as this process requires high intakes of essential amino acids. The non-essential amino acids are less able to stimulate protein synthesis in skeletal muscle, making plant proteins inferior in this regard.
Comparisons of the non-essential versus essential amino acids have been made in human subjects with regard their ability to stimulate positive protein balance after resistance training. For example, in one study, researchers administered either 6 grams of essential amino acids or 3 grams of essential amino acids with 3 grams of non-essential amino acids to subjects following exercise. The essential amino acids were able to significantly increase positive protein balance when compared to the mixture of essential and non-essential amino acids, supporting the hypothesis that it is the essential amino acids that are required to stimulate skeletal muscle growth. This would also tend to suggest that some protein sources are better at stimulating muscle growth compared to others. Sources of protein with a high content of essential amino acids, such as animal proteins including meat, milk and eggs would therefore be better sources of protein compared to plant proteins for maximally stimulating skeletal muscle growth.
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