Protein ingestion can strongly increase muscle protein synthesis rates in vivo in humans. The increase in muscle protein synthesis rate is attributed to the post-prandial increase in plasma essential amino acid concentrations, with the post-prandial increase in plasma leucine concentration being of particular interest. Different proteins and protein sources can have distinct protein digestion and amino acid absorption kinetics and differ in their amino acid composition. Consequently, proteins can differ in their ability to stimulate post-prandial muscle protein synthesis rates. We conducted a pair of studies to determine how the ingestion of different proteins impact post-exercise muscle protein synthesis rates in vivo in humans.

Milk protein is a blend of whey (20%) and casein (80%). Whey and casein both contain relative large amounts of leucine, but whey is digested more rapidly when compared with (micellar) casein. In the first study, we investigated whether the ingestion of 20 g milk, whey, or micellar casein protein with carbohydate differ in their capacity to increase muscle protein synthesis rates during recovery from concurrent exercise. We found that protein plus carbohydrate ingestion increased muscle protein synthesis rates in comparison to the ingestion of carbohydrate only. However, the ingestion of 20 g milk protein, whey protein, or micellar casein protein did not differ in their capacity to increase post-exercise muscle protein synthesis rates in healthy, active males.

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Soy protein is rapidly digested, much like whey protein, but contains less leucine. In the second study, we investigated whether whey protein, soy protein, or soy protein plus added leucine (to match the leucine content of whey protein) differ in their capacity to increase post-exercise muscle protein synthesis rates. We observed that the ingestion of 20 g whey protein, soy protein, or soy protein plus added leucine with carbohydrate did not differ in their capacity to stimulate post-exercise muscle protein synthesis rates in healthy, active males.

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From these studies, we can conclude that the ingestion of high quality protein sources such as milk protein, whey protein, micellar casein protein, soy protein, and soy plus added leucine do not differ in their capacity to increase muscle protein synthesis rates when an ample amount (20 g) of protein is ingested with carbohydrate following cessation of exercise. Exercise makes the muscle so sensitive to the anabolic properties of amino acids that the protein source may be of lesser importance when ample (20 g) protein is ingested with carbohydrate during recovery from exercise. More research is warranted to assess this for protein sources of a lesser quality (i.e. many other plant based proteins).

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