Muscle protein synthesis peaks 1-2 hours after exercise depending on the protein source

Milk proteins (whey and casein) and soy protein are both considered high quality protein sources that can be used to support recovery from exercise. However, a growing body of evidence indicates that each protein source has unique characteristics. The techniques to study muscle protein synthesis accurately are sophisticated which limits human experiments to relatively small studies. 

A recent study from Japan used rats, which allowed them to assess muscle protein synthesis more directly and make more comparisons. They had rats perform a swimming exercise for 2 hours followed by ingestion of either whey protein concentrate, caseinate (casein), milk protein concentrate (which is a combination of whey and casein in a 1:4 ratio) or soy protein concentrate.


All the proteins increased the rate of muscle protein synthesis but the timing of when they peaked differed. Whey caused the most rapid increase in muscle protein synthesis peaking 60 minutes after exercise, compared to 90 minutes for milk protein, 90 minutes for soy protein and 120 minutes for casein. Each of the three milk-based proteins all increased muscle protein synthesis to greater magnitude than soy protein.

What does this tell us about the ideal way to use different protein sources?

These findings confirm the concept in prior human studies that whey is a fast-acting protein, milk protein is intermediate and casein is a slow protein. Soy might be considered intermediate as well, but soy protein’s magnitude of impact on protein synthesis was lower than each of the three milk proteins. 

Because of the rapid digestion/absorption and quick spike in muscle protein synthesis, whey is ideal for use before and after exercise to maximize the anabolic response to exercise. Milk protein can be effectively used as well to prolong the benefits of muscle protein synthesis. Using casein alone may be useful if you don’t plan on consuming a protein meal for several hours after exercise, but consider adding leucine to the casein to drive up protein synthesis further as casein has slightly lower naturally-occurring levels of this muscle-building amino acid compared to whey.

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