Casein and soy protein, on the other hand, have a different amino acid profile. They have a more moderate digestion rate and may have other antioxidant or anti-inflammatory effects. Therefore, it might be of value to combine both these high quality protein sources with whey, or at least that is what researchers hypothesized.
Researchers had young healthy participants perform a single bout of resistance exercise and then 1 hour later consume either whey protein alone or a blend of whey, casein, and soy. The supplements both contained 19 grams of protein. The blend consisted of 50% casein, 25% whey, and 25% soy.
Both supplements equally increased muscle protein synthesis during the first 2 hour post-exercise time period. During the 2 to 4 hour post-exercise time period, only the protein blend increased protein synthesis. Thus, the effects of whey protein alone were not sustained over a 4-hour time period after exercise.
Does this mean you should change your protein supplement? Results of this study are consistent with the large amount of research supporting the superiority of whey consumed right after exercise. However, it highlights the limitation of using whey as the only protein source when examined over a longer time frame. Based on these new findings, it would be prudent to either combine whey with a slower digesting protein source like casein or soy, or consume a meal containing protein during the first couple hours after exercise. The protein in milk is about 80% casein and 20% whey, so mixing whey with milk may also be a good option to help extend the muscle-building window.