When you go shopping for protein supplements, you’ll probably encounter a lot of sales copy that discusses the “speed” of different proteins. “Fast” proteins are digested and absorbed quickly, leading to a rapid rise and a high peak in plasma amino acid levels, followed by a (relatively) quick decline in plasma amino acid levels. “Slow” proteins are digested and absorbed more slowly, leading to a more gradual rise and lower peak in plasma amino acid levels, followed by a slower decline.

All of the marketing material will tell you that “fast” proteins, such as whey, are ideal most of the time: the rapid digestion and high peak in blood amino acid levels will stimulate greater muscle protein synthesis. Conversely, the marketing material will tell you that “slow” proteins, such as casein, are ideal to consume before sleep. Since blood amino acid concentrations will stay elevated for a longer period of time, slow-digesting proteins consumed before bed will keep stimulating muscle growth all night, ensuring you never spend a single moment in a net catabolic state.

However, the research suggests that the first claim is mostly incorrect: whey does cause a higher peak in blood amino acid concentration than casein, but despite that, whey and casein are similarly effective at stimulating muscle protein synthesis (one, two, three). In other words, whey is a faster-digesting protein than casein, but that advantage in digestion speed doesn’t actually result in greater muscle protein synthesis.

I’m sure you can already tell where I’m going with this: a recent study suggests that the claimed advantages of consuming a slower-digesting before sleep are also overblown. Casein does elevate plasma amino acid levels for a longer duration than whey, but it doesn’t do a better job of stimulating muscle protein synthesis overnight.