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Protein, fast and slow

Fast-digesting whey helps with post-workout muscle protein synthesis, and some studies suggest slower-digesting casein reduces muscle protein breakdown. This opens the possibility that their combination could be better than either alone

Study under review: Effects of Post-Exercise Protein Intake on Muscle Mass and Strength During Resistance Training: is There an Optimal Ratio Between Fast and Slow Proteins?


Protein comes from many different sources, including meat, peas, eggs, hemp, soy, and dairy. However, dairy proteins tend to get the most attention in the world of sport nutrition. For years, protein researchers have studied[1] and compared the effects of fast (whey) and slow (casein) digesting proteins on muscle protein synthesis, hypertrophy, and performance.

Both whey and casein are complete proteins, since both of these dairy byproducts contain sufficient amounts of all essential amino acids. They also both have[2] the same protein digestibility-corrected amino acid score (a measurement of protein quality). Whey and casein are not identical, though; fast-digesting whey protein comes from a liquid milk byproduct formed during cheese production, and casein is a solid milk byproduct. In normal milk, nearly 80% of the protein is casein and approximately 20% is whey. Whey is high in many essential and branched-chain amino acids[3] that can be beneficial for preventing the catabolic effects of training. Unlike whey, casein works slowly and releases amino acids[1] into the bloodstream for up to several hours as seen in Figure 1, thus possibly providing enhanced nitrogen retention and use by the body.

Figure 1: Leucine kinetics of whey (fast) vs casein (slow)

Reference: Boirie et al. Proc Natl Acad Sci USA. 1997 Dec.

But does the speed of protein digestion translate to real-world effects on gains from training? To answer this question, two processes need to be considered: muscle protein breakdown (MPB) and muscle protein synthesis (MPS). Resistance training both induces MPS[4] immediately after training and up to 48 hours afterwards. Protein ingestion enhances[5] this processes. Additionally, resistance training also acutely promotes[6] a state of hyperinsulinemia that prevents MPB.

Fast-digesting protein sources, which are high in essential amino acids like leucine, are thought the be the most beneficial protein sources to consume following training to acutely increase MPS. Whey is digested fast enough[1] so that amino acid levels peak as soon as 60 minutes following ingestion, and maximally increases amino acid bioavailability at around 90 minutes. Its fast digestibility, which further facilitates aminoacidemia-driven increases in MPS[7], and its high levels of MPS-activating leucine[8], have made it a popular post-workout protein supplement. However, there is also evidence to suggest that slower digesting proteins like casein may also have post-workout benefits, as casein ingestion has been shown to be sufficient to prevent MPB[9]. Thus, the complementary effects these two protein sources have on MPS and MPB suggest that their combined consumption would be optimal for enhancing resistance training adaptations, at least in theory. However, there is relatively little evidence that directly addresses this hypothesis.

In an attempt to answer this question, the study under review was designed to measure the effects of resistance training combined with post-workout consumption of different ratios of fast- and slow-digesting protein.

It is possible that whey and casein have differing effects on post-exercise muscle adaption due to their fast and slow release, respectively, of amino acids into the bloodstream. Muscle protein synthesis (MPS), which is stimulated by both amino acids and training, begins acutely post-exercise and persists for up to 48 hours. Given the complementary roles of whey in acutely increasing MPS and casein in preventing muscle protein breakdown (MPB), it is possible that a combination of slow- and fast-digesting proteins post-workout may improve training adaptations. However, until the study under review, limited research had been done to examine this question.

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