by Jerry Brainum Iron Man Magazine
You’ve no doubt heard about the different absorption times of the two primary milk proteins, whey and casein. Study after study has shown that whey is absorbed rapidly, while casein is absorbed at a much more gradual rate.
Specifically, whey proteins peak in the blood at about the 90-minute mark after you take them, while it takes up to seven hours for casein’s amino acid content to be fully released into the blood. Basically, casein curdles in the stomach and is digested more gradually than whey. In addition, casein has protein bioactive elements that work to slow its digestion. Most of the prior studies that examined the differences between whey and casein in relation to muscle protein synthesis have attributed them to their varying amino acid contents.
A new study sought to eliminate the effects of the different amino acid contents of whey and casein by manipulating the intake of whey protein alone.1 Specifically, the whey was supplied as either one large dose—a bolus—or as 10 small doses over a longer time period, which mimics the effects of casein. The total dose of protein was the same for both, 25 grams.
The subjects were eight young men. For the time-released version they got 10, 2.5-gram protein drinks taken every 20 minutes. The authors measured rates of muscle protein synthesis as well as the effects on signaling factors involved in protein synthesis. Both styles of protein intake were used following a resistance-exercise workout.
As you would expect, one large dose of whey protein increased blood amino acids 162 percent compared to the 53 percent rise from the timed-release intake within an hour after exercise. The timed-released intake, called “pulse” by the authors, produced a smaller but more sustained bump in amino acids that remained elevated above the bolus intake 180 to 220 minutes after exercise. That is precisely what happens with casein protein.
Despite the identical amounts of protein, the bolus intake led to a 95 percent greater rise in protein synthesis at one to three hours after exercise compared to the 42 percent rise produced by the pulse intake. At the three-to-five-hour mark after exercise the bolus was still producing a 193 percent increase in protein synthesis compared to the 121 percent rise from the casein-type pulse intake. The muscle-signaling factors that promote protein synthesis were also more affected by the bolus than the pulse.
The key factors that encourage increased muscle protein synthesis after training are the essential amino acids, particularly leucine, which is the primary amino involved in turning on those signaling factors mentioned above. This study showed that the highest rate of muscle protein synthesis occurred three to five hours after exercise, when the amino acids in the blood had dropped to lower levels. Despite that, the larger dose of whey protein still produced greater rates of protein synthesis at that time than the smaller doses taken at regular intervals. This study further underscores the importance of taking in the bulk of essential amino acids right after a workout to promote the most muscle protein synthesis. The authors note that getting casein alone right after the workout would likely delay muscle protein synthesis responses, as would taking in fat or carbs, which would slow stomach emptying and thus delay the delivery of required essential amino acids.
Furthermore, they say, “We speculate that, over time, the habitual practice of consuming rapidly digested proteins after resistance exercise would provide an anabolic advantage that leads to greater hypertrophy.” They also suggest that taking in a larger single dose of rapidly absorbed protein like whey would help overcome the anabolic amino acid resistance common in older people
1 West, D., et al. (2011). Rapid aminoacidemia enhances myofibrillar protein synthesis and anabolic intramuscular signaling responses after resistance exercise. Am J Clin Nutr. 94(3):795-803.