Milk protein consists of two primary proteins: casein and whey, with casein accounting for 80 percent and whey 20 percent. In recent years studies have shown that both proteins have varying uptake rates in the body. Whey digests rapidly, reaches peak blood uptake in about 90 minutes, and then rapidly declines. In contrast, casein curdles in the stomach, leading to an extended release of amino acids lasting for up to seven hours.
Based on those findings, scientists have suggested that whey protein is superior to casein for purposes of stimulating muscle protein synthesis. The reason is that the high essential-amino-acid content of whey, including branched-chain amino acids, spurs muscle protein synthesis, with one particular BCAA, leucine, perhaps the most important player of all in relation to muscle protein synthesis. Scientists also suggest that casein, because of its relatively slow release of essential amino acids, is better than whey for generating an extended anticatabolic effect. Since muscle growth is based on a balance between increased protein synthesis and reduced catabolism, it would appear that whey and casein provide the perfect synergistic combination for helping build muscle when combined with weight training.
One problem with studies that originally revealed the properties of casein and whey was that they measured whole-body protein synthesis, which differs from muscle protein synthesis specifically. Would the findings about casein and whey be confirmed if the focus was only on muscle protein synthesis? That was the major focus of a new study.1 Seventeen healthy young men, average age 28, were randomly chosen to participate in either of two protein trials or in a control group that didn’t take any supplemental protein. Immediately after a weight workout, the men in the protein groups drank a whey or casein drink at a dose of 0.3 gram per kilogram of body mass. The controls got a calorie-free drink. To measure amino behavior and rate of muscle protein synthesis, researchers tagged leucine and the proteins with radioactive tracers.
The primary finding was that both whey and casein stimulated about the same level of muscle protein synthesis over a one-to-six-hour period. Whey, however, did it more rapidly, a result that confirmed previous studies. Casein showed a more moderate but prolonged increase in muscle protein synthesis. Measures of downstream muscle-protein-synthesis signals revealed similar responses to the two proteins, except for one factor that responded more strongly to whey.
The type of experimental casein used made a difference. This study used calcium caseinate; another form, micellar casein, produces a more prolonged release of amino acids. One reason is that the micellar form retains certain peptides, which are short chains of amino acids, that help partition the release of amino acids over an extended time.
”Taken together,” the authors note, “it could be argued that the combination of whey and casein would be an optimal choice postexercise, as whey stimulates the protein synthesis machinery with its high digestibility and consequently high peak concentrations of insulin and amino acids, including a high proportion of leucine in its composition. Casein provides amino acids for a prolonged time, fulfilling the amino acid need for the increased protein synthesis machinery.” They also confirm that whereas whey is the protein of choice immediately after a workout, the ideal supplement would contain a 50/50 blend of casein and whey.
While leucine is considered a key amino acid in muscle protein synthesis, the authors note that adding leucine to a postworkout protein-and-carb combo doesn’t extend the effects in subjects of any age. It does, however, appear to uptick whole-body protein synthesis, which, as noted above, doesn’t directly translate to increased muscle protein synthesis. Additional leucine does increase muscle protein synthesis in older people—but only under resting, not postexercise, conditions.
All this suggests that you don’t need supplemental leucine and probably not branched-chain amino acids if you use a supplement that contains both whey and casein, preferably micellar casein. Any excess leucine will simply be oxidized in the liver and not used to synthesize muscle protein. One interesting effect observed in the new study was that the cheaper form of casein, calcium caseinate, brings on an insulin release, while micellar casein does not. That’s moot if you use a whey-and-casein combo because whey produces a pronounced insulin release. In the presence of a high blood count of amino acids, insulin leads to muscle protein synthesis, but otherwise it’s more involved in keeping muscle from breaking down. The study reported here measured muscle protein synthesis only, not anticatabolic activity, but found that after about six hours muscle protein synthesis was similar for both whey and casein, with whey being better than casein in that respect.
Speaking of leucine, a few more newly published studies have found some interesting effects related to it and other amino acids. One study compared plasma leucine in men and women following sprinting.2 In some respects sprinting is similar to high-intensity weight training because both involve high-intensity activity followed by brief rests. Men produce more ammonia than women after either type of exercise. One reason is that ammonia is produced as a waste product of the metabolism of adenosine triphosphate, the immediate energy source for muscle contraction, and men have more muscle and ATP than women. They also have more type 2 muscle fibers, making for larger muscles, and ammonia is produced mainly in type 2 muscle fibers.
Women produce less ammonia than men during exercise for another reason. It turns out that after high-intensity exercise, excess ammonia is taken up in fat tissue, where it is buffered and converted into glutamine. Since women usually have more fat tissue than men, more ammonia buffering goes on in their bodies after exercise. Meanwhile, men experience twice the decrease of plasma leucine than women after exercise.
The significance is that the drop in blood or plasma leucine directly correlates to blood ammonia, and because women produce less ammonia, their plasma leucine drops less after exercise. Estrogen comes into the picture as well; when men are given estrogen, they show decreased leucine oxidation after exercise. The higher leucine counts in women after sprinting has implications for muscle growth, since only women show increased mass after sprinting exercise alone. That’s based on the higher leucine retention in women than men after that activity.
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1 Reitelseder, S., et al. (2011). Whey and casein labeled with L[1-13C] leucine and muscle protein synthesis: Effect of resistance exercise and protein ingestion. Am J Physiol Endocrinol Metabol. 300(1):E231-42.
2 Esbjornsson, M., et al. (2010). Reduction in plasma leucine after sprint exercise is greater in males than in females. Scand J Med Sci Sports. In press.