Protein can have very different physiological effects depending on its source, and these differences relate to the varying amino acid profiles as well as the speed of digestion. For example, casein and whey protein are two milk proteins with very different physiological effects when administered following exercise. However, comparisons of the absorption rate effects of these proteins is confounded by the fact that they contain very different amino acid profiles. Whey protein is known to stimulate insulin release, and can also induce an anabolic muscle protein synthesis effect because of its high concentration of leucine. However, it is likely that the benefits of whey protein come not just from its unique amino acid profile, but from its rapid digestion and absorption that causes a pronounced aminoacidaemia. The effects of this aminoacidaemia have been studied by infusing whey protein at various rates following resistance exercise.
For example, researchers1 have administered whey protein as a single 25g infusion or as a repeated infusions of 2.5g 10 times in 20 minutes, in a randomised crossover trial involving 8 healthy male subjects. The repeated infusion was to mimic the physiological process of consuming a more slowly digested protein. The muscle protein synthesis rate and the phosphorylation of signalling proteins was then assessed both at rest and after resistance exercise on a leg press machine. Following resistance exercise, the single dose of whey protein increased blood essential amino acid (EAA) concentration by 162%, whereas the multiple dose only increased EAA concentration by 53%. However, the multiple dose raised EAA concentrations in a more sustained manner which remained elevated above the single dose 180 to 220 min after exercise. Plasma aminoacidaemia can therefore be effected by the speed of administration of whey protein following resistance exercise.
Interestingly, the total net area under the curve for EAAs was identical between the two infusion techniques, but muscle protein synthesis was elevated more after the single dose that compared to the multiple dose. This occurred both earlier (1 to 2 hours post infusion) when muscle protein synthesis was raised by 95 versus 42%, and later (2 to 5 hours post infusion) when muscle protein synthesis was raised 193 versus 121%. The authors also reported a significantly greater phosphorylation in the Akt-mammalian target of rampicin pathway (mTOR) following the single infusion, compared to the repeated infusion. These results highlight that the increase in aminoacidaemia is more important than the area under the curve as the defining factor in stimulation of muscle protein synthesis. It is likely that this effect is related to the peak leucine concentrations, and may have also involved stimulation of insulin.
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