Journal Title (Medline/Pubmed accepted abbreviation): Am. J.Physiol Endocrinol. Metab.
Primary article they are referring to:
Reitelseder S, Agergaard J, Doessing S, Helmark IC, Lund P, Kristensen NB, Frystyk JF, Flyvbjerg A, Schjerling P, van Hall G, Kjaer M, & Holm L. Whey and casein labeled with L-[1-13C]leucine and muscle protein synthesis: effect of resistance exercise and protein ingestion. Am J Physiol Endocrinol Metab 300: E231–E242, 2011.
Reference to Dr. Phillips’ letter:
Phillips, S. M. (2011). A comparison of whey to caseinate. American J Physiol Endrocrinol Metab, 300(3), E610; author reply E611-2. doi: 10.1152/ajpendo.00647.2010.
Summary of letter:
The authors acknowledge that micellarized casein and acidic caseinate have different physiological properties, and agree that their results cannot be translated to casein. However, they took exception with Phillips’ claim that the caseinate and whey showed nearly identical postprandial amino acid increases. The argued that their results did show amino acid concentrations in the blood following ingestion of caseinate that were significantly different than following ingestion of whey (Figures 4E and 4F in the paper). In the end, despite the differences in protein digestion rates they say they did, in fact, observe in their study, Reitelseder et al. still support their general conclusion that the both protein sources supported roughly equal rates of protein synthesis over the course of their study. Thus, they maintain that differences in rate of protein digestion are a relatively unimportant factor in determining the rate of protein synthesis, especially in a study in which protein synthesis is measured over a longer postprandial time frame. Future studies ought to compare exactly how micellarized casein, casein from milk, and caseinate are absorbed at rest and after exercise.
Based on their studies, the authors propose a dose of 20 g of high quality protein (20 g of egg protein + 10 g of essential amino acids (corresponding to 20 g of quality protein) to promote maximal muscle protein synthesis.