Journal Title (Medline/Pubmed accepted abbreviation): Am. J. Physiol Endocrinol. Metab
Article that they are referencing:
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.
Summary of letter:
In the above article, the authors concluded that blood amino acid levels are similar after ingestion of whey protein and casein. Phillips is concerned that the authors used caseinate, an acid form of casein, and not casein itself and therefore should be careful how they report their data and interpret their results. Casein, a milk protein, is present in micellar form in milk, meaning that it is present as a lipid-soluble globule within a spherical shape that makes it somewhat more water-soluble. When this structure enters the acidic environment of the stomach, it forms large, thick globules that move slowly through the stomach, thus experiencing slower rates of digestion. The amino acids are thus taken up into the blood more slowly. The acidic caseinate, however, is much more soluble in the stomach and may exit the stomach at a rate more comparable to that of whey. Because casein and caseinate have very different solubility properties, results generated from caseinate should not be translated to micellar casein.