Muscle protein synthesis is dependent upon a supply of the required amino acids, and existing research demonstrates the ability of protein ingestion to promote muscle protein synthesis. Protein digestion and the kinetics of amino acid absorption dictate both the magnitude and duration of the increase in protein synthesis. Milk proteins (whey and casein) differ in their digestibility, and much is known concerning their abilities to influence protein synthesis. However, relatively little is known about the digestion and absorption kinetics of intact milk protein containing both whey and casein. Gorissen and colleagues assessed the impact of protein type, dose, and age of subject on protein digestion, and amino acid absorption kinetics in humans, and the results of their study are published in the August 2020 issue of The Journal of Nutrition.

Data for this work were derived from 18 randomized controlled trials that included subjects (n=602) who were provided intrinsically labeled ([13C]-phenylalanine) whey, casein, or milk, as well as infusions of [2H]-phenylalanine. This approach allowed the researchers to determine protein digestion and absorption kinetics of phenylalanine, as well as the release of diet-derived phenylalanine into the circulation. The effect of aging on these variables was determined using data from a subset of young (n=82) and older (n=83) adults.

Within the first 5 hours after consumption, phenylalanine appearance into circulation was greatest for milk protein (65%), followed by whey protein (57%) and casein (45%). Consuming more protein led to more diet-derived phenylalanine in circulation. Older adults had lower levels of protein digestion and phenylalanine absorption kinetics than the younger individuals, which caused them to only have 45% of the consumed amount appearing in circulation versus 51% for the younger subjects. The authors concluded that both the type and dose of a milk protein, as well as age have impacts upon protein digestion, amino acid absorption kinetics, and the resulting circulating levels of available amino acids.

In a commentary on this article, West and Mitchell agree that the work of Gorissen and colleagues adds to the literature demonstrating that whole food protein sources may be more effective in supporting anabolism than their individual components. They emphasize the need for long-term intervention studies using different protein sources and doses that also document muscle mass/quality to fully understand the significance of the relatively small changes in amino acid availability detected in this study on muscle protein synthesis.


Gorissen SHM, Trommelen J, Kouw IWK, Holwerda AM, Pennings B, Groen BBL, Wall BT, Churchward-Venne TA, Horstman AMH, Koopman R, Burd NA, Fuchs CJ, Dirks ML, Res PT, Senden JMG, Steijns JMJM, de Groot LCPGM, Verdijk LB, van Loon LJC. Protein type, protein dose, and age modulate dietary protein digestion and phenylalanine absorption kinetics and plasma phenylalanine availability in humans. The Journal of Nutrition, Volume 150, Issue 8, August 2020, Pages 2041–2050,

Commentary provided by:

West DWD, Mitchell CJ. Tracking the fate of milk proteins: Better in whole or in part? The Journal of Nutrition, Volume 150, Issue 8, August 2020, Pages 2001–2002,

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