The Effect of Casein Ingestion within a Milk Matrix on Muscle Protein Synthesis


  • Sarah Reiners



Isolated micellar casein has been classified as a slow protein because of its slow digestion and amino acid absorption kinetics. These result further in a more moderate and sustained amino acid availability in the plasma and subsequently lower muscle protein synthesis (MPS) rate when, for example, compared with whey. However, the milk matrix with lactose, fat and various minerals may modulate the effect of casein on MPS. Therefore, this study aimed to compare the effects of the ingestion of casein dissolved in bovine milk serum to the ingestion of isolated casein on myofibrillar protein synthesis. In this parallel group randomized trial, 32 healthy older men (age: 71 ± 1 y) received a primed continuous infusion of L-[ring-2 H5 ]phenylalanine, L-[1-13C]leucine, and L-[ring-3,5-2 H2 ]tyrosine and blood and muscle samples were taken to assess myofibrillar fractional synthetic rate (FSR) under basal postabsorptive conditions and after a single bolus of 25 g intrinsically L-[1-13C]phenylalanine and L-[1-13C]leucine-labeled casein either in an isolated form (ISOCAS; n = 16) or dissolved in bovine milk serum (MILK-CAS; n = 16). The ingestion of 25 g of casein significantly (P < 0.05) increased MPS rates in both groups when assessed over the late postprandial period (t = 120 – 300 min). Ingestion of MILK-CAS and ISO-CAS did not significantly stimulate MPS rates when assessed over the early (t = 0 – 120 min) and overall (t = 0 – 300 min) postprandial FSR compared to postabsorptive (t = -120 – 0 min) FSR. No significant differences were observed between the two groups over the early, late or entire postprandial period (P > 0.05). The ingestion of additional normal milk matrix to micellar casein does not modulate overall myofibrillar protein synthesis rates in older men when compared to the ingestion of micellar casein dissolved in water. Therefore, there is neither a benefit, nor a detrimental effect of ingesting micellar casein within a normal milk matrix instead of the isolated form to increase MPS.


Pennings B, Boirie Y, Senden JM, Gijsen AP, Kuipers H, van Loon LJ. Whey protein stimulates postprandial muscle protein accretion more effectively than do casein and casein hydrolysate in older men. Am J Clin Nutr. 2011;93(5):997-1005.

Boirie Y, Dangin M, Gachon P, Vasson MP, Maubois JL, Beaufrere B. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci U S A. 1997;94(26):14930-5.

Fouillet H, Gaudichon C, Mariotti F, Bos C, Huneau JF, Tome D. Energy nutrients modulate the splanchnic sequestration of dietary nitrogen in humans: a compartmental analysis. Am J Physiol Endocrinol Metab. 2001;281(2):E248-60.

Park YW. Relative buffering capacity of goat milk, cow milk, soy-based infant formulas and commercial nonprescription antacid drugs. J Dairy Sci. 1991;74(10):3326-33.

Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. Diabetes Care. 2003;26(suppl 1):s5-s20.

Abumrad NN, Rabin D, Diamond MP, Lacy WW. Use of a heated superficial hand vein as an alternative site for the measurement of amino acid concentrations and for the study of glucose and alanine kinetics in man. Metabolism. 1981;30(9):936-40.

Bergstrom J. Percutaneous needle biopsy of skeletal muscle in physiological and clinical research. Scand J Clin Lab Invest. 1975;35(7):609-16.

Calbet JA, MacLean DA. Role of caloric content on gastric emptying in humans. J Physiol. 1997;498 ( Pt 2):553-9.

Gorissen SH, Burd NA, Hamer HM, Gijsen AP, Groen BB, van Loon LJ. Carbohydrate coingestion delays dietary protein digestion and absorption but does not modulate postprandial muscle protein accretion. J Clin Endocrinol Metab. 2014;99(6):2250-8.

Dangin M, Boirie Y, Guillet C, Beaufrere B. Influence of the protein digestion rate on protein turnover in young and elderly subjects. J Nutr. 2002;132(10):3228S-33S.

Koopman R, Crombach N, Gijsen AP, Walrand S, Fauquant J, Kies AK, et al. Ingestion of a protein hydrolysate is accompanied by an accelerated in vivo digestion and absorption rate when compared with its intact protein. Am J Clin Nutr. 2009;90(1):106-15.

Bohe J, Low JF, Wolfe RR, Rennie MJ. Latency and duration of stimulation of human muscle protein synthesis during continuous infusion of amino acids. J Physiol. 2001;532(Pt 2):575-9.

Cynober LA. Plasma amino acid levels with a note on membrane transport: characteristics, regulation, and metabolic significance. Nutrition. 2002;18(9):761-6.

Kimball SR, Jefferson LS. Control of protein synthesis by amino acid availability. Curr Opin Clin Nutr Metab Care. 2002;5(1):63-7.

Volpi E, Ferrando AA, Yeckel CW, Tipton KD, Wolfe RR. Exogenous amino acids stimulate net muscle protein synthesis in the elderly. J Clin Invest. 1998;101(9):2000-7.

Pennings B, Groen BB, van Dijk JW, de Lange A, Kiskini A, Kuklinski M, et al. Minced beef is more rapidly digested and absorbed than beef steak, resulting in greater postprandial protein retention in older men. Am J Clin Nutr. 2013;98(1):121-8.

Mitchell WK, Phillips BE, Williams JP, Rankin D, Lund JN, Smith K, et al. A Dose- rather than Delivery Profile– Dependent Mechanism Regulates the “Muscle-Full” Effect in Response to Oral Essential Amino Acid Intake in Young Men. The Journal of Nutrition. 2015;145(2):207-14.