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

Abstract

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 H 5 ]phenylalanine, L-[1-13 C]leucine, and L-[ring-3,5-2 H 2 ]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-[1-13 C]phenylalanine and L-[1-13 C]leucine-labeled casein either in an isolated form (ISO-CAS; 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.