The muscle protein synthetic response to the combined ingestion of protein and carbohydrate is not impaired in healthy older men

Abstract

Aging is associated with a progressive decline in skeletal muscle mass. It has been hypothesized that an attenuated muscle protein synthetic response to the main anabolic stimuli may contribute to the age-related loss of muscle tissue. The aim of the present study was to compare the muscle protein synthetic response following ingestion of a meal-like amount of dietary protein plus carbohydrate between healthy young and older men. Twelve young (21 ± 1 years) and 12 older (75 ± 1 years) men consumed 20 g of intrinsically L-[1-(13)C]phenylalanine-labeled protein with 40 g of carbohydrate. Ingestion of specifically produced intrinsically L-[1-(13)C]phenylalanine-labeled protein allowed us to assess the subsequent incorporation of casein-derived amino acids into muscle protein. Blood samples were collected at regular intervals, with muscle biopsies obtained prior to and 2 and 6 h after protein plus carbohydrate ingestion. The acute post-prandial rise in plasma glucose and insulin concentrations was significantly greater in the older compared with the younger males. Plasma amino acid concentrations increased rapidly following drink ingestion in both groups. However, plasma leucine concentrations were significantly lower at t = 90 min in the older when compared with the young group (P < 0.05). Muscle protein-bound L-[1-(13)C]phenylalanine enrichments increased to 0.0071 ± 0.0016 and 0.0072 ± 0.0013 mole percent excess (MPE) at 2 h and 0.0229 ± 0.0016 and 0.0213 ± 0.0024 MPE at 6 h following ingestion of the intrinsically labeled protein in the young and older males, respectively, with no differences between groups (P > 0.05). We conclude that the use of dietary protein-derived amino acids for muscle protein synthesis is not impaired in healthy older men following intake of protein plus carbohydrate.

Fig. 1

a Mean (±SEM) plasma glucose and b insulin concentrations following ingestion of 20 g of intrinsically l-[1-¹³C]phenylalanine-labeled protein plus 40 g of carbohydrate in healthy young (n = 12) and older (n = 12) men. a Significant time (P < 0.0001), group (P < 0.0001), and time*group interaction (P < 0.0001) effect. b Significant time (P < 0.0001), group (P < 0.0001), and time*group interaction (P < 0.0001) effect. * = P < 0.05 compared with corresponding time point in the Young group

Fig. 2

a Mean (±SEM) plasma phenylalanine, b leucine, c total branched chain amino acid (BCAA), d total essential amino acid (EAA), e total non-essential amino acid (NEAA), and f total amino acid (AA) concentrations following ingestion of 20 g of intrinsically l-[1-¹³C]phenylalanine-labeled protein plus 40 g of carbohydrate in healthy young (n = 12) and older (n = 12) men. a Significant time (P < 0.0001) effect. b Significant time (P < 0.0001) and time*group interaction (P = 0.0018) effect. c Significant time (P < 0.0001), group (P = 0.0050), and time*group interaction (P < 0.0001) effect. d Significant time (P < 0.0001) and time*group interaction (P = 0.0032) effect. e Significant time (P < 0.0001) and group (P = 0.0032) effect. f Significant time (P < 0.0001) and group (P = 0.0005) effect. * = P < 0.05 compared to corresponding time point in the Young group

Fig. 3

Mean (±SEM) plasma l-[1-¹³C]phenylalanine enrichments (MPE) following ingestion of 20 g of intrinsically l-[1-¹³C]phenylalanine-labeled protein plus 40 g of carbohydrate in healthy young (n = 12) and older (n = 12) men. Significant time (P < 0.0001) and time*group interaction (P = 0.0006) effect. No significant differences between groups at individual time points

Fig. 4

Mean (±SEM) muscle protein-bound l-[1-¹³C]phenylalanine enrichments (MPE) at 2 and 6 h following the ingestion of 20 g of intrinsically l-[1-¹³C]phenylalanine-labeled protein plus 40 g of carbohydrate in healthy young (n = 12) and older (n = 12) men. No significant differences were observed between groups