Amino acid ingestion improves muscle protein synthesis in the young and elderly

Abstract

We recently demonstrated that muscle protein synthesis was stimulated to a similar extent in young and elderly subjects during a 3-h amino acid infusion. We sought to determine if a more practical bolus oral ingestion would also produce a similar response in young (34 Ϯ 4 yr) and elderly (67 Ϯ 2 yr) individuals. Arteriovenous blood samples and muscle biopsies were obtained during a primed (2.0 mol/kg) constant infusion (0.05Muscle protein kinetics and mixed muscle fractional synthetic rate (FSR) were calculated before and after the bolus ingestion of 15 g of essential amino acids (EAA) in young (n ϭ 6) and elderly (n ϭ 7) subjects. After EAA ingestion, the rate of increase in femoral artery phenylalanine concentration was slower in elderly subjects but remained elevated for a longer period. EAA ingestion increased FSR in both age groups by ϳ0.04%/h (P Ͻ 0.05). However, muscle intracellular (IC) phenylalanine concentration remained significantly higher in elderly subjects at the completion of the study (young: 115.6 Ϯ 5.4 nmol/ml; elderly: 150.2 Ϯ 19.4 nmol/ml). Correction for the free phenylalanine retained in the muscle IC pool resulted in similar net phenylalanine uptake values in the young and elderly. EAA ingestion increased plasma insulin levels in young (6.1 Ϯ 1.2 to 21.3 Ϯ 3.1 IU/ml) but not in elderly subjects (3.0 Ϯ 0.6 to 4.3 Ϯ 0.4 IU/ml). Despite differences in the time course of plasma phenylalanine kinetics and a greater residual IC phenylalanine concentration, amino acid supplementation acutely stimulated muscle protein synthesis in both young and elderly individuals.

Amino acid metabolism and regulatory effects in aging

“…Since muscle protein degradation has been consistently reported to remain essentially unchanged with advancing age [12][13][14][15][16], there has been an emphasis on studies examining the influence of age on muscle protein synthesis in the basal (postabsorptive) and fed (post-prandial) state. While some researchers have reported a decrease in basal muscle protein synthesis rate with age [17,18], others [14,19] could not confirm those findings in older individuals exhibiting a reduction in muscle mass. The reasons for these discrepancies are still unclear, but it is likely that differences in the health, nutritional status and physical activity level of the different older cohorts enrolled in the various studies [14,19] may have played a significant role.…”

“…While some researchers have reported a decrease in basal muscle protein synthesis rate with age [17,18], others [14,19] could not confirm those findings in older individuals exhibiting a reduction in muscle mass. The reasons for these discrepancies are still unclear, but it is likely that differences in the health, nutritional status and physical activity level of the different older cohorts enrolled in the various studies [14,19] may have played a significant role. Furthermore, because in the studies reporting a reduced muscle protein synthesis with aging muscle protein breakdown had only been indirectly estimated using whole-body approaches, it is not possible to establish if the subjects were truly experiencing a reduction in net muscle protein balance with aging (i.e.…”

“…There is clear evidence that increased amino acid or protein availability can enhance muscle protein synthesis and anabolism in young and older subjects [19][20][21]22 • ]. It has, however, been suggested that the recommended dietary allowance for protein (0.8 g/kg/day) may not be sufficient for older adults to maintain their muscle mass.…”

“…A number of studies have shown that pure amino acids can stimulate muscle protein synthesis and improve net protein balance in older as well as in younger individuals [19,[32][33][34][35]. Although older persons have an increased splanchnic extraction of orally administered amino acids at first pass (i.e.…”

Leucine elicits myotube hypertrophy and enhances maximal contractile force in tissue engineered skeletal muscle in vitro

“…mTORC1 signaling has consistently shown to be activated in response to both muscle loading (e.g., resistance exercise), and amino acid consumption/treatment (Marcotte, West, & Baar, 2015), and as such these stimuli represent excellent candidates as therapies for attenuating the muscle wasting associated with a number of disease states and ageing. Indeed, acute human studies have observed activation of mTORC1 and its downstream targets (e.g., p70S6K, rpS6, and 4EBP‐1) following ingestion of mixed amino acids, and this is coupled with an increase in muscle protein synthesis (MPS) in the ensuing 60–120 min (Atherton, Etheridge et al, 2010; Koopman et al, 2006; Paddon‐Jones et al, 2004; Volpi, Kobayashi, Sheffield‐Moore, Mittendorfer, & Wolfe, 2003). Furthermore, when human skeletal muscle undergoes loading prior to amino acid ingestion this effect on mTORC1 signaling and MPS is potentiated (Moore, Atherton, Rennie, Tarnopolsky, & Phillips, 2011; Witard et al, 2014).…”