Amino acid supplementation increases lean body mass, basal muscle protein synthesis, and insulin-like growth factor-I expression in older women

Abstract

Context: Inadequate dietary protein intake has been implicated in sarcopenia.

Objective and design: The objectives of this study were to determine whether: 1) chronic essential amino acid (EAA) supplementation improves postabsorptive muscle protein fractional synthesis rate (FSR), lean body mass (LBM), and one-repetition maximum muscle strength, and androgen receptor and IGF-I muscle protein expression; and 2) the acute anabolic response to EAA ingestion is preserved after a 3-month supplementation period. Using a randomized, double-blinded, placebo-controlled design, older women (68 +/- 2 yr) were assigned to receive either placebo (n = 7), or 15 g EAA/d [supplemented treatment group (SUP)] (n = 7) for 3 months. Metabolic outcomes were assessed in association with stable isotope studies conducted at 0 and 3 months.

Setting: The study was performed at The University of Texas Medical Branch General Clinical Research Center.

Results: Ingestion of 7.5 g EAA acutely stimulated FSR in both groups at 0 months (P < 0.05). Basal FSR at 3 months was increased in SUP only. The magnitude of the acute response to EAA was unaltered after 3 months in SUP. LBM increased in SUP only (P < 0.05). One-repetition maximum strength remained unchanged in both groups. Basal IGF-I protein expression increased in SUP after 3 months (P = 0.05), with no changes in androgen receptor or total and phosphorylated Akt, mammalian target of rapamycin, S6 kinase, and 4E-binding protein.

Conclusions: EAA improved LBM and basal muscle protein synthesis in older individuals. The acute anabolic response to EAA supplementation is maintained over time and can improve LBM, possibly offsetting the debilitating effects of sarcopenia.

Figures

Figure 1

Baseline and 3-month infusion protocol. Arterial and venous (AV) blood samples were obtained at 15-min intervals during an infusion of l-[ring 2H5]phenylalanine. Basal muscle biopsies from the vastus lateralis were obtained at 60 and 180 min. Post-EAA muscle biopsies were obtained at 60 and 240 min after a drink containing 7.5 g EAA.

Figure 2

Phenylalanine (Phe) enrichment (A), concentration (B), and Rd (C) throughout the infusion studies at months 0 and 3.

Figure 3

A, Basal-mixed muscle FSR at months 0 and 3. Dotted lines are representative of pooled averages from both groups at month 0. B, Percent change in FSR from months 0–3. #, Significant increase in FSR from baseline to 3 months (P < 0.05). *, Significant increase in FSR after ingestion of an EAA bolus (P < 0.05). PLA, Placebo.

Figure 4

IGF-I protein expression in skeletal muscle at months 0 and 3 of EAA supplementation. *, Significant increase in IGF-I expression in the SUP group (P = 0.05). Top, Representative Western blot from a placebo (PLA) and SUP subject. Bottom, Mean densitometry data from the seven subjects receiving 3 months placebo and seven subjects receiving 3 months 15 g EAA (SUP). Months 0 (white bars) and 3 (black bars). The arbitrary units result from the IGF-I densitometric data expressed as a ratio with the densitometric data for the actin control.