Circulating branched-chain amino acid concentrations are associated with skeletal muscle mass and thyroid function in young Japanese men

抄録

Abstract
Background Few nutritional markers reflect the hypermetabolic state of athletes with high levels of skeletal muscle. Although branched-chain amino acids (BCAA) play crucial roles in protein metabolism in skeletal muscle, the relation between circulating BCAA concentration and skeletal muscle mass has not been fully understood. The aim of this study is to evaluate the association between skeletal muscle mass and serum BCAA to tyrosine ratio (BTR), a convenient indicator of circulating BCAA, in young Japanese men including wrestling athletes with high skeletal muscle mass. Methods The study enrolled 111 young Japanese men: 70 wrestling athletes and 41 controls. None were taking medications or undergoing extreme dietary restriction or an intense exercise regimen. Each participant’s body composition, serum concentrations of albumin and rapid turnover proteins, BTR, and thyroid function were assessed. Results Compared to the controls, the athletes had significantly higher weight-adjusted skeletal muscle mass (SMM) (p < 0.001) and skeletal muscle index (SMI) (p < 0.001), and lower serum albumin concentration (p < 0.001) and BTR (p < 0.001). Kruskal–Wallis tests with Bonferroni multiple comparison tests showed that serum albumin concentration and BTR were significantly lower in the participants with higher SMM and SMI. Spearman's correlation analyses showed that SMM and SMI were inversely correlated with serum albumin concentration (SMM, ρ = −0.315, p = 0.001; SMI, ρ = −0.511, p < 0.001) and BTR (SMM, ρ = −0.424, p < 0.001; SMI, ρ = −0.511, p < 0.001). Serum albumin concentration was positively correlated with BTR (ρ = 0.438, p < 0.001), transthyretin (ρ = 0.281, p = 0.003) and transferrin (ρ = 0.299, p = 0.001), but BTR was not correlated with other nutritional indicators. Serum concentration of free 3,5,3′-triiodothyronine was inversely correlated with BTR (ρ = −0.308, p = 0.001), but not with serum albumin or skeletal muscle mass. Conclusions These results suggest that the consumption of circulating BCAAs increases with increased skeletal muscle mass and is facilitated by thyroid hormones independent of skeletal muscle mass. Serum BTR may be a useful biomarker to assess the hypermetabolic state of athletes.