Significance of serum branched-chain amino acid to tyrosine ratio measurement in athletes with high skeletal muscle mass

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 relationship 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 including transthyretin and transferrin, BTR, and thyroid function were assessed. Results Compared to the controls, the athletes had significantly higher skeletal muscle index (SMI) (p < 0.001), and lower serum albumin concentration (p < 0.001) and BTR (p < 0.001). Kruskal–Wallis tests showed that serum albumin concentration and BTR were significantly lower in the participants with higher SMI. Serum albumin concentration and BTR were inversely correlated with SMI by multiple regression analysis (logarithmic albumin, β = −0.358, p < 0.001; BTR, β = −0.299, p = 0.001). SMI was inversely and transthyretin was positively correlated with serum albumin (SMI, β = −0.554, p < 0.001; transthyretin, β = 0.379, p < 0.001). Serum concentration of free 3,5,3'-triiodothyronine (FT₃) was inversely correlated with BTR, and, along with SMI and albumin, was independent predictor of BTR (SMI, β = −0.321, p < 0.001; FT₃, β = −0.253, p = 0.001; logarithmic albumin, β = 0.261, p = 0.003), but was not correlated with SMI or serum albumin. Serum concentrations of rapid turnover proteins were not correlated with BTR. Conclusions These results suggest that increased skeletal muscle mass enhances the consumption of circulating BCAAs, facilitated by thyroid hormones independently. Serum BTR may be a useful biomarker to assess the hypermetabolic state of wrestling athletes with high levels of skeletal muscle.