This study investigated associations between skeletal muscle mass and nutritional indicators, including serum concentrations of albumin, rapid turnover proteins, and BTR, as well as the associations between these indicators and thyroid function tests, in young Japanese men. The wrestling athletes with high levels of skeletal muscle mass had significantly lower serum albumin concentrations and BTR than the control participants. In all the participants, serum albumin concentration and BTR were inversely correlated with SMI, and serum FT3 concentration was inversely correlated with serum BTR, independently.
Previous reports have compared concentrations of circulating albumin and rapid turnover proteins between athletes and controls. Serum albumin concentrations tended to be lower in male professional cyclists and skiers than in controls [16]. Among athletes, rowers had lower plasma albumin concentrations than intermittent fasted athletes, such as Ramadan-fasted runners and boxers, and lightweight rowers had lower albumin concentrations than heavyweight rowers [17]. Serum transthyretin concentrations were higher in elite marathon runners than in controls, but there were no differences in retinol binding protein or transferrin concentrations between these groups [18]. These indicators are influenced by many exercise-related factors, including dehydration and inflammation, so no definite findings have been obtained for these indicators as markers for assessing the conditions of athletes who are undergoing continuous training. In addition, the relationship between skeletal muscle mass and these indicators remains unclear. For example, the relationship between levels of circulating BCAAs and the skeletal muscle mass of athletes in a resting state has not been investigated, although the effects of BCAA supplementation on exercise have been extensively studied in athletes [6–8].
In the present study, serum albumin concentration and BTR were lower in the wrestling athletes than in the controls and were inversely correlated with skeletal muscle mass. Although serum albumin concentration was positively correlated with levels of rapid turnover proteins, there was no significant correlation between serum BTR and rapid turnover proteins. These results suggest that, even at rest, albumin and BCAAs may be used as sources of protein in skeletal muscles to a greater extent than other marker proteins. The serum albumin concentration also reflects liver synthesis ability, but the serum BTR does not. Therefore, the serum BTR is considered to more strongly reflect the state of increased energy consumption within skeletal muscles, especially in athletes. In contrast to the correlations found in the present study, patients with chronic heart failure showed lower serum BCAA concentrations and Fisher's ratios than controls, with positive correlations between the values of their SMI and their BCAA concentrations and Fisher's ratios [19]. In patients with chronic liver diseases, it was observed that lower BCAA to tyrosine ratios (BTRs) were associated with decreased skeletal muscle mass [20]. In those studies, the diseases may have led to increased BCAA catabolism or insufficient BCAA intake, resulting in decreased concentrations of circulating BCAAs, which in turn may lead to a reduction in skeletal muscle mass. Future studies are needed to determine whether the concentration of circulating BCAAs reflects energy expenditure in skeletal muscle, through a detailed assessment of BCAA intake and measurement of other biomarkers of skeletal muscle mass and function.
Muscle cross-sectional area has been reported to be lower in elderly subclinical hypothyroid patients than in age-matched euthyroid controls [21]. A recent study reported that serum FT3 concentrations were positively correlated with appendicular skeletal muscle mass, handgrip strength, and the results of a short physical performance battery in elderly Chinese euthyroid subjects [22]. In that study, the FT3 concentrations were especially low in subjects with sarcopenia. In contrast to those results, we did not observe a correlation between skeletal muscle mass and thyroid function tests although, unlike the previous studies, our study involved young participants with adequate skeletal muscle mass. However, we observed an inverse correlation between serum FT3 concentration and serum BTR. A study of rats with hyperthyroidism showed that leucine supplementation improved their swimming performance [23]; this suggests that thyroid hormones increase BCAA metabolism in skeletal muscle, which may be relevant to our results. Further studies are needed to elucidate the roles of thyroid hormones on BCAA metabolism in skeletal muscle.
This study had several limitations. One was its relatively small sample size. During the enrollment, we specifically recruited elite wrestling athletes with high skeletal muscle mass at a time when they were not undergoing intense training or extreme restriction of nutrition and water intake for a tournament. Another limitation was that the participants' diets prior to the sample collection were not fully standardized. It has been reported that plasma BCAA concentrations in healthy male students increase immediately after BCAA ingestion, peak at 30 min, and gradually decrease to the initial level by 180 min after ingestion [24]. In the present study, nutritional status, including BCAA metabolism, in the athletes and the controls who did not exercise and have breakfast from wake-up to sample collection after overnight fasting may have been at baseline without any immediate influence. Further studies are needed to confirm our hypothesis through investigations of athlete with various types of specialized skeletal muscle function, such as instantaneous power or endurance strength.