Brown adipose tissue (BAT) dissipates energy as heat and contributes to energy expenditure. In animal model, myostatin (Mst) knockout has been shown to drive browning of white adipose tissue, promote lean mass and increase total energy expenditure. We explored whether BAT explain the variation in the relationship between lean mass and energy expenditure in humans, and examined the relationship between myostatin, ethnicity and BAT.
We recruited 186 males, mean age(SD) 26.1(4.4) years, BMI 24.0 (2.9) kg/m2. There were 44.6% Chinese, 23.7% Malays and 31.7% Asian-Indians. We measured total lean mass (LM) using DEXA, and resting metabolic rate (RMR) using the indirect calorimetry. Plasma myostatin was measured by commercially available ELISA. We recalled 13 subjects with upper (mean 1.68), 12 subjects with lower (mean -1.32) and 13 mid (mean 0.06) standardized residuals for the linear relationship between LM and RMR for BAT imaging. They underwent 2-hour cold-exposure followed by 18-FDG PET CT-scan for BAT detection.
Proportion of detectable BAT, mean BAT volume and standard uptake volume (SUV) did not differ between those with high and low residuals for the relationship between RMR and LM. Myostatin levels were higher in those without compared to those with detectable BAT (vs. (57.34(3.15) vs 49.15(3.11), p=0.07). There were no differences in the BMI and RMR between those with or without detectable BAT. Asian-Indians were younger and had higher insulin resistance compared to Chinese. LM and RMR were similar between the two ethnic groups. However, Asian-Indians had significantly higher detectable BAT compared to Chinese (66.7% vs 31.2%, p=0.049).
BAT does not explain the variation in the relationship between lean mass and energy expenditure. Asian-Indians have higher detectable brown adipose tissue despite having a less favorable metabolic profiles compared to Chinese. Myostatin levels appear higher among individuals without detectable brown adipose tissue.