Swick AG et al. Metabolism. 2013.
Fat-free mass (FFM) explains about 80 percent of the variance in 24-hour energy expenditure (EE), but there is significant variability between individuals with similar FFM that cannot be explained by age, body mass index (BMI) or sex. The 20 percent variation in EE has a potentially large impact on the prevalence and severity of obesity in the general population. One potential source of variability in EE is brown adipose tissue (BAT). The regulation of BAT in humans is not well understood; however, it has been recently reported that the discovery of irisin, a myokine that stimulates the development of BAT in mice and in cell culture, is decreased in patients with type 2 diabetes, while positively correlated with BMI, fat mass and muscle mass in humans. Post-menopausal women (n=17) spent 24 hours in a whole room calorimeter to measure EE, and plasma irisin levels were measured. There was a significant correlation between irisin levels and EE/kg/FFM for subjects whose EE was greater than predicted. For subjects whose EE was equal to or less than predicted by FFM, there was not a significant correlation with irisin levels.  The authors proposed that since irisin stimulates BAT, BAT-derived thermogenesis may explain the differences in EE and thus contribute to the difference in EE that is not accounted for by FFM. Increasing irisin may be a promising avenue for future prevention and treatment of obesity.