Abstract
Some studies indicate that basal metabolic rate is greater in winter than in the summer,
suggesting a role for brown fat in human thermogenesis. We examined whether there
are clinically meaningful differences in basal metabolic rate under thermoneutral
conditions between winter and summer months in inhabitants of Rochester, Minnesota.
We collated data from 220 research volunteers studied in the winter (December 1 –
February 28) and 214 volunteers studied in the summer (June 1 – August 31), 1995–2012.
Basal metabolic rate was measured by indirect calorimetry and body composition by
dual-energy X-ray absorptiometry. The effect of season on basal metabolic rate was
tested using multivariate regression analysis with basal metabolic rate as the dependent
variable and fat-free mass, fat mass, age, sex, and season as the independent variables.
The groups were comparable with respect to age, body mass index, fat mass, and fat-free
mass. There was no significant difference in basal metabolic rate between winter and
summer groups (1 667±322 vs. 1 669±330 kcal/day). Both winter and summer basal metabolic
rates were strongly predicted by fat-free mass (Pearson’s r=0.75 and r=0.77, respectively,
p <0.0001). Using multiple linear regression analysis, basal metabolic rate was significantly,
independently predicted by fat-free mass, fat mass, age, and sex, but not season.
We conclude that the lack of seasonal variation of thermoneutral basal metabolic rate
between winter and summer suggests that modern, Western populations do not engage
thermogenically detectable brown fat activity during periods of living in a cold climate.
Key words
indirect calorimetry - body composition - brown fat
