Brown adipose tissue and seasonal variation in humans

Abstract

Objective: Brown adipose tissue (BAT) is present in adult humans where it may be important in the prevention of obesity, although the main factors regulating its abundance are not well established. BAT demonstrates seasonal variation relating to ambient temperature and photoperiod in mammals. The objective of our study was therefore to determine whether seasonal variation in BAT activity in humans was more closely related to the prevailing photoperiod or temperature.

Research design and methods: We studied 3,614 consecutive patients who underwent positron emission tomography followed by computed tomography scans. The presence and location of BAT depots were documented and correlated with monthly changes in photoperiod and ambient temperature.

Results: BAT activity was demonstrated in 167 (4.6%) scans. BAT was demonstrated in 52/724 scans (7.2%) in winter compared with 27/1,067 (2.5%) in summer months (P < 0.00001, chi(2) test). Monthly changes in the occurrence of BAT were more closely related to differences in photoperiod (r(2) = 0.876) rather than ambient temperature (r(2) = 0.696). Individuals with serial scans also demonstrated strong seasonal variation in BAT activity (average standardized uptake value [SUV(max)] 1.5 in July and 9.4 in January). BAT was also more common in female patients (female: n = 107, 7.2%; male: n = 60, 2.8%; P < 0.00001, chi(2) test).

Conclusions: Our study demonstrates a very strong seasonal variation in the presence of BAT. This effect is more closely associated with photoperiod than ambient temperature, suggesting a previously undescribed mechanism for mediating BAT function in humans that could now potentially be recruited for the prevention or reversal of obesity.

FIG. 1.

Seasonal variation in the occurrence of BAT in adults. Results are expressed as either the number of individual depots recorded (A) or the percentage of all scans (B) together with the variation in ambient temperature (http://www.tutiempo.net/en/climate/united_kingdom/gb.html) (C) and photoperiod i.e., night length (http://www.timeanddate.com/worldclock/astronomy.html) (D) over the time period in which BAT-positive scans were recorded. Summary showing the percentage occurrence of BAT according to calendar month (E). Significant effect of season assessed by either Mann-Whitney U test; **P < 0.01) or χ² test ***P < 0.00001. The number of patients with positive scans for each month is given and includes 16 patients who were scanned more than once.

FIG. 2.

Effect of month of year on individual changes in SUV_max of BAT. Each line represents an individual patient who underwent serial PET/CT scans throughout the study period, with the total number of patients being 16.

FIG. 3.

Effect of age on the occurrence of BAT in adults. Results are expressed as either the mean total intensity (A) or the number of individual depots recorded (B). A total of 71 scans were analyzed because not all were available for this type of analysis.