Abstract

Objective: Brown adipose tissue (BAT) is established to be 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.

Materials and Methods: 3614 consecutive patients who underwent positron emission tomography followed by computed tomography scans were studied. The presence and location of BAT depots were documented and correlated with monthly changes in photoperiod and ambient temperature.

Results: 167 (4.6%) scans demonstrated BAT activity. 52/724 scans (7.2%) demonstrated BAT in winter compared to 27/1067 (2.5%) in summer months (p<0.00001, Chi-square test). Monthly changes in the occurrence of BAT were more closely related to differences in photoperiod (r² = 0.876) rather than ambient temperature (r² = 0.696). Individuals with serial scans also demonstrated strong seasonal variation in BAT activity (Average SUV_max 1.5 in July, 9.4 in January). BAT was also more common in females (female: n=107, 7.2%, male: n=60, 2.8% p<0.00001, Chi squared 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.