High Incidence of Metabolically Active Brown Adipose Tissue in Healthy Adult Humans: Effects of Cold Exposure and Adiposity

Abstract

Objective: The significant roles of brown adipose tissue (BAT) in the regulation of energy expenditure and adiposity are established in small rodents, but have been controversial in man. The objective is to examine the prevalence of metabolically active BAT in healthy adult humans, and to clarify the effects of cold exposure and adiposity.

Research Design and Methods: In vivo 2-[¹⁸F ]fluoro-2-deoxyglucose (FDG) uptake into adipose tissue was measured in 56 healthy volunteers (31 males and 25 females) aged 23 - 65 y by positron emission tomography (PET) combined with X-ray computed tomography (CT).

Results: When exposed to cold (19 °C) for 2 hr, 17 out of 32 younger subjects (23-35 y.o.) and 2 out of 24 elderly subjects (38-65 y.o.) showed a substantial FDG uptake into adipose tissue of the supraclavicular and paraspinal regions, whereas they showed no detectable uptake when kept in warm (27 °C). Histological examinations confirmed the presence of brown adipocytes in these regions. The cold-activated FDG uptake was increased in winter compared to summer (p<0.001), and inversely related to body mass index (p<0.001), and total (p<0.01) and visceral (p<0.001) fat areas estimated from CT image at the umbilical level.

Conclusions: Our findings, being against the conventional view, indicate the high incidence of metabolically active BAT in adult humans, and suggest a role in the control of body temperature and adiposity.