Magnetic resonance imaging reveals human brown adipose tissue is rapidly activated in response to cold

Context. In rodents, cold exposure induces the activation of brown adipose
tissue (BAT) and the induction of intracellular triacylglycerol (TAG)
lipolysis. However, in humans, the kinetics of supraclavicular (SCV) BAT
activation and the potential importance of TAG stores remain poorly
defined. Objective. To determine the time course of BAT activation and
changes in intracellular TAG using magnetic resonance imaging (MRI)
assessment of the SCV (i.e. BAT depot) and fat in the posterior neck
region (i.e. non BAT). Design. Cross-sectional. Setting. Clinical research
centre. Patients or Other Participants. Twelve healthy male volunteers
ages 18-29 years [BMI=24.7±2.8kg/m2 and body fat percentage = 25.0±7.4%
(both mean±SD)]. Intervention(s). Standardized whole-body cold exposure
(180 minutes at 18°C) and immediate re-warming (30 minutes at 32°C). Main
Outcome Measure(s). Proton density fat fraction (PDFF) and T2* of the SCV
and posterior neck fat pads. Acquisitions occurred at 5-15 minute
intervals during cooling and subsequent warming. Results. SCV PDFF
declined significantly after only 10 minutes of cold exposure [-1.6%
(standard error (SE) 0.44%), p=0.007) and continued to decline until 35
minutes after which time it remained stable until 180 minutes. A similar
time course was also observed for SCV T2*. In the posterior neck fat
(non-BAT) there were no cold-induced changes in PDFF or T2*. Re-warming
did not result in a change in SCV PDFF or T2*. Conclusions. The rapid
cold-induced decline in SCV PDFF suggests that in humans, BAT is activated
quickly in response to cold and that TAG is a primary substrate.