Differences in Sympathetic Nervous Stimulation of Brown Adipose Tissue Between the Young and Old, and the Lean and Obese.

UNLABELLED: Brown adipose tissue (BAT) could facilitate weight loss by increasing energy expenditure. Cold is a potent stimulator of BAT, activating BAT primarily through the sympathetic nervous system (SNS). Older or overweight individuals have less metabolic BAT activity than the lean and young, but the role of the SNS in this decline is unknown. We aimed to determine whether this lower metabolic BAT activity in older or overweight individuals can be explained by a lower SNS response to cold.

METHODS: This was a prospective observational study. We included 10 young obese, 11 old lean, and 14 young lean healthy men. All subjects underwent (18)F-FDG PET/CT and (123)I-meta-iodobenzylguanidine ((123)I-mIBG) SPECT/CT after an overnight fast and 2 h of cold exposure. Metabolic BAT activity was expressed as volume and as SUVmax of (18)F-FDG. BAT SNS activity was expressed as volume and as the ratio between (123)I-mIBG uptake in BAT and a reference region (SQUVmax of (123)I-mIBG).

RESULTS: SUVmax, BAT volume, and SQUVmax were significantly different between young and old (SUVmax, 7.9 [range, 4.2-17.3] vs. 2.9 [range, 0.0-4.0]; volume, 124.8 [range, 10.9-338.8] vs. 3.4 [range, 0.0-10.9]; and SQUVmax, 2.7 [range, 1.9-4.7] vs. 0.0 [range, 0.0-2.2], respectively) (all P < 0.01) but not between lean and obese (SUVmax, 7.9 [range, 4.2-17.3] vs. 4.0 [range, 0.0-13.5] [P = 0.69]; volume, 124.8 [range, 10.9-338.8] vs. 11.8 [range, 0.0-190.2] [P = 0.64]; and SQUVmax, 2.7 [range, 1.9-4.7] vs. 1.7 [range, 0-3.5] [P = 0.69], respectively). We found a strong positive correlation between BAT activity measured with (18)F-FDG and (123)I-mIBG in the whole group of BAT-positive subjects (ρ = 0.82, P < 0.01).

CONCLUSION: Both sympathetic drive and BAT activity are lower in older but not in obese men.