Differences in Sympathetic Nervous Stimulation of Brown Adipose tissue between the young and old and the lean and obese.

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 males. All subjects underwent a ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG) PET-CT and a ¹²³I-meta-iodobenzylguanidine (¹²³I-mIBG) SPECT-CT after an overnight fast and two hours of cold exposure. Metabolic BAT activity was expressed as volume and as maximal standardised uptake value (SUV_max) of ¹⁸F-FDG. BAT SNS activity was expressed as volume and as the ratio between ¹²³I-mIBG uptake in BAT and a reference region (SQUVmax of ¹²³I-mIBG). Results: SUV_max, BAT volume and SQUVmax were significantly different between young and old (SUV_max 7.9[4.2-17.3] vs. 2.9[0.0-4.0], volume 124.8[10.9-338.8] vs 3.4 [0.0-10.9] and SQUVmax 2.7[1.9-4.7] vs 0.0[0.0-2.2] all p<0.01) but not between lean and obese (SUV_max 7.9[4.2-17.3] vs 4.0[0.0-13.5] P = 0.69; volume 124.8[10.9-338.8] vs 11.8 [0.0-190.2] P = 0.64 and SQUVmax 2.7[1.9-4.7]vs 1.7[0-3.5] P = 0.69). We found a strong positive correlation between BAT activity measured with ¹⁸F-FDG and ¹²³I-mIBG in the whole group of BAT positive subjects (=0.82, p<0.01). Conclusion: We conclude that both sympathetic drive and BAT activity are lower in older but not in obese males.

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