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T h e ne w engl a nd jour na l o f medicine

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BAT Activity (kBq)

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r=0.60 P=0.002

brown-adipose-tissue activity and BMI as well as between brown-adipose-tissue activity and per- centage of body fat. However, only one obese subject had no brown-adipose-tissue activity. This finding indicates that it may be possible to in- crease brown adipose tissue in most overweight or obese men, and we speculate that methods of increasing brown adipose tissue in such men may be of therapeutic interest. Studies of the effects of regular exposure to cold, weight-loss diets, and increased physical activity on brown-adipose- tissue activity will also be of interest.

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Figure 3. Activity of Brown Adipose Tissue in Relation to BMI and Percentage of Body Fat.

Panel A shows the relation between brown­adipose­tissue (BAT) activity and BMI. Panel B shows the relation between BAT activity and percentage of body fat.

In our study, the rate of detection and level of activity of brown adipose tissue were much high- er than those in earlier studies. The high detec- tion rate in the current study is most likely due to the use of a standardized test for cold exposure. Previous studies measured brown adipose tissue in patients with cancer and related the occur- rence of brown adipose tissue to outside tem- peratures or the season.23 One early study used ephedrine rather than cold to stimulate brown- adipose-tissue activity in men.9 However, that study focused on perirenal brown adipose tissue, assuming that all depots have the same level of activity.

Our study showed that in all the subjects, the level of brown-adipose-tissue activity was high- est in the supraclavicular region. However, other sites with substantial brown-adipose-tissue activ- ity were evident. Most sites have been described in previous autopsy studies8 or in studies using PET–CT scanning with 18F-FDG.14 A second rea- son for the high incidence of brown-adipose- tissue activity in our study is the relatively young age of our subjects, ranging from 18 to 32 years. It is likely that the amount of brown adipose tis- sue decreases with age,17 although no relation be- tween brown-adipose-tissue activity and age was observed in our study.

(the supraclavicular area of the body is the pri- mary location for active brown adipose tissue). Moreover, the results of PET–CT scanning per- formed just after the subjects had spent 2 hours in thermoneutral conditions revealed no brown- adipose-tissue activity, indicating that brown adi- pose tissue is activated by cold exposure.

The relation between brown-adipose-tissue ac- tivity and BMI observed in our study is similar to the results of previous studies, which showed that the level of brown-adipose-tissue activity was highest in subjects with a low BMI.15,16 However, other studies, which were retrospective and were not designed to assess brown-adipose-tissue ac- tivity, did not confirm this finding.12,17

Our study showed that brown-adipose-tissue activity was significantly lower in overweight or obese male subjects than in lean male subjects. There was a significant negative relation between

Brown-adipose-tissue activity was negatively correlated with a change in distal skin tempera- ture, suggesting that the activity during cold ex- posure was involved in the immediate response

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n engl j med 360;15

nejm.org

april 9, 2009

The New England Journal of Medicine Downloaded from nejm.org on February 3, 2015. For personal use only. No other uses without permission. Copyright © 2009 Massachusetts Medical Society. All rights reserved.

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