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Brown adipose tissue (BAT) plays an important role in thermoregulation in rodents. Its role in temperature homeostasis in people is less studied. To this end, we recruited 18 men [8 subjects with no/minimal BAT activity (BAT−) and 10 with pronounced BAT activity (BAT+)]. Each volunteer participated in a 6 h, individualized, non-shivering cold exposure protocol. BAT was quantified using positron emission tomography/computed tomography. Body core and skin temperatures were measured using a telemetric pill and wireless thermistors, respectively. Core body temperature decreased during cold exposure in the BAT− group only (−0.34°C, 95% CI: −0.6 to −0.1, p = 0.03), while the cold-induced change in core temperature was significantly different between BAT+ and BAT− subjects (BAT+ vs. BAT−, 0.43°C, 95% CI: 0.20–0.65, p = 0.0014). BAT volume was associated with the cold-induced change in core temperature (p = 0.01) even after adjustment for age and adiposity. Compared to the BAT− group, BAT+ subjects tolerated a lower ambient temperature (BAT−: 20.6 ± 0.3°C vs. BAT+: 19.8 ± 0.3°C, p = 0.035) without shivering. The cold-induced change in core temperature (r = 0.79, p = 0.001) and supraclavicular temperature (r = 0.58, p = 0.014) correlated with BAT volume, suggesting that these non-invasive measures can be potentially used as surrogate markers of BAT when other methods to detect BAT are not available or their use is not warranted. These results demonstrate a physiologically significant role for BAT in thermoregulation in people. This trial has been registered with Clinaltrials.gov: NCT01791114 (https://clinicaltrials.gov/ct2/show/NCT01791114).

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Objective Prolonged hospitalization due to burn injury results in physical inactivity and muscle weakness. However, how these changes are distributed among body parts is unknown. The aim of this study was to evaluate the degree of body composition changes in different anatomical regions during intensive care unit hospitalization (ICUh). Design Retrospective chart review. Setting Children’s burn hospital. Patients Twenty-four severely burned children admitted to our institution between 2000 and 2015. Interventions All patients underwent a dual-energy x-ray absorptiometry (DEXA) within 2 weeks after injury and 2 weeks before discharge to determine body composition changes. No subject underwent anabolic intervention. We analyzed changes of bone mineral content, bone mineral density, total fat mass, total mass, and total lean mass of the entire body and specifically analyzed the changes between the upper and lower limbs. Measurements and Main Results In the 24 patients, age was 10±5 years, total body surface area burned was 59±17%, time between DEXAs was 34±21 days, and length of stay was 39±24 days. We found a significant (p<0.001) average loss of 3% of lean mass in the whole body; this loss was significantly greater (p<0.001) in the upper extremities (17%) than in the lower extremities (7%). We also observed a remodeling of the fat compartments, with a significant whole-body increase in fat mass (p<0.001) that was greater in the truncal region (p<0.0001) and in the lower limbs (p<0.05). Conclusions ICUh is associated with greater lean mass loss in the upper limbs of burned children. Mobilization programs should include early mobilization of upper limbs to restore upper extremity function.