Endocrine Abstracts

Background: Humans and other mammals need to maintain a stable core body temperature. Energy expenditure increases in response to a mild cold stimulus, this is called cold induced thermogenesis (CIT). Recently, thermogenic brown adipose tissue (BAT) has been found to play an important role for CIT in human adults. It is known that energy expenditure and CIT are acutely influenced by ambient temperature. In the present study, we investigated the effect of seasonal temperature variation on CIT in human adults.

Methods: We collected data from two prospective observational studies and the screening data from an interventional trial. The analysed 89 participants all underwent measurement of CIT. Of these, 56 participants were healthy volunteers and 33 were hypothyroid patients at the time of the measurement sufficiently substituted with thyroxin. CIT was measured by indirect calorimetry during warm conditions and after a mild cold stimulus of 90 min. CIT was determined as the difference between energy expenditure (EE) during warm (EE_warm) and cold (EE_cold) conditions. Skin temperature was measured in the supraclavicular region (TempSC) adjacent to the major human BAT depot and compared with parasternal temperatures (TempPS); the difference between TempSC and TempPS was calculated as an indicator of BAT activity. We analysed the relation ofEE, CIT and skin temperatures to the outdoor temperatures, which were recorded by the Institute for Meteorology, Climatology and Remote Sensing at the University of Basel. Daily temperatures were averaged over a period of 7 (TempMax7d) or 30 days (TempMax30d) prior to the corresponding study visit.

Results: CIT was inversely associated with the average maximum outdoor temperature during the week (R²=0.1737, P<0.0001) and the month (R²=0.1424, P=0.0003) before the visit date. EE_warm and EE_cold were not significantly related to outdoor temperatures, EE_warm; R²=0.0074, P=0.4218 (TempMax7d), R²=0.0168, P=0.2252 (TempMax30d) and EE_cold; R²=0.0132, P=0.2840 (TempMax7d), R²=0.0035, P=0.5831 (TempMax30d). The difference between TempSC and TempPS was also inversely related to TempMax7d, R²=0.07575, P=0.0221, indicating increased activation of BAT in response to longer periods of cold temperatures.

Conclusion: CIT is strongly and inversely correlated to outdoor temperatures indicating dynamic adaption of thermogenesis and BAT activity to environmental stimuli in adult humans.