Cold exposure and thermoneutrality similarly reduce supraclavicular brown adipose tissue fat fraction in fasted young lean adults

Brown adipose tissue (BAT) is a metabolically highly active tissue that dissipates energy stored within its intracellular triglyceride droplets as heat. Others have previously utilized MRI to show that the fat fraction of human supraclavicular BAT (scBAT) decreases upon cold exposure, compared with baseline (i.e., pre-cooling). However, comparisons to a control group that was not exposed to cold are largely lacking. We recently developed a non-invasive dynamic MRI protocol that allows for quantifying scBAT fat fraction changes over time. Here, we aimed to study the effect of cold exposure versus thermoneutrality on fat fraction changes in human scBAT. Ten young (mean age: 21.5 ± 0.7 years), lean (mean BMI: 21.7 ± 0.5 kg/m²), 12 h-fasted volunteers (9 females; 1 male) underwent up to 70 consecutive MRI scans each on two separate study visits in a cross-over design. Participants were exposed to a temperature of 32°C for 10 scans (i.e., ±16 min), which was then either lowered to 18°C (i.e., cold exposure) or was maintained at 32°C (i.e., thermoneutrality). Dynamic fat fraction changes were quantified, and self-reported thermal perception scores were monitored. The fat fraction in scBAT decreased over time upon cold exposure (r = -.222, p < .001). Interestingly however, we also observed a decrease in scBAT fat fraction over time upon thermoneutrality (r = -.212, p < .001). No difference was observed between the two temperature conditions (p = .55), while self-reported thermal perception scores were consistently higher (i.e., colder) upon cold exposure. In the trapezius muscle and the humerus bone as control tissues, the fat fraction was unaffected in both temperature conditions. The fat fraction in subcutaneous white adipose tissue (sWAT) however, also decreased over time upon cold exposure (r = -.270, p < .001) and during thermoneutrality (r = -.190, p < .001), again with no difference (p = .92) between the two temperature conditions. In conclusion, our results show that in 12 h-fasted, healthy individuals cold exposure and thermoneutrality similarly reduce the fat fraction within scBAT and sWAT. While we interpret that the cold exposure was sufficient to induce thermogenesis, we suggest that an increased lipolytic activity within adipocytes, as a consequence of fasting, may be the primary cause of the decreased fat fraction in both sWAT and scBAT in our study. The current study highlights the potential influence of fasting on the fat fraction in scBAT and stresses the importance of inclusion of a thermoneutral control group in studies investigating the BAT-modulating effect of cold exposure.