Based on 11C-acetate tissue kinetics, the present results demonstrate significant cold-induced activation of BAT oxidative metabolism in all subjects studied under well-controlled cold exposure conditions designed to minimize shivering. We also determined for what we believe is the first time in vivo in humans that plasma NEFA uptake is increased in cold-activated BAT compared with resting skeletal muscles and subcutaneous adipose tissues.
A cold-induced total BAT glucose uptake averaging 10.8 ± 2.9 μmol/min lies within the range reported by Virtanen et al. (6
). In addition, the present results demonstrate the ability of human BAT to also increase its NEFA uptake during cold exposure. Whole-body 18
FTHA uptake at the end of the protocol predicts NEFA uptake by BAT amounting to 2.3 ± 0.8 μmol/min (range, 0.3 to 4.4 μmol/min). One limitation of our experimental design was the impossibility of assessing 18
FDG and 18
FTHA BAT uptake at ambient temperature, given the complexity of design and acceptable limits of radiation exposure of research participants. However, significant BAT glucose and NEFA uptake is not detectable in the vast majority of individuals at ambient temperature (12
). In rodents, the contribution from glucose and NEFA uptake has proven to be a relatively small fraction of total BAT metabolism during acute cold-induced thermogenesis compared with intracellular brown adipocyte triglycerides (21
). Also, we did not assess BAT utilization of fatty acids from circulating lipoproteins, another potential source of BAT energy substrates (22
). However, unlike what has been observed in mice in the latter study, we did not find any reduction in circulating triglycerides in the present study (Table ). Assuming a volume of distribution of 0.45 dl/kg of body weight (23
), with an average body weight of 80.7 kg and a plasma triglyceride level of 0.93 mmol/l (82 mg/dl), mean total circulating triglyceride content is estimated at approximately 3 g in the participants of the present study. As the half-life of circulating triglycerides during fasting (mostly VLDL) is relatively long (approximately 2 hours, i.e., a fractional clearance rate of 0.5/h; ref. 23
) compared with the length of cold exposure in the present study, it is very unlikely that the pool of circulating triglycerides, even if it was fully utilized, could account for the 250 extra kcal of energy expenditure observed (i.e., approximately 28 g of triglycerides). It is also noteworthy that our subjects were not, in contrast with the mice in Bartelt’s study (22
), adapted to a temperature below thermoneutrality, which may render BAT adipocytes more prepared to clear circulating glucose and lipid.
The 6 subjects of the present study all exhibited increases in BAT metabolism upon cold exposure, albeit to varying degrees. The variability in the response occurred despite application of a cold exposure protocol that not only restricted shivering, but also dictated the strength of the cold response from strict body and skin temperature monitoring and indirect calorimetry measurements. Such variation would seem to be accounted for by interindividual differences in BAT capacity (volume). The wide interindividual differences in detectable BAT volume of activity (from to 31 to 329 ml) observed in young healthy men in the present study suggests that unknown factors may modulate BAT volume and thermogenic capacity in addition to age, sex, body mass index, and diabetes (24
). Furthermore, we found a significant inverse relationship between BAT volume of activity and shivering. Interestingly, BAT precursor cells are present in supraclavicular fat independently of the presence of spontaneous 18
FDG uptake (25
). Of note, a very recent publication demonstrated cold-induced increased blood flow in BAT that was associated with increased energy expenditure (26
). The latter is consistent with the present results, which demonstrate cold-induced BAT thermogenesis in humans.
Our demonstration of BAT oxidative metabolism with trivial rates of plasma glucose and NEFA utilization and rapid increase in BAT radio density during cold exposure is suggestive of increased intracellular triglyceride utilization as the main source of energy for BAT thermogenesis. Intracellular triglycerides are the main fuel to sustain BAT energy metabolism during cold exposure in animal models (2
). In room temperature–acclimated rats, short-term acute exposure to cold has been reported to lead to a near-complete depletion of BAT lipid (28
). A similar depletion of BAT lipid has been reported at necropsy in newborn infants and adults who died from hypothermia (27
). Intracellular brown adipocyte triglycerides represent half of the brown adipocyte volume (27
). From a mass of 168 g (the average BAT mass seen in the present study), one can predict a BAT fat content of approximately 84 g. Mobilization of one-third of that lipid reserve (28 g), which seems possible based on previous investigations (28
), would be sufficient to account for the extra energy (250 ± 45 kcal) expended during the 3-hour period of cold exposure in the present study. Although the 80% increase in total energy expenditure (TEE) that we observed during acute cold exposure appears important, it is therefore very likely that BAT thermogenesis accounted for an important fraction of this increase in TEE. Future studies will need to determine the contribution of intracellular triglycerides to BAT thermogenesis.
In addition, we cannot exclude the contribution of other tissues to cold-induced thermogenesis. We recorded increased glucose, NEFA, and acetate uptake in the longus colli. Nonetheless, the observation that significant cold-induced increase in 11C-acetate oxidative metabolism was only seen in BAT demonstrates a significant role of this tissue in the nonshivering thermogenic response to cold. The limited field of view of our PET/CT scanner (18 cm length) during dynamic acquisitions prevents us from assessing oxidative metabolism from the 11C-acetate method in other internal organs, such as the heart and other deep central muscles that might have been activated by cold. The number of subjects in the present study is small due to the complexity of our measurements. Despite this limitation, we are nonetheless confident that our findings apply to other populations, since they were very consistent between subjects. Finally, it is not possible to exclude an effect of the 2-hour daily time difference in 11C-acetate injection between the 2 protocols in the present study. To our knowledge, the effect of nycthemeral cycle on BAT metabolism has not been previously studied in humans.
The present findings support a role of BAT for nonshivering thermogenesis in humans with intracellular triglycerides as the main source of energy for this process, as observed in rodents. However, it remains to be demonstrated whether chronic and frequent bouts of cold exposure may contribute to increase BAT capacity and/or activity and may be a viable adjunct therapeutic strategy to other lifestyle interventions to prevent or treat obesity and its metabolic complications. It is also possible that energy substrate uptake by BAT could be substantially increased once intracellular triglyceride stores are depleted and/or BAT is fully cold adapted, as recently shown in rodents (22
). Quantitative assessment of the contribution of intracellular triglyceride oxidation in BAT thermogenesis awaits further methodological developments.
In summary, the present study demonstrates that a cold-exposure stimulus designed to minimize muscle-mediated shivering thermogenesis enhances BAT oxidative metabolism as well as glucose and NEFA uptake in adult humans. The enhanced BAT activity was associated with a 1.8-fold increase in whole-body energy expenditure. We found a significant inverse relationship between BAT volume of activity and shivering and a significant increase in BAT radio density within 3 hours of cold exposure, indicating rapid reduction in BAT triglyceride content. The present results demonstrate that BAT is undoubtedly involved in nonshivering thermogenesis in humans.