Almost all views about BAT thermogenesis have come from studies using small rodents such as the mouse, rat, and hamster. In humans, significant amounts of BAT are present in newborns and may contribute to body temperature regulation during the neonatal period, probably in the same way as in small rodents. However, BAT seems to disappear rapidly during postnatal periods and in adults is rather difficult to identify by conventional anatomical examinations. Thus, it has been a general contention that BAT is absent or of minute amounts and plays negligible, if any, roles in adult humans (6–8).

The existence of metabolically active BAT in adult humans has been suggested by the current clinical studies using fluoro-deoxyglucose (FDG)–positron emission tomography (PET), one of the powerful diagnostic tools for malignant tumors; that is, PET sometimes detects symmetrical FDG uptake in the shoulder and thoracic spine regions, where no tumor is present. By simultaneous examinations with PET and X-ray computed tomography (CT), the site of FDG uptake was identified as adipose tissue (9). Such FDG uptake is increased at lower environmental temperatures (10–12) and reduced by pretreatment with β-adrenergic blockers (13,14). These findings collectively suggest that the FDG uptake in adipose tissue at the specific regions reflects the metabolic activity of BAT (15).

However, almost all human studies thus far reported seemed for more accurate diagnosis of cancer and not for the detection and evaluation of BAT itself. In this study, we performed FDG-PET/CT examinations in adult healthy volunteers under a warm and cold condition to clarify the effects of cold exposure, age, season, and other body parameters including body fat. Our data indicate an unexpected high incidence of cold-activated BAT in adult healthy humans and suggest a role in the regulation of metabolic thermogenesis and body fat content.

During a 7-month period from August of 2006 to the following March, we performed a total of 71 FDG-PET/CT examinations under the 2-h cold condition for 56 healthy volunteers aged 23–65 years. As summarized in Table 2, cold-activated FDG uptake in the supraclavicular region was detected in 18 of 55 subjects (33%) in winter from January to March. Although the incidence of such FDG uptake seemed not to show apparent sex difference, it changed with age (P < 0.01 by χ² test), being 52% (16 of 31) and 8% (2 of 24) in younger (aged 23–35 years) and elderly (aged 38–65 years) subjects, respectively. Table 2 confirms again the stimulatory effect of acute cold exposure on FDG uptake as typically shown in Fig. 1; that is, eight subjects who showed clear FDG uptake under the cold condition were tested again under the warm condition at 28°C in 2 weeks, but none of them showed detectable FDG uptake in the adipose tissue, including supraclavicular and paraspinal regions, regardless of the test seasons.

Table 2 also suggests some seasonal variations of the total prevalence, being low in summer from August to September. This was clearly demonstrated from the results of some individual subjects; that is, in summer, the FDG uptake was detected in two of eight subjects. When the same subjects were examined again in winter, it was found in six subjects, four of which showed no FDG uptake in summer and the other two showed lower uptake in summer (Fig. 2B). Moreover, FDG uptake was sometimes detected both at the supraclavicular and paraspinal regions in winter but only in the supraclalvicular region in summer (Fig. 2A). Thus, the incidence and intensity of cold-activated FDG uptake showed seasonal variations, being higher in winter.

In small rodents, BAT thermogenesis is recognized as a significant component of whole-body energy expenditure and thereby contributes to the regulation of body fat content (1,2). To examine whether this is also the case in humans, we first analyzed the relationship between the cold-activated FDG uptake into BAT and adiposity in a total of 19 subjects bearing detectable BAT. As shown in Fig. 3, FDG uptake into the supraclavicular adipose tissue showed significant inverse relations to BMI (r = −0.674, P < 0.001), total fat (r = −0.564, P < 0.01), and visceral fat (r = −0.681, P < 0.001). Weaker but significant inverse relationships were also found to subcutaneous fat (r = −0.492, P < 0.05) and plasma insulin level (r = −0.473, P < 0.05) but not to plasma levels of leptin, adiponectin, and T3 (data not shown).

We also compared the similar parameters in subjects bearing detectable and undetectable amounts of cold-activated BAT (Table 3). As expected from the results in Table 2, the mean age was significantly lower in the group bearing detectable BAT than that without BAT. Body weight, BMI, and visceral and subcutaneous fat areas estimated from CT tended to be lower in the group bearing detectable BAT, but the difference was not statistically significant. Since these parameters of adiposity are much influenced by aging, being higher in elderly subjects, next, we compared the two groups of 32 younger subjects (aged 23–35 years) (Table 2). The mean age was comparable in the two groups (aged 27.3 vs. 29.1 years in male subjects, aged 30.3 vs. 29.5 years in female subjects); the effects of aging may be canceled. There was no difference in height, body weight, and blood parameters examined between the two groups in both male and female subjects. The group bearing detectable BAT tended to show decreased BMI, body fat content, and visceral fat area compared with that without BAT, but again the difference was not statistically significant (data not shown). In a separate study, we examined histologically an autopsy sample of fat depots obtained from the supraclavicular region and found numerous multilocular adipocytes expressing UCP1 protein (Fig. 4).

It has been reported in rodents that cold exposure markedly increased 2-deoxy-d-glucose (2-DG) uptake into BAT but only slightly into WAT (17–20). The stimulatory effects of cold exposure were mimicked by electrical stimulation of sympathetic nerves into BAT and β-adrenergic agonist administration but were abolished by surgical severing of the sympathetic nerves or β-adrenergic blockade.

We (21) demonstrated that the β-adrenergically stimulated 2-DG uptake into BAT is totally dependent on the activation of UCP1 and is thereby a metabolic marker of BAT. Our present findings of increased FDG uptake into adipose tissues of the specific regions after 2-h cold exposure are quite similar to those in rodents and thus support the idea that it reflects the activation of BAT and that adult humans have, more or less, metabolically active BAT. These seem consistent with previous case reports about patients with malignant lymphoma and some other types of tumors, where the incidence of FDG uptake in the supraclavicular region is higher when the examination was performed at lower room temperatures (10–12).

In our study, cold-activated FDG uptake was markedly increased when examined in winter compared with summer. Cohade et al. (10) reported that the incidence of FDG uptake into the supraclavicular region showed seasonal variations, being higher from January to March. It is known in mice and rats that chronic cold exposure (cold acclimation) results in hyperplasia of BAT and increased 2-DG uptake (1,18–20). Moreover, cold acclimation has been shown in rodents to induce an apparent transdifferentiation of WAT to BAT (22–24). Considering that the average outdoor temperature in our study was −1.6°C in winter and 21.3°C in summer, it is quite likely that the increased FDG uptake in winter is attributable to hyperplasia of BAT and/or transdifferentiation of WAT to BAT. Collectively, it can be concluded that the cold-activated FDG uptake in the supraclavicular and paraspinal regions is an index of the amount and activity of BAT.

It is to be noted that about half of younger (aged 23–35 years) subjects have metabolically active BAT, being quite against the widely accepted view that the amount and activity of BAT are negligible in adult humans. Although the prevalence of BAT was much decreased in elderly subjects (aged 38–65 years), the prevalence in our study is much higher than those previously reported in clinical studies (10), where FDG uptake in the shoulder region is detected in some patients (<10%) at relatively low room temperatures (~22°C) but not found at higher temperatures (>25°C). The reason for the high prevalence in our study must be that our subjects were kept at a lower temperature (19°C) and stimulated by intermittent ice cooling of legs. In other words, our condition of cold exposure may be more appropriate to detect BAT. More interesting in our results is the relationship between BAT and adiposity of the subjects; that is, cold-activated FDG uptake into BAT showed significant inverse relations to BMI, body fat, and visceral fat. Weaker but significant inverse relations were also found to subcutaneous fat and plasma insulin level.

This seems quite compatible with the observation in rodents that the amount and activity of BAT is decreased in obesity. There is ample evidence that BAT thermogenesis is a significant component of whole-body energy expenditure and thereby contributes to the regulation of energy balance and body fat content (1). Our present results thus imply a significant role of BAT in the control of adiposity via the regulation of energy expenditure in humans in the same way as in rodents. Being consistent with this idea, the subjects bearing detectable BAT tended to be low in BMI and body fat content compared with those without BAT, although the difference did not reach statistically significant levels.

Another point to be noted is that the incidence of cold-activated BAT decreased in elderly subjects. In a total of 24 elderly subjects, BAT was detected in only 2 male subjects (Table 2). It is interesting that these 2 male subjects are rather lean, with BMI 22.2 and 20.6 kg/m², respectively, compared with the other 10 male subjects bearing no BAT with BMI of 24.4 kg/m². Although these results are preliminary and are yet to be confirmed in larger numbers of subjects, they may imply that the disappearance of BAT accelerates the development of obesity with aging. This idea seems consistent with our previous observation that UCP1 deficiency in mice increases susceptibility to diet-induced obesity with age (25).