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A correction has been published: N Engl J Med 2009;360(18):1917.
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Background Studies in animals indicate that brown adipose tissue is important in the regulation of body weight, and it is possible that individual variation in adaptive thermogenesis can be attributed to variations in the amount or activity of brown adipose tissue. Until recently, the presence of brown adipose tissue was thought to be relevant only in small mammals and infants, with negligible physiologic relevance in adult humans. We performed a systematic examination of the presence, distribution, and activity of brown adipose tissue in lean and obese men during exposure to cold temperature. Brown-adipose-tissue activity was studied in relation to body composition and energy metabolism.
Methods We studied 24 healthy men — 10 who were lean (body-mass index [BMI] [the weight in kilograms divided by the square of the height in meters], <25) and 14 who were overweight or obese (BMI,
Results Brown-adipose-tissue activity was observed in 23 of the 24 subjects (96%) during cold exposure but not under thermoneutral conditions. The activity was significantly lower in the overweight or obese subjects than in the lean subjects (P=0.007). BMI and percentage of body fat both had significant negative correlations with brown adipose tissue, whereas resting metabolic rate had a significant positive correlation.
Conclusions The percentage of young men with brown adipose tissue is high, but its activity is reduced in men who are overweight or obese. Brown adipose tissue may be metabolically important in men, and the fact that it is reduced yet present in most overweight or obese subjects may make it a target for the treatment of obesity.
25) — under thermoneutral conditions (22°C) and during mild cold exposure (16°C). Putative brown-adipose-tissue activity was determined with the use of integrated 18F-fluorodeoxyglucose positron-emission tomography and computed tomography. Body composition and energy expenditure were measured with the use of dual-energy x-ray absorptiometry and indirect calorimetry.
Cold-induced thermogenesis and diet-induced thermogenesis have recently been shown to be related, suggesting that they have a similar underlying mechanism.6 In small mammals, brown adipose tissue serves as a thermogenic organ, in which mitochondrial respiration is uncoupled from ATP production (through the action of uncoupling protein 1 [UCP1]) to dissipate energy. Although several early anatomical studies suggested that brown adipose tissue is present in adult humans,7,8,9 its physiologic relevance was believed to be marginal for most.9,10 In the past few years, however, studies conducted for other reasons that made use of integrated positron-emission tomography and computed tomography (PET–CT) indicated that brown adipose tissue is occasionally present and active in adult humans,11,12,13,14 with the prevalence ranging from 2.5%11 to 45%.15 Some studies indicate that brown adipose tissue is related to the body-mass index (BMI),15,16 and others do not.12,17
We systematically examined the presence, distribution, and activity of brown adipose tissue in healthy volunteers in relation to body composition and energy metabolism. The activity of brown adipose tissue was assessed with the use of PET–CT scanning with 18F-fluorodeoxyglucose (18F-FDG), body composition with dual-energy x-ray absorptiometry (DEXA), and energy expenditure with indirect calorimetry. Brown adipose tissue was activated by a standardized cold-exposure test.
Methods
Subjects and Procedures
Between October 2007 and December 2008, we studied 24 healthy male volunteers; 10 were classified as lean (BMI [the weight in kilograms divided by the square of the height in meters], <25), and 14 as overweight or obese (BMI, 25) (Table 1). The ethics committee of Maastricht University approved the protocol. All subjects provided written informed consent. In addition, a female patient provided written informed consent for the removal of samples of brown adipose tissue and white adipose tissue during surgery for multinodular goiter.
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The PET–CT scanning protocol (Gemini TF PET–CT, Philips) included confirmation of the serum glucose level and the intravenous injection of 2 mCi (74 MBq) of 18F-FDG. Subjects rested comfortably, with the head, neck, and shoulders supported from the outset of the experiment until imaging began. Imaging was performed in three-dimensional mode, with emission scans of 6 minutes per bed position. Six to seven bed positions per subject were needed to cover the areas where brown adipose tissue is usually found. Imaging started with a low-dose CT scan (30 mAs), immediately followed by a PET scan. The scanner was equipped with time-of-flight electronics, which allows the use of a relatively low amount of radioactivity (2 mCi). The resulting total radiation dose from the low-dose CT scan and the injected radioactive tracer was approximately 2.8 mSv. The CT scan was used for attenuation correction and localization of the 18F-FDG uptake sites. Both image sets were reconstructed in transaxial, coronal, and sagittal images with a slice thickness of 4 mm. Two nuclear-medicine physicians interpreted the PET–CT images using PMOD software, version 2.85 (PMOD Technologies, May 2007). Brown-adipose-tissue activity (measured in kilobecquerels) and brown-adipose-tissue volume (measured in cubic centimeters) were quantified in the region of interest by autocontouring the brown-adipose-tissue areas with a set threshold.
Energy expenditure was measured by means of a respiratory gas analyzer with the use of a ventilated hood system (Oxycon, Jaeger). The resting metabolic rate was corrected for fat-free mass by means of linear regression, since 80% of individual differences in energy expenditure can be explained by fat-free mass.18 The residuals of the regression between energy expenditure and fat-free mass were added to the average energy expenditure. Skin temperatures were measured continually at 16 positions by means of iButtons (type DS1921H, Maxim).19 The mean skin temperature was calculated with a 14-point method (ISO-9886).20 Underarm–fingertip temperature gradients were calculated to obtain an estimate of blood flow in the fingers.21 Core temperature was measured in the small intestine with the use of a telemetric pill (HQ). In three subjects (two lean and one overweight), the core-temperature measurement failed.
The protocol for temperature (mildly cold, 16°C) was validated earlier.22 Every 15 minutes, each subject was asked whether he had experienced shivering. Body composition was measured with the use of a DEXA whole-body scanner (QDR 4500A, Hologic).
To ascertain whether brown adipose fat in the supraclavicular region could be identified with classic microscopy and an immunofluorescence assay for UCP1, samples were obtained during surgery for a multinodular goiter in a 46-year-old woman with normal thyroid function (body mass, 72 kg; height, 1.72 m; thyroid-stimulating hormone, 1.7 mU per milliliter; and free thyroxine, 12.8 pmol per liter [1.0 ng per deciliter]). Biopsy samples of both brown and white adipose tissues were obtained from the supraclavicular region on the right side. In this area, both fat types were observed in patches measuring approximately 0.5 cm3. Sections (5 µm thick) were fixed for 15 minutes with 3.7% formaldehyde in phosphate-buffered saline, and UCP1 was detected with the use of rabbit polyclonal human UCP1 antiserum and with goat antirabbit fluorescein isothiocyanate as the secondary antibody, together with the nuclei-staining dye 4',6-diamidino-2-phenylindole (0.5 mg per milliliter).
Statistical Analysis
Data are reported as means ±SD. Data analysis was performed with the use of SPSS software, version 11, and Statview SE for MacIntosh (SAS Institute). Comparisons were made by means of a paired t-test (for findings under thermoneutral conditions as compared with mild cold exposure) and an unpaired t-test (for findings in lean men as compared with overweight or obese men). Linear regression analyses were conducted to identify correlations between variables. For the relation between brown-adipose-tissue activity and body composition, exponential curve fitting was applied; therefore, log (brown-adipose-tissue activity) was plotted against body composition (BMI or percentage of body fat). Cook's distance values that were larger than 0.5 and Studentized deleted residuals larger than 4.0 were considered to be outliers. P values of less than 0.05 were considered to indicate statistical significance.
Results
Twenty-three of the 24 subjects had definite, albeit highly variable, amounts of 18F-FDG activity in the neck, supraclavicular region, chest, and abdomen (Figure 1A). On the fused PET–CT images, the activity was located not in muscle tissue but in fat tissue, which was therefore considered to be brown adipose tissue (Figure 1B through 1D). All lean subjects and 13 of the 14 overweight or obese subjects had evidence of brown-adipose-tissue activity in response to cold exposure. Mean brown-adipose-tissue activity was significantly lower in the overweight or obese subjects (102±93 kBq) than in the lean subjects (428±394 kBq) (P=0.007) (Table 2). The volume of brown adipose tissue tended to be lower in the overweight or obese subjects than in the lean subjects, but the difference was not significant (P=0.14). Only one subject had no 18F-FDG uptake in areas of adipose tissue. This subject had the highest BMI (38.7) and percentage of body fat (41.8%). In all subjects with brown-adipose-tissue activity, the area with the most evidence of activity was the supraclavicular region (Figure 1). However, other sites also had substantial putative brown-adipose-tissue activity.
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Studies of brown adipose tissue obtained from the supraclavicular region during surgery in the patient with multinodular goiter revealed the presence of UCP1 (Figure 2). The protein was not detected in white adipose tissue taken from the same region in this patient.
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Nonlinear regression analysis showed that the activity of whole-body brown adipose tissue was negatively correlated with BMI (r=–0.60, P=0.002) and percentage of body fat (r=–0.60, P=0.001) (Figure 3A and 3B). There was no correlation between brown-adipose-tissue activity and age, and in a stepwise regression analysis, BMI but not age was significantly associated with brown-adipose-tissue activity.
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We observed a very high rate of occurrence of brown adipose tissue in normal young men (>95%). In tissue samples taken from the supraclavicular area of a surgical patient, an immunofluorescence assay revealed the presence of UCP1, confirming the presence of brown adipose tissue (the supraclavicular area of the body is the primary location for active brown adipose tissue). Moreover, the results of PET–CT scanning performed just after the subjects had spent 2 hours in thermoneutral conditions revealed no brown-adipose-tissue activity, indicating that brown adipose tissue is activated by cold exposure.
Our study showed that brown-adipose-tissue activity was significantly lower in overweight or obese male subjects than in lean male subjects. There was a significant negative relation between brown-adipose-tissue activity and BMI as well as between brown-adipose-tissue activity and percentage of body fat. However, only one obese subject had no brown-adipose-tissue activity. This finding indicates that it may be possible to increase brown adipose tissue in most overweight or obese men, and we speculate that methods of increasing brown adipose tissue in such men may be of therapeutic interest. Studies of the effects of regular exposure to cold, weight-loss diets, and increased physical activity on brown-adipose-tissue activity will also be of interest.
In our study, the rate of detection and level of activity of brown adipose tissue were much higher than those in earlier studies. The high detection rate in the current study is most likely due to the use of a standardized test for cold exposure. Previous studies measured brown adipose tissue in patients with cancer and related the occurrence of brown adipose tissue to outside temperatures or the season.23 One early study used ephedrine rather than cold to stimulate brown-adipose-tissue activity in men.9 However, that study focused on perirenal brown adipose tissue, assuming that all depots have the same level of activity.
Our study showed that in all the subjects, the level of brown-adipose-tissue activity was highest in the supraclavicular region. However, other sites with substantial brown-adipose-tissue activity were evident. Most sites have been described in previous autopsy studies8 or in studies using PET–CT scanning with 18F-FDG.14 A second reason for the high incidence of brown-adipose-tissue activity in our study is the relatively young age of our subjects, ranging from 18 to 32 years. It is likely that the amount of brown adipose tissue decreases with age,17 although no relation between brown-adipose-tissue activity and age was observed in our study.
The relation between brown-adipose-tissue activity and BMI observed in our study is similar to the results of previous studies, which showed that the level of brown-adipose-tissue activity was highest in subjects with a low BMI.15,16 However, other studies, which were retrospective and were not designed to assess brown-adipose-tissue activity, did not confirm this finding.12,17
Brown-adipose-tissue activity was negatively correlated with a change in distal skin temperature, suggesting that the activity during cold exposure was involved in the immediate response to cold temperatures. Thus, subjects with the highest level of brown-adipose-tissue activity had the lowest drop in distal temperature, suggesting that these subjects were able to maintain core body temperature through an increase in body heat. However, contrary to our expectation, there was no significant relation between brown-adipose-tissue activity and cold-induced thermogenesis. The most likely reason is that other tissues, such as skeletal muscle,1 may be involved in the thermogenic response. Indeed, we recently found that mitochondrial uncoupling in skeletal muscle is significantly related to cold-induced adaptive thermogenesis in humans.24 Given the findings of an earlier study, in which subjects were exposed to cold for 1 hour,2 we expected less cold-induced thermogenesis in the group of overweight or obese subjects than in the lean group, but the groups did not differ significantly in this respect. Even though the overweight or obese subjects had an increase in thermogenesis on exposure to cold that was similar to the increase in the lean subjects, they had less brown-adipose-tissue activity (or none).
Recent studies conducted at the molecular level have identified a mechanism by which PR domain containing 16 (PRDM16), a zinc-finger protein selectively expressed in brown adipose tissue, can induce gene expression in brown adipose tissue while suppressing it in white adipose tissue, thereby controlling the induction of brown adipose tissue.25,26 Moreover, there is recent evidence that brown adipose tissue is more closely related to skeletal muscle than to white adipose tissue.27,28 A putative shared lineage between brown adipose tissue and skeletal muscle may provide clues about the origins of stem cells that lead to the formation of brown fat and to adaptive thermogenesis.
Our study suggests that the relatively high level of brown-adipose-tissue activity in young men is related to energy metabolism. Further work is needed to understand brown-adipose-tissue activity in relation to skeletal-muscle metabolism; ideally, this work will be conducted along with molecular studies that may delineate the relative contribution of each to adaptive thermogenesis.
No potential conflict of interest relevant to this article was reported.
We thank Loek Wouters, Paul Schoffelen, and Emiel Beijer for their technical support; David Creytens and Gert Schaart for the analyses of biopsy specimens; and the members of our Temperature Literature Club, especially Sander Wijers, for helpful suggestions.
Source Information
From the Department of Human Biology, Nutrition and Toxicology Research Institute Maastricht (W.D.M.L., J.W.V., J.M.A.F.L.D., P.S.), the Department of Nuclear Medicine (N.M.S., G.J.K., G.J.J.T.), and the Department of General Surgery (N.D.B.), Maastricht University Medical Center, Maastricht, the Netherlands.
Address reprint requests to Dr. van Marken Lichtenbelt at the Department of Human Biology, Maastricht University, P.O. Box 616, 6200 MD, Maastricht, the Netherlands, or at markenlichtenbelt{at}hb.unimaas.nl.
References
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Related Letters:
The Importance of Brown Adipose Tissue
Timmons J. A., Pedersen B. K., Stefan N., Pfannenberg C., Häring H.-U., Villarroya F., Domingo P., Giralt M., Jacene H. A., Wahl R. L., Lee P., Ho K. K.Y., Fulham M. J., Sacks H. S., van Marken Lichtenbelt W. D., Schrauwen P., Teule G.J. J., Cypess A. M., Kahn C. R., Enerbäck S., Oksi J., Nuutila P.
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N Engl J Med 2009;
361:415-421, Jul 23, 2009.
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