Adults with active brown fat show ‘distinct fuel metabolism’ after cold exposure
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A small cohort of healthy adults with brown adipose tissue expended 15% more energy after a cold exposure challenge compared with adults who did not have brown adipose tissue, according to findings published in The Journal of Clinical Endocrinology & Metabolism.
“We found that people with active brown fat compared with those without have a very distinct fuel metabolism,” Florian W. Kiefer, MD, PhD, associate professor of medicine at the division of endocrinology and metabolism at the Medical University of Vienna, told Healio. “They burn more energy in the cold and have a more favorable fatty acid composition in the blood.”
Kiefer and colleagues analyzed data from 16 age- and BMI-matched healthy volunteers from the same geographical region previously identified as brown adipose tissue-positive (n = 8) or brown adipose tissue-negative (n = 8) during a cross-sectional brown adipose tissue imaging study. Researchers invited the participants for a follow-up study to analyze total cold-induced energy expenditure after a single bout of cold exposure.
Participants were examined during three separate study visits in the morning after an overnight fast. For the first visit, participants underwent the first PET scan at room temperature to detect any basal brown fat activity. For the second visit, participants underwent a second PET scan after cold exposure, using a personalized cooling protocol with a water-perfused cooling vest (CoolShirt Systems) to determine cold-induced brown fat activity.
“The water temperature was kept slightly above the shivering temperature, and muscle activity was monitored by electromyography,” the researchers wrote.
For the third visit, participants underwent repeated indirect calorimetry before, during and after cold exposure to determine the magnitude and duration of cold-induced energy expenditure. All examinations started between 8 a.m. and 9 a.m. and followed a 10-hour fast.
Researchers found that brown adipose tissue is the “major determinant” of cold-induced energy expenditure, as only adults who were brown adipose tissue-positive experienced significantly increased energy expenditure in response to cold.
“At baseline, resting energy expenditure and the respiratory quotient were similar in both groups,” the researchers wrote. “However, cold-induced energy expenditure increased significantly in brown adipose tissue-positive adults, reaching a maximum during the 90-minute cold exposure period. In contrast, energy expenditure was unaltered in brown adipose tissue-negative adults in response to cold, suggesting that brown adipose tissue status is the primary determinant of cold-mediated energy consumption.”
After the participants were exposed to cold, researchers found that cold-induced energy expenditure slowly declined to baseline levels within 150 minutes among those who were brown adipose tissue-positive; however, those who were brown adipose tissue-negative did not experience any changes in energy expenditure during the test period.
“The incremental area under the curve of energy consumption during cooling and rewarming was approximately 20 kcal per day in brown adipose tissue-positive adults,” the researchers wrote.
In an analysis of cold-mediated changes in lipid species, the researchers also found that presence of brown adipose tissues was associated with a cold-induced change in 11 lipid molecules, such as the polyunsaturated fatty acids docosahexaenoic acid and docosapentaenoic acid.
“These data support the recent concept that targeting brown fat in humans may have therapeutic potential in the fight against metabolic complications such as obesity and diabetes,” Kiefer said. “More large-scale clinical studies are needed to evaluate the role of brown fat activation in the prevention of obesity and related sequelae. The identification of potential pharmacological targets for increasing brown fat activity and/or transforming white fat into brown fat should be a high priority.” – by Regina Schaffer
For more information:
Florian W. Kiefer, MD, can be reached at the Clinical Division of Endocrinology and Metabolism, Department of Medicine III, Medical University of Vienna, 1090 Vienna, Austria; email: florian.kiefer@meduniwien.ac.at.
Disclosures: The authors report no relevant financial disclosures.
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Kirsi A. Virtanen, MD, PhD
These findings are interesting but not surprising; they are expected based on research in this area published previously. However, it was interesting that the researchers compared people with high brown fat activity vs. no brown fat activity, and matched volunteers for age and BMI. This is important because BMI impacts brown fat activity significantly. That said, it is not clear, among lean adults, why some have active brown and others not. There are different etiologies behind this.
Of course, every study has its limitations, and this study has a very small sample size. There is also debate about whether women have more active brown fat vs. men, and this remains an open question. If women would have more active brown fat than men — as the PET image in this article may suggest — women with a BMI similar to men have higher whole body fat mass. Thus, it is reasonable to consider fat free mass, and that is something we have done with our study population assessing brown adipose tissue function in humans (Orava J, et al. Obesity. 2013; doi:10.10.102/oby.20456.). However, the between-group differences in polyunsaturated fatty acid levels reported here were important, indicating more favorable lipid profile among people with highly active brown fat. In animal models, researchers have demonstrated brown fat is capable of attracting lipids from the bloodstream. If human brown fat is also capable of “clearing” lipids from the blood, that could prove very beneficial for metabolic health.
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