Brown adipose tissue is present in adults and appears to play an active role in metabolism, rather than being vestigial and without physiologic purpose, according to three reports.
Both the presence and the activity of reservoirs of brown adipose tissue seem to be inversely associated with overall adiposity and with measures of the metabolic syndrome, the three studies' investigators said.
“These studies … are a powerful proof of concept that this tissue might be used as a target for interventions, pharmacologic and environmental, aimed at modulating energy expenditure,” Dr. Francesco S. Celi of the National Institute of Diabetes and Digestive and Kidney Diseases said in an accompanying editorial (N. Engl. J. Med. 2009;360:1553-6).
Brown adipose tissue helps newborns maintain body temperature but has been thought to regress with age until it is all but lost in adulthood. It was thought to have marginal physiological relevance after infancy. But recent studies using combined PET and CT techniques have suggested that pockets of brown adipose tissue are present in some adults and may be metabolically active in response to cold exposure.
In the first report, Dr. Kirsi A. Virtanen of the University of Turku, Finland, and associates scanned and sampled tissue from putative adipose pockets in five healthy volunteers using PET-CT scanning after cold provocation. Previous research had identified the supraclavicular area as the most prominent site for deposits of brown adipose cells.
Glucose uptake in these pockets increased by a factor of 15 upon exposure to cold, compared with glucose uptake in adjacent white adipose tissue. The pockets of fat also expressed several markers of brown, but not white, adipose tissue, and the biopsy samples showed multilocular, intracellular lipid droplets characteristic of brown fat.
“These findings constitute direct identification of functional human brown adipose tissue,” the researchers said. “We speculate that in humans, activated brown adipose tissue has the potential to contribute substantially to energy expenditure,” they added (N. Engl. J. Med. 2009;360:1518-25).
In the second study, Dr. Aaron M. Cypess of the Joslin Diabetes Center, Boston, and associates analyzed PET-CT images from 1,972 patients who had undergone whole-body scanning for a variety of diagnostic reasons in 2003-2006. They focused on the supraclavicular region after finding that brown adipose tissue had been present in samples from 34 previous patients who had undergone neck surgery for different indications.
They found substantial pockets of brown adipose tissue in 7.5 % of women and 3.1% of men, but noted that this likely underestimates the actual prevalence in the general population because the tissue was not being stimulated by exposure to cold at the time of scanning in these subjects (N. Engl. J. Med. 2009;360:1509-17).
Dr. Cypess and his colleagues found an inverse correlation between the metabolic activity of brown adipose tissue and subject age, body mass index, and fasting plasma glucose level.
In the third report, Wouter D. van Marken Lichtenbelt, Ph.D., and associates at Maastricht (the Netherlands) University Medical Center used PET-CT to assess cold-stimulated activity of brown adipose tissue in relation to metabolic measures in 24 healthy men. Ten of the men were lean and 14 were overweight or obese.
Brown adipose tissue was present in all but the most obese of these men, a 95% rate that is much higher than has previously been reported. The tissue's activity, measured by energy expenditure at rest and core vs. surface body temperatures, correlated inversely with BMI and percentage of body fat.
Dr. Cypess reported receiving grant support from the Eli Lilly Foundation and being the sole inventor on a pending patent application to use infrared thermography to monitor brown adipose tissue. His study was supported in part by Pfizer, Merck, and the Eli Lilly Foundation. No conflicts of interests were reported by the other researchers.