Obesity results from an imbalance between energy intake and expenditure. In rodents

and newborn humans, brown adipose tissue helps regulate energy expenditure by

thermogenesis mediated by the expression of uncoupling protein 1 (UCP1), but brown

adipose tissue has been considered to have no physiologic relevance in adult humans.

tomographic and computed tomographic (PET–CT) scans performed for various

diagnostic reasons in 1972 patients for the presence of substantial depots of putative

brown adipose tissue. Such depots were defined as collections of tissue that were

more than 4 mm in diameter, had the density of adipose tissue according to CT,

indicating high metabolic activity. Clinical indexes were recorded and compared

with those of date-matched controls. Immunostaining for UCP1 was performed on

biopsy specimens from the neck and supraclavicular regions in patients undergoing

surgery.

Substantial depots of brown adipose tissue were identified by PET–CT in a region

extending from the anterior neck to the thorax. Tissue from this region had UCP1-

immunopositive, multilocular adipocytes indicating brown adipose tissue. Positive

scans were seen in 76 of 1013 women (7.5%) and 30 of 959 men (3.1%), corresponding

to a female:male ratio greater than 2:1 (P<0.001). Women also had a greater mass

detection of brown adipose tissue was inversely correlated with years of age (P<0.001),

outdoor temperature at the time of the scan (P = 0.02), beta-blocker use (P<0.001), and

among older patients, body-mass index (P = 0.007).

Defined regions of functionally active brown adipose tissue are present in adult humans,

are more frequent in women than in men, and may be quantified noninvasively

tissue is inversely correlated with body-mass index, especially in older people,

suggesting a potential role of brown adipose tissue in adult human metabolism.