NEJM: PET/CT is newest weapon in adult obesity, diabetes battle
Using FDG PET/CT, researchers have shown the presence of brown adipose tissue in adults is inversely correlated with body-mass index, and can burn large numbers of calories when triggered by cold temperatures, according to a study in this week's New England Journal of Medicine.

In rodents and newborn humans, the presence of brown adipose tissue helps regulate energy expenditure through thermogenesis mediated by the expression of uncoupling protein 1 (UCP1). Historically, brown adipose tissue has been considered to have no physiologic relevance in adults.

C. Ronald Kahn, MD, from the section on obesity and hormone action at Joslin Diabetes Center in Boston, and colleagues analyzed 3,640 consecutive 18F-fluorodeoxyglucose (18F-FDG) PET/CT scans performed for various diagnostic reasons in 1,972 patients for the presence of substantial depots of putative brown adipose tissue.

Depots were defined as collections of tissue that were more than 4 mm in diameter, had the density of adipose tissue according to CT, and had maximal standardized uptake values of 18F-FDG of at least 2 g per milliliter, 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.

The researchers reported that PET/CT identified substantial depots of brown adipose tissue 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 1,013 women and 30 of 959 men, corresponding to a female:male ratio greater than 2:1. Women also had a greater mass of brown adipose tissue and higher 18F-FDG uptake activity.

The probability of the detection of brown adipose tissue was inversely correlated with years of age, outdoor temperature at the time of the scan, beta-blocker use, and among older patients, body-mass index, according to the authors.

The researchers concluded that the use of 18F-FDG PET/CT shows that functional brown adipose tissue is prevalent in adults, with a significant female predominance.

"We are hopeful that with increasing capability to measure the mass and activity of brown adipose tissue in humans in vivo, we will better understand its role in physiology and its potential as a therapeutic target in the treatment of obesity and other metabolic disorders," Kahn and colleagues wrote.