Quantitative assessment of oxidative metabolism in brown fat

Objectives The presence of functional brown adipose tissue (BAT) depots in humans has been recently demonstrated by the uptake of F18-deoxyglucose (FDG). However, it is unclear whether uptake of FDG in BAT represents an increase in oxidative metabolism (i.e. thermogenesis) and, if so, whether BAT oxidative metabolism contributes to the overall metabolic rate of young adults.

Methods Our study was designed to include 20 young adults who undergo dynamic oxygen-15 (O15) water injection and O15 oxygen inhalation PET imaging on an EXACT HR PET scanner. Coinciding with a short (10s) inhalation of O15 labeled oxygen (80 mCi), a dynamic scan (60 x 2s) is initiated. This is followed by an injection of O15 water (50 mCi) and dynamic imaging (60 x 2s). These two scans are then repeated during cold stress. To induce cold stress, subjects are exposed to a temperature of 60 degrees F in minimal clothing. Subsequently, FDG (5 mCi) is injected and following a 40min uptake period during cold stress, a 2-bed position scan is performed on a GE DSTE PET/CT scanner. ROIs representing BAT and muscle tissue are defined on CT images and transferred to coregistered dynamic O15 scans, moreover the arterial input function is derived from a small ROI over the aortic arch. A 10min calorimetric measurement is performed prior to PET imaging as well as during the uptake period to obtain calorie consumption at rest and during cold stress.

Results Initial studies in 2 young adult males (24, 34 yrs) showed BAT activation (as determined based on FDG uptake) only in the older subject. In this subject, the blood flow in BAT increased from 10 to 22 ml/100g/min with no change in muscle blood flow (5.6 vs 4.5 ml/100g/min). A relatively large increase in MRO from 0.73 to 1.40 ml/100g/min was accompanied by only a small increase in the daily calorie consumption (1915 to 2164 kcal/day).

Conclusions This study establishes the magnitude of BAT thermogenesis in individuals and may lead to a better understanding of the underlying mechanism of BAT oxidative metabolism, possibly resulting in more effective approaches to weight control in obese subjects