Context. Uncoupling protein-1 [UCP-1] is the inner mitochondrial membrane protein that is a specific marker for and mediator of non-shivering thermogenesis in brown adipocytes.

Objective. This study was performed to better understand the putative thermogenic function of human epicardial fat.

Design. We measured the expression of UCP-1 and brown adipocyte differentiation transcription factors PR-domain-missing16 [PRDM16] and peroxisome-proliferator-activated receptor co-activator-1 [PGC-1] in epicardial, substernal and subcutaneous thoracic, abdominal and leg fat.

Setting. Tertiary care hospital cardiac center.

Patients. Coronary artery bypass (n = 44) and valve replacement (n = 6).

Interventions. Fat samples were taken at open heart surgery.

Results. UCP-1 expression was 5 fold higher in epicardial fat than substernal fat and barely detectable in subcutaneous fat. Epicardial fat UCP-1 expression decreased with age, increased with BMI, was similar in women and men and patients on and not on statin therapy and showed no relationship to epicardial fat volume or waist circumference. UCP-1 expression was similar in patients without and with severe coronary atherosclerosis and Metabolic Syndrome or Type 2 Diabetes. PRDM16 and PGC-1 expression was 2 fold greater in epicardial than subcutaneous fat. Epicardial fat UCP-1, PRDM16 and PGC1- mRNAs were similar in diabetics treated with thiazolidenediones compared to diabetics not treated with thiazolidenediones.

Conclusion: Since UCP-1 is expressed at high levels in epicardial fat as compared to other fat depots, the possibility should be considered that epicardial fat functions like brown fat to defend the myocardium and coronary vessels against hypothermia. This process could be blunted in the elderly.