The metabolic function of the nuclear receptor peroxisome proliferator-activated receptor delta (PPAR(delta)) has been established by transfer of the PPAR(delta) gene into adipose tissue of mice in vivo and into adipocytes in culture. Investigators found that PPAR(delta) activation by such transfer leads to up-regulation of energy expenditure by fatty acid oxidation. PPAR(delta) activation also results in lowered serum triglyceride and free fatty acid levels and decreased lipid accumulation. In vivo activation of PPAR(delta) in adipose tissue protects against obesity and fatty liver in mice fed a high-calorie diet. PPAR(delta) also activates the heat-producing uncoupling enzymes in brown adipose tissue (UCP1 and 3) and muscle (UCP2).