OBEJECTIVE: To investigate whether the expression of uncoupling proteins (UCP2 and UCP3) was affected by a very low calorie diet (VLCD) and growth hormone (GH) treatment for 4 weeks. DESIGN: A randomized, placebo-controlled intervention study of VLCD with or without concomitant GH-treatment. SUBJECTS: Seventeen obese women (body mass index, BMI = 42.1 ± 1.4 kg/m2 (range 31.8-54.5 kg/m2)) treated with VLCD for 4 weeks and randomized to concomitant placebo treatment (n = 9) or GH treatment (n = 8). MEASUREMENTS: Fat mass and lean body mass were measured by dual-energy X-ray absorptiometry. Energy expenditure (EE) was measured by indirect calorimetry. UCP2 and UCP3 mRNA were measured in adipose tissue and skeletal muscle biopsies before VLCD and after VLCD ± GH-treatment by reverse transcription polymerase chain reaction (RT-PCR). RESULTS: VLCD treatment resulted in a mean weight loss of 5.23 kg ± 0.8 (P < 0.01). a 4.1% decrease in EE (P < 0.05) and a 24% decrease in UCP3 mRNA in adipose tissue (P < 0.03), whereas adipose tissue UCP2 mRNA and skeletal muscle UCP2 and UCP3 mRNA levels were unchanged. GH-treatment had no effects on EE, changes in body weight or UCP mRNA level. In multiple regression analysis the change in EE caused by VLCD was significantly correlated with changes in adipose tissue UCP2 mRNA (r = 0.66, P < 0.02) and a tendency towards a significant association with the change in adipose tissue UCP3 mRNA (r = 0.45, P = 0.09), but not with change in body weight, skeletal muscle UCP2 or UCP3 mRNA levels. CONCLUSION: VLCD for 4 weeks decreased UCP3 mRNA expression in human adipose tissue, whereas GH-treatment had no effect on UCP expression. Multiple regression analysis demonstrated that changes in adipose tissue UCP2 and probably UCP3 mRNA were correlated with the change in EE, These findings indicate that UCPs in adipose tissue in very obese individuals might play a role for the reduction in EE observed during energy restriction.