Objectives: The involvement of skeletal muscle mitochondrial uncoupling protein-3 (UCP3) in the control of energy expenditure in skeletal muscle and at the whole-body level is still a matter of debate. We previously reported that UCP3 downregulation is linked to an enhanced mitochondrial energy metabolism in rat skeletal muscle as a result of acute capsiate treatment. Here, we aimed at investigating noninvasively the effects of chronic capsiate ingestion on metabolic changes occurring in exercising gastrocnemius muscle and at the whole-body level. Methods: We used an original experimental setup allowing a complete noninvasive investigation of gastrocnemius muscle function in situ using 31-phosphorus magnetic resonance spectroscopy. Whole-body fat composition was determined using magnetic resonance imaging and UCP3 gene expression was measured by quantitative real-time RT-PCR analysis. Results: We found that a 14-day daily administration of capsiate (100 mg kg À1 body weight) reduced UCP3 gene expression and increased phosphocreatine level at baseline and during the stimulation period in gastrocnemius muscle. During muscle stimulation, pH i showed a larger alkalosis in the capsiate group suggesting a lower glycolysis and a compensatory higher aerobic contribution to ATP production. Although the capsiate-treated rats were hyperphagic as compared to control animals, they showed a lower weight gain coupled to a decreased abdominal fat content. Conclusion: Overall, our data indicated that capsiate administration contributes to the enhancement of aerobic ATP production and the reduction of body fat content coupled to a UCP3 gene downregulation.