Leptin is a 167-amino-acid product of the ob gene, which is produced primarily by adipocytes. Although originally associated with the central regulation of satiety and energy metabolism, there is now increasing evidence that leptin may be an important mediator of several peripheral physiological phenomena. Although there is a clear relationship between body mass index (BMI) and plasma leptin concentrations, there are great individual variations indicating that other factors than fat mass regulates the production. How do life style parameters like physical activity affect leptin?Several studies have been performed looking at the effect of exercise, both after single events and after long-term exercise training with duration up to one year, on circulating leptin levels. The results suggest that there might be a reduction in circulating leptin levels after a single bout of exercise, but this reduction is seen after a 24-48 hour period and is related to energy imbalance. Long term training may alter leptin levels, but there are disparate findings. Some studies report no effect of training on leptin concentrations when correcting for fat-/weight loss, while others do observe a reduction in leptin concentrations also after adjusting for changes in fat mass. The different studies vary greatly in number of individual included, gender and the overall experimental design, and only a few studies have an individual control group. My overall conclusion is that exercise reduces circulating levels of leptin, however the effect is linked to energy status and metabolism.