Leptin and endurance exercise: implications of adiposity and insulin*

Obesity currently is qualified as a worldwide health epidemic and its consequences include diabetes mellitus as far as cardiac disease. Genetic and environmental factors contribute to obesity, although the genetic component is still poorly understood in humans. With the cloning of mouse ob gene and its receptor, leptin was discovered, the “satiety hormone”. Leptin is expressed and secreted primarily by adipose tissue and is highly correlated to body fat mass. Nevertheless, many factors can regulate leptin synthesis and expression, such as fasting, sympathetic activity, insulin, exercise and changes in energy balance. Aerobic physical activity effects on leptin are still not very clear, seeing that there are contradictory studies about its effects on leptin regulation. Transversal studies suggest that leptin concentrations are not acutely affected after an exercise bout. However, reductions in leptin concentrations are observed following extreme bouts of exercise such as ultramarathons, where the extenuating physical activity induces a deficit in energy balance. Also, long-term (≥ 60 min) exercise seems to be associated with a delayed reduction in leptin concentrations 48 hr after the exercise bout, possibly due to an energy imbalance. Some longitudinal studies show that aerobic exercise training does not affect leptin levels, others that any changes in leptin levels are due to possible changes in body fat, and, lastly, some studies show a reduction in leptin levels and/or expression independently of any changes in adiposity. That suggests there may be other factors besides adiposity that regulate the reduction in leptin levels after exercise training, being insulin the main candidate for such role. Therefore, this review analyses the main aspects of leptin, its action, function and regulation, its association with insulin, and also the effects of acute and chronic endurance exercise on leptin synthesis and secretion and possible implications of insulin and adiposity.