'Runners High': Here's Why You're So Happy When You Workout

"Fun" or "happiness" isn't the first word that comes to mind when when we think of exercise. Yet staying fit can boost endorphin levels, or neuropeptides, which help boost mood and lower rates of depression.

A new study published in the journal Cell Metabolism examines why people get "runner's high," the result of dopamine, a critical neurotransmitter for motivation.

"We discovered that the rewarding effects of endurance activity are modulated by leptin, a key hormone in metabolism. Leptin inhibits physical activity through dopamine neurons in the brain," said Stephanie Fulton, a researcher at the CRCHUM and lead author of the study.

In this recent study, researchers examined voluntary wheel running of mice in cages. The mice can run up to seven kilometres a day. In a laboratory setting, the physical activity of normal mice was compared with that of mice who underwent a genetic modification to suppress a molecule activated by leptin, STAT3. The STAT3 molecule is found in the neurons that synthesize dopamine in the midbrain.

"Mice that do not have the STAT3 molecule in the dopaminergic neurons run substantially more. Conversely, normal mice are less active because leptin then activates STAT3 in the dopamine neurons, signalling that energy reserves in the body are sufficient and that there is no need to get active and go looking for food," added Maria Fernanda Fernandes, first author of the study.

And is leptin as important for motivation to be active in humans? Yes. "Previous studies have clearly shown a correlation between leptin and marathon run times. The lower leptin levels are, the better the performance. Our study on mice suggests that this molecule is also involved in the rewarding effects experienced when we do physical exercise. We speculate that for humans, low leptin levels increase motivation to exercise and make it easier to get a runner's high," Fulton concluded.

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