Exercising means more than strong muscles and bones: it means a much better health. Exercising fights against impotence, high cholesterol, diabetes, heart disease, bone diseases, arthritis, cancer, while boosting mood, memory and longevity.

Physical exercises are also known to have an effect similar to that of antidepressants, fact proven by many researches. There are many hypotheses, including a supposed rise in serotonin ("happiness" hormone) levels. Now, Astrid Bjørnebekk at Karolinska Institutet has come with a new explanation for this: exercise stimulates the formation of new neurons (brain cells).

She has looked in a series of researches for the underlying biological mechanisms explaining why exercise can act as a therapy against depression, comparing it to the pharmacological treatment for an SSRI drug. She made the tests on lab rats. The results revealed that both exercise and antidepressants sped up the production of new cells in a brain zone linked to memory and learning processes.

Bjørnebekk's studies reached the same results as previous researches did, and she has come with a model to explain how exercise can induce an antidepressant effect in mild to moderately severe clinic depression, pointing out that exercise is a very good complement to chemicals.

"What is interesting is that the effect of antidepressant therapy can be greatly strengthened by external environmental factors," she said.

Previous researches revealed that drug abusers have decreased amounts of the dopamine D2 receptor in the brain's reward system, and it has been thought that this may be greatly involved in the depressive symptoms experienced often by drug abusers. These rat researches demonstrated that genetic factors may influence how external environmental factors can tune the amount of the brain's dopamine D2 receptor.

"Different individuals may have differing sensitivity to how stress lowers dopamine D2 receptor levels, for example. This might be significant in explaining why certain individuals develop depression more readily than others," she said.