For the study, researchers have carried out three experiments which revealed that daily exercise alone could be as effective in treating severe depression as antidepressant pills, the 'Daily Mail' reported.

The first experiment carried out in Portugal involving 150 depressed patients, showed that moderate exercise improved symptoms in those who hadn't responded well to drugs.

Participants kept taking their present medication, but half were also given aerobic exercises -- 30 to 45-minute walks, five days a week, for 12 weeks. Results showed a 26 per cent remission rate among patients in the exercise group compared with a zero remission rate in the others.

Another experiment of 400 volunteers in Brazil found that men who had the highest levels of physical activity were 68 per cent less likely to have symptoms of depression. Women who exercised had a 24 per cent lower risk.

In a third experiment, at University of Rochester in New York, more than 1,000 people were equipped with monitors that measured physical movements, and the researchers found that the more active individuals were, the lower the levels of depressive symptoms.

According to them, one theory is that those diagnosed with depression have higher levels of inflammation in their body tissues; and previous studies have shown when depression is successfully treated, inflammation levels drop.

Dr Glenn Wilson, of Gresham College, London, said: "Exercise can make a difference for some people."