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People who take regular exercise during their free time are less likely to have symptoms of depression and anxiety, a study of 40,000 Norwegians has found.

But physical activity which is part and parcel of the working day does not have the same effect, it suggests.

Writing in the British Journal of Psychiatry, the researchers said it was probably because there was not the same level of social interaction.

The charity Mind said that exercise and interaction aids our mental health.

Higher levels of social interaction during leisure time were found to be part of the reason for the link.

Researchers from the Institute of Psychiatry at King's College London teamed up with academics from the Norwegian Institute of Public Health and the University of Bergen in Norway to conduct the study.

Participants were asked how often, and to what degree, they undertook physical activity in their leisure time and during the course of their work.

Researchers also measured participants' depression and anxiety using the Hospital Anxiety and Depression Scale.

People who were not active in their leisure time were almost twice as likely to have symptoms of depression compared to the most active individuals, the study found.

But the intensity of the exercise did not seem to make any difference.

Lead researcher Dr Samuel Harvey, from the Institute of Psychiatry, said: "Our study shows that people who engage in regular leisure-time activity of any intensity are less likely to have symptoms of depression.

"We also found that the context in which activity takes place is vital and that the social benefits associated with exercise, like increased numbers of friends and social support, are more important in understanding how exercise may be linked to improved mental health than any biological markers of fitness.

"This may explain why leisure activity appears to have benefits not seen with physical activity undertaken as part of a working day."

Paul Farmer, chief executive of the mental health charity Mind, said that lifestyle factors such as diet and exercise are known to have a positive impact on mental well-being.

"Exercise gives you a natural high and is a great way to boost your mood. However, another mental health benefit of physical activity is derived from social interaction.

"So going out with a running club, taking part in a team sport or working on a communal allotment is far better for your mental well-being than a physically demanding job.

"Mind has found that after just a short country walk 90% of people had increased self-esteem," Mr Farmer said.