Physical activity combats depression by purging the body of a harmful substance that accumulates during stress, research suggests.

Exercised muscle produces enzymes that convert the stress chemical kynurenine into a form that cannot pass into the brain, tests on mice have shown.

Although its precise function is unknown, high levels of kynurenine are found in patients with mental illness. Scientists believe taking exercise may reduce symptoms of depression by the way it affects the stress compound.

Lead researcher Jorge Ruas, from the Karolinska Institute in Sweden, said: “Well-trained muscle produces an enzyme that purges the body of harmful substances.

“So in this context the muscle’s function is reminiscent of that of the kidney or the liver.

“It’s possible that this work opens up a new pharmacological principle in the treatment of depression, where attempts could be made to influence skeletal muscle function instead of targeting the brain directly.

“Skeletal muscle appears to have a detoxification effect that, when activated, can protect the brain from insults and related mental illness.”

The research appears in the journal Cell.