The link between a sound body and a sound mind has been demonstrated by scientists in a groundbreaking new study on the impact of exercise.

Researchers from Karolinska Institutet in Sweden found that the same mechanisms behind the beneficial effects of exercise training on the brain also help counteract fat and strengthen the immune system.

"We've linked the two parts of the expression 'sound mind, sound body'," says Jorge Ruas, principal investigator at the Department of Physiology and Pharmacology at Karolinska Institutet.

"Our research adds to the understanding of why exercise training benefits the body and in the long-run can lead to the development of new treatments for obesity or diabetes."

The research examined the function of kynurenic acid, following an initial study in 2014 which identified a mechanism behind the beneficial effect of exercise training on the brain. Scientists were able to show that trained muscles help to clean the blood in a way similar to the kidneys and liver.

Through exercise training, the muscles convert the stress marker kynurenine into kynurenic acid. High levels of kynurenine have been measured in people with depression and mental illness.

Now, a follow-up study shows that the same process also boosts fat metabolism and strengthens the anti-inflammatory properties of the immune system.

Using mice fed on a fat-rich diet that made them overweight and raised their blood sugar levels, researchers found that a daily dose of kynurenic acid stopped the mice putting on weight and gave them better glucose tolerance, despite no change in their food intake.

In their findings, published in the journal Cell Metabolism, the researchers posit the explanation that kynurenic acid activated the cell receptor GPR35, which is found in both fat cells and immune cells.

In fat cells, this led to a conversion of white fat into energy-burning brown fat, and in immune cells to an enhancement of their anti-inflammatory properties.

"We've shown that kynurenic acid prevents weight gain despite excessive energy intake," says Dr Ruas. "Our next step is to identify the complex chain of interacting molecules that's affected by diet and training."