The benefits of going for a run to alleviate stress after a tough day in the office are well known.
But a new study has found out why working up a sweat is so relaxing and mood-boosting.
Exercise actually detoxes harmful chemicals from the body and can alleviate depression.
Researchers at the Karolinska Institute in Sweden have discovered that physical activity purges the blood of a substance which accumulates during stress and can be harmful to the brain.
Previous studies have suggested that people feel more positive after exercise because it releases a rush of endorphins.
But it now appears that during exercise, the muscles begin to act like the liver or kidneys and produce an enzyme which clears out a molecule linked to depression.
The team is hopeful that eventually a pill could be produced which would trigger the same effect to help the mentally ill.
“Our initial research hypothesis was that trained muscle would produce a substance with beneficial effects on the brain,” said Dr Jorge Ruas, principal investigator at the Department of Physiology and Pharmacology.
“We actually found the opposite: well-trained muscle produces an enzyme that purges the body of harmful substances. So the muscle’s function is reminiscent of that of the kidney or the liver.”
Dr Ruas said cardiovascular exercise would probably have the biggest impact on mood and reducing stress.
“It is possible that other kinds of exercise will also have an effect, like resistance training such as weight lifting. But our results support the use of aerobic exercise like biking and running.
“Skeletal muscle appears to have a detoxification effect that, when activated, can protect the brain from mental illness.”
The study also demonstrates why people who do not exercise end up feeling sluggish, depressed and are more prone to disease.
GPs can currently prescribe exercise for depression, but are far more likely to prescribe anti-depressants. There were 53?million prescriptions were issued for antidepressants in England alone last year, nearly double the number prescribed a decade ago.
“Our modern, sedentary lifestyles that don’t include sufficient physical activity, might have made us mode susceptible to diseases such as stress-induced depression,” Dr Ruas added.
“Physical exercise is already prescribed as a therapy or co-therapy for mild to moderate depression. We think that our findings will help support the use of physical exercise in the prevention and treatment of depression.”
Researchers had known that the protein PGC-1a1 increases in skeletal muscle during exercise but were unclear about what it was doing.
The team genetically engineered mice to have high levels of the protein and then exposed them, and a control group of normal mice, to a stressful environment of loud noises and flashing lights.
They found that after five weeks the normal mice had become depressed but the engineered mice appeared to be protected.
It is thought that the protein produces an enzyme called KAT which turns the harmful kynurenine molecule into harmless kynurenic acid which can be passed easily out of the body.
Carmine Pariante, Professor of Biological Psychiatry at Kings College London said the finding was ‘very important’ to the understanding of exercise and depression.
“Exercise is always good for mental and physical health,” he said.
“This study shows one of the mechanisms by which exercise is beneficial but is not the only one good thing – people should exercise anyway.”
Dr Clare Stanford, Reader in Experimental Psychopharmacology, UCL, said: “The suggestion that kynurenin is a casual factor in depression is not new, but linking it with muscle metabolism and the beneficial effects of exercise on mood is really interesting. That said, I think the interpretation of the behavioural data should be more cautious.
“Although stress-induced immobility in mice is used as a screen for antidepressant drugs, there is evidence that it is not a mouse version of depression (anti-depression and depression probably target completely brain processes). A reduction in sucrose preference is a more plausible analogue of anhedonia (the inability to experience pleasure), but that is only one aspect of depression and, by itself, would not qualify for a diagnosis of depression in humans.
“In summary, this is interesting research but does not necessarily explain the role of exercise in reducing depression in humans.”
The study was published in the journal Cell.