Exercise could increase the risk of developing motor neurone disease (MND), research suggests.
People who are physically active are more likely to go on to suffer from the condition, with those who exercise vigorously at the greatest risk, according to a study published in the Journal Of Neurology Neurosurgery And Psychiatry.
However experts said a link between the two was not conclusive and stressed that exercise has been found to prevent many other diseases.
MND, also known as amyotrophic lateral sclerosis (ALS), is an incurable neurodegenerative disease which attacks the nerves in the brain and spinal cord, leading to progressive paralysis.
The condition affects up to 5,000 adults in the UK.
The researchers compared the lifestyles of 1,557 adults diagnosed with MND in their mid-60s in Ireland, Italy and the Netherlands with 2,922 people of a similar age without the condition.
Lifetime physical activity was associated with a 6% increased risk of MND, with the link strongest in Italian and Irish participants, they found.
Those who were the most physically active had a greater chance of developing the disease in later life.
"This is in line with reports that describe a higher prevalence of patients with ALS among former professional athletes," the authors wrote.
While exercise is unlikely to be the main factor leading to the development of MND, the researchers said it could be important in those genetically predisposed to the disease.
They wrote: "Overall, physical activity has been demonstrated to be protective against many diseases including cardiovascular disease, diabetes and a variety of cancers.
"Decreasing the risk of these common conditions may be a trade-off with increasing the risk of a relatively rare disease such as ALS."
Commenting on the findings of the observational study, Professor Tara Spires-Jones, from the University of Edinburgh, said it did not prove physical activity causes MND and that other factors that increase with physical activity levels could be the cause of the risk.
She said: "It is important to keep in mind that ALS is a relatively rare disease affecting around two in every 100,000 people and that physical activity protects us from much more common diseases including Alzheimer's, heart disease, diabetes and cancer, which together affect more than 10 million people in the UK today."
Nick Cole, head of research at the Motor Neurone Disease Association, said the link between exercise and the development of the condition "is a very subtle one".
He said: "It does not mean that exercise causes MND.
"Put in context, it is a small increased risk and one of multiple factors, from genetic to environmental, likely to be needed in a combination to develop MND.
"Given that exercise has been shown to offer significant protection against many diseases it would not be advisable to adopt a sedentary lifestyle in order to avoid a very small increased risk of developing MND."
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