Exercise does not slow down mental decline and may even make dementia worse, a new study suggests.

Oxford University found that people with mild to moderate dementia who went to the gym twice a week for up to 90 minutes went downhill faster than those who abstained.

Although the difference between the two groups was small, the researchers say exercise should not be recommended for people with dementia and called for future trials to ‘consider the possibility that some types of exercise intervention might worsen cognitive impairment.’

Previous research had suggested that exercise could prevent mental decline, and stave off diseases like Alzheimer’s, so experts and charities said they were surprised by the findings.

Commenting on the study, which was published in the BMJ, Rob Howard, Professor of Old Age Psychiatry at University College London said: “Had this been instead an improvement in cognitive functioning with exercise we would all have been excited about finding something positive in the, so far, depressing fight against dementia.

“On this basis, I don’t think we should ignore the possibility that exercise might actually be slightly harmful to people with dementia.”

Dr James Pickett, Head of Research and Development at Alzheimer's Society, added :“The results are somewhat surprising as we would anticipate that exercise would have positive effects.”