Moderate exercise, long thought to be a tool in the battle against Alzheimer’s disease, may actually hasten symptoms if done regularly after diagnosis, a new study suggests.

Researchers at Oxford University found people with mild to moderate dementia who worked out in a gym twice weekly for up to 90 minutes saw their condition decline more quickly than their peers.

Although authors of a study appearing in The BMJ Wednesday noted the differences were small and of “uncertain importance,” they suggested moderate- to high-intensity workouts be nixed as a treatment option for cognitive impairment in Alzheimer’s patients.

“The exercise improved physical fitness in the short term, but this did not translate into improvements in activities of daily living, behavioural outcomes, or health-related quality of life,” they wrote. “There is the possibility that the intervention could worsen cognition.”

Led by professor Sarah E. Lamb, the team suggested future trials explore whether other forms of exercise, such as psychomotor protocols used to address long-term neurological conditions. They reported that future investigators should consider the possibility that some exercise types might worsen cognitive impairment or cause a “rebound” effect if exposure is limited.

For the study, 494 English Alzheimer’s patients with an average age of 77 were assigned to a supervised exercise and support program or standard care.  The support program included 60-90 minute group sessions in a gym twice a week for four months, plus one hour of at-home exercises weekly.

All were assessed at study’s start, after 6 months and after a year using an Alzheimer’s disease assessment score.

By 12 months, the average score in the exercise group increased by 12.3 points, 2.1 points more than that of the non-exercise group, meaning the exercise group saw their impairment worsen more.

The findings were surprising, given that previous studies have shown moderate exercise and overall fitness in midlife may offer some protection against cognitive decline.

“The evidence that exercise may reduce the risk of developing dementia is convincing,” Martin Rossor, a professor of clinical neurology at University College London, told Newsweek. “It is not much less clear whether exercise can delay the decline of those with an established dementia. This study would suggest not.”