Look at any of the multitude of articles of the past few years on how to avoid dementia and you’ll almost certainly read that exercise delays onset. Not so, according to the most recent research, published this week in the BMJ.
The 28-year study followed over 10,000 middle-aged British civil servants, noting at seven-year intervals whether participants were doing the “recommended” amount of exercise, defined as moderate or vigorous physical activity for 2.5 or more hours per week.
Surprisingly, the researchers found no correlation between how much exercise a patient did and whether they experienced cognitive decline over the study period, identified through a battery of cognitive tests, along with dementia diagnoses from hospital and mental health services.
The finding runs counter to several recent meta-analyses of observational studies which concluded that physical activity is neuroprotective in cognitive decline and dementia risk.
What the researchers did find was that in participants who eventually developed dementia, a decline in physical activity started around nine years before diagnosis.
This finding could be key to why previous observational studies have found a correlation between exercise and dementia risk, say the French researchers from the Centre for Research in Epidemiology and Population Health in Paris.
It’s now well known that brain changes start happening many years before dementia symptoms become apparent, and a decrease in physical activity is probably part of the cascade of changes in this preclinical phase of dementia, the researchers say.
The upshot is that findings of a lower risk of dementia with exercise may be attributable to reverse causation – in other words, decline in physical activity is due to the dementia, and not the other way around.
The researchers say that two problems with some of the earlier observational studies were that their duration was too short and their participants were too old. This made them more liable to be confounded by participants with preclinical dementia, who for that reason had lower levels of physical exercise.
They also point out a difference between observational and randomised trials, with the latter less likely to find a protective effect with exercise.
The recommendation of exercise for the prevention of dementia has already become enshrined in a number of international guidelines, including in Australia.
You can access the study here.