This is part of a series on aging well.
Time and again, we've heard that exercise is one of the best defences against dementia. But do we know for sure?
Just last month, researchers from Johns Hopkins University challenged this assumption after tracking physical activity in 646 adults for 30 years. In a study published in the Journal of Alzheimer's Disease, they found no connection between exercise in midlife and cognitive fitness later on.
But that doesn't mean we should throw in the gym towel. For one thing, their findings may not apply to the rest of us.
Participants in the study were healthy, highly educated physicians – mostly men. In midlife, nearly 75 per cent said they worked up a sweat at least once a week. A mere 8.8 per cent had high blood pressure at an age when 32.8 per cent is the norm.
Having normal blood pressure, high education levels and being male are all associated with lower rates of dementia, regardless of exercise habits. Sure enough, at the end of the study period, just 4.5 per cent of participants – 28 individuals – had Alzheimer's disease.
With so few cases in such a physically active group, we can hardly draw a clear conclusion about the effects of exercise versus a couch-potato lifestyle on the risk for dementia.
It's a tricky thing to study, since prevention is a long-term process and the causes of Alzheimer's remain poorly understood. Individuals recruited for studies may have genetic risk factors or protective habits other than exercise that researchers may not always pick up. Moreover, exercise studies – including the Johns Hopkins research – tend to rely on participants' recollections of physical activity, which may be spotty or too rosy.
Even so, the Johns Hopkins research is significant as one of few studies to look at exercise and dementia over the long haul.
Participants answered a questionnaire about their physical activity at regular intervals starting in 1978, when their median age was 46 years old. Thirty years later, they completed cognitive tests and had their medical records reviewed to flag dementia.
The long-term follow-up allowed the team to study which came first: low levels of physical activity, or cognitive decline, said lead author Alden Gross, an epidemiologist at Johns Hopkins Bloomberg School of Public Health. In the past, researchers have connected a lack of exercise with dementia in numerous studies of shorter duration. But these associations could be explained by "reverse causation," he said. It is possible that the people who eventually developed dementia began exercising less as their cognitive functioning declined.
In this study, all participants were students at Johns Hopkins School of Medicine between 1948 and 1964, a common academic background that made it easier to rule out major differences in cognitive abilities earlier in life. Having such a homogeneous group "is actually an advantage when you approach studies of cognitive aging," Gross said.
He noted that two other long-term studies – one involving Swedish twins, the other, Finnish twins – did not find a strong link between exercise and dementia. Twin studies help researchers isolate habits such as exercise from family characteristics and genetics as they compare dementia rates in twins with different lifestyles.
On the other hand, at least one long-term study has shown that exercise does prevent cognitive decline. After following 387 women for two decades, a 2016 Australian study found that those who did light exercise every day were far less likely to suffer memory loss in their 60s and 70s than their sedentary peers. In this study, published in the American Journal of Geriatric Psychiatry, researchers singled out regular exercise as the lifestyle factor with the greatest protective effect on short-term memory.
Gross said he was unfamiliar with the Australian study, which did not include men. Could it be that men and women respond differently to exercise in midlife? Some researchers have hazarded this guess, but so far, the evidence is inconclusive.
The same could be said of the role of exercise in safeguarding against dementia. Scientists generally agree that the theory makes sense, though. Exercise can reduce hypertension, Type 2 diabetes and obesity – all known to increase the risk of dementia. Physical activity improves blood flow to the brain, which is linked to better cognitive functioning. And in mouse studies, exercise is associated with less accumulation of beta-amyloid plaques, thought to play a role in Alzheimer's disease.
Exercise makes us healthier, no question. But we still lack hard data that exercise in midlife has a direct impact in reducing Alzheimer's disease. Without sufficient evidence, health authorities run the risk of giving false hope that regular workouts can ward off dementia – or worse, misleading the public into thinking Alzheimer's patients are to blame for not exercising enough when they were younger.
"I personally would love for physical exercise to be that magic bullet that's going to prevent me from getting dementia," Gross said. "But I think we need a lot more data before we come to that conclusion."