No matter how you spin it, the news that exercise may help prevent and treat Disease X doesn't really qualify as a surprise. That's the default assumption these days.
But when Disease X is Alzheimer's, a progressive and irreversible degenerative brain condition with few effective treatment options, the finding is worth a closer look. Drawing on data from more than 150,000 participants in the National Runners' and Walkers' Health Studies over a 17-year period, a new U.S. study shows that regular exercise lowers the risk of dying from Alzheimer's by as much as 40 per cent – an indication that the disease's progression is not unchangeable.
"Currently, doctors do not screen for Alzheimer's disease susceptibility because of the belief that nothing can be done for those at risk," says Dr. Paul Williams, a staff scientist at Lawrence Berkeley National Laboratory in California and the author of the study. "However, our results add to the growing body of scientific evidence suggesting that people can be proactive in lowering Alzheimer's disease risk."
Williams began enrolling subjects in his study in 1991, and has used the massive database of health records to publish dozens of papers on the link between exercise levels and health conditions ranging from brain cancer and heart disease to cataracts and gout. The average age in the study was 45 at baseline; the subjects were all recreational walkers or runners, with roughly equal numbers of men and women.
Studying Alzheimer's presents particular challenges, since its diagnosis can only be fully confirmed by examining brain tissue after death. To get around this problem, Williams used the U.S. National Death Index to identify 175 subjects for whom the disease was listed as an underlying or contributing cause of death, publishing the results in the Journal of Alzheimer's Disease.
While this approach can't distinguish between preventing or delaying the onset of the disease and slowing its progression, it does allow him to calculate a dose-response function showing that those who exercised the most had the lowest probability of dying from Alzheimer's during the study period. While health authorities recommend getting at least 150 minutes of moderate exercise per week, Williams found the greatest benefits – a 40-per-cent reduction in mortality – in those who did twice that much exercise, equivalent to running about 25 kilometres a week.
The results also showed that running and walking are equally effective as long as you burn the same amount of energy overall. That means you need to spend about twice as much time (or cover 50 per cent more distance) walking briskly compared to running, Williams says.
That doesn't mean that lesser amounts of exercise are useless, cautions Dr. Jordan Antflick of the Ontario Brain Institute, who co-ordinated a 2013 report on the role of exercise in Alzheimer's prevention and treatment.
"You don't have to run a marathon," he says. "Even raking the leaves or going for a walk after dinner can help."
After reviewing more than 800 studies and selecting the 45 highest-quality trials, Antflick and his colleagues concluded that more than one in seven cases of Alzheimer's could be prevented if everyone simply met the minimum guidelines of 150 minutes of exercise per week, in doses as short as 10 minutes at a time. Given that approximately 10 per cent of Canadians over 65 have Alzheimer's, that would save tens or even hundreds of millions of dollars in health-care costs.
Moreover, the benefits continue for those who do develop the disease, which means "it's never too late to start exercising," Antflick says. Alzheimer's patients who exercise are less likely to suffer from depression, have better balance to avoid falls and also have improved general cognition.
"Most importantly, they're also able to maintain their independence and live at home longer," he says.
The precise mechanisms through which exercise fights Alzheimer's remain unclear, but Antflick points to several possible contributors. Exercise boosts cardiovascular health through mechanisms like improved blood flow to the brain – "so if your heart is healthy, your brain is healthy," he says. Exercise also boosts levels of brain-derived neurotrophic factor, which stimulates the growth of new neurons and their integration into the network of existing neurons.
There's also recent evidence that exercise stimulates the production of a protein that helps protect the brain from stress-related damage linked to depression, which often accompanies Alzheimer's.
So is early screening for Alzheimer's risk the answer? A gene variant known as APOE-e4 is thought to play a role in up to one-quarter of Alzheimer's cases; Williams argues that early knowledge of this risk factor could be the kick in the pants needed to convince someone to take exercise seriously. Critics, on the other hand, worry that current screening techniques don't have enough predictive power to be useful, giving false assurance to those without the gene variant and an overly negative view to those who do have it.
For the moment, our imperfect knowledge of the causes of Alzheimer's leaves us with just one safe conclusion: If you want to improve your odds, you should exercise no matter what your genes say.
Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com.