Thursday October 14 2010
Walking is recognised as having several health benefits
“Walking just one mile a day could keep dementia at bay,” according to the Daily Express. The newspaper said that pensioners who walk between six and nine miles a week are 50% less likely to suffer from future memory problems.
The story comes from a study which looked at how physical activity (measured by walking) in elderly adults might be associated with any brain volume and risk of cognitive decline. It found that elderly people who reported walking the most at the start of the study showed higher volumes of grey matter in particular areas of the brain when tested nine years later. This increased grey matter was also associated with a 50% reduction in cognitive impairment.
This study is of interest but it has some important limitations, including its design and the fact that it only measured brain volumes at a single point rather than over time. These limitations mean we cannot confirm that the participants’ walking affected their brain volume or rule out that ill health had contributed to both reduced walking and a shrinkage in brain volume. That said, there are plenty of good reasons to be physically active, and walking is a form of physical activity that is recognised as having health benefits.
Where did the story come from?
The study was carried out by researchers from the University of Pittsburgh, the University of Nevada and the University of California. It was funded by a number of US governmental health institutions: the National Institute on Aging, the National Heart Lung and Blood Institute, and the National Institute of Neurological Disorders and Stroke. The study was published in the peer-reviewed journal Neurology.
The study was widely reported in the media. The Daily Mail’s headline, suggesting that even a short walk can reduce the risk of Alzheimer’s disease, was misleading. In fact, increased grey matter volume was associated with walking a minimum of 6-9 miles per week. In addition, the study did not look at any specific association between walking and Alzheimer’s, but rather between walking, grey matter and cognitive impairment, including both dementia and mild cognitive impairment.
Overall, the newspapers have overstated the certainty of the findings from this study, and they did not report on its weaknesses.
What kind of research was this?
This was a cohort study, which looked at whether walking in older people at the start of the study had any association with the volume of grey matter measured nine years later, or with levels of cognitive impairment 13 years later.
Cohort studies are often used to investigate possible associations between lifestyle (in this case, the amount of walking people did) and health outcomes (in this case, grey matter volume and cognitive status). However, on their own, they rarely prove cause and effect. A different study design, such as a randomised controlled trial, would be better for proving this type of relationship.
The researchers point out that grey matter shrinks in late adulthood, often preceding and leading to cognitive impairment. Some researchers have hypothesised that physical activity may protect against deterioration of brain tissue, but this has not been adequately tested in studies. In addition, some studies have identified a lack of physical activity as a risk factor for developing dementia.
The researchers set out to look at whether walking had any association with greater grey matter volume and whether they could identify the threshold at which walking distance made a difference to grey matter volume. They also assessed whether greater grey matter volume was associated with reduced cognitive impairment.
What did the research involve?
Between 1989 and 1990, the researchers enrolled 1,479 adults aged 65 and older. They assessed the amount of physical activity they did (in particular walking) using a standardised questionnaire. Participants were divided into four equal-sized bands (quartiles), based on increasing number of blocks walked weekly. Of these original adults, 924 met the criteria for having an MRI scan.
Between 1992 and 1994, the participants had a low-resolution MRI scan. Then in 1998/99, researchers took a second, high-resolution, MRI scan of their brains. Only 516 of the original participants who had returned for follow-up had this second MRI. The MRI scans were used to assess grey matter volume through established techniques.
Of these 516 participants, 299 with an average age of 78 met the criteria for the study. The criteria included normal cognition and the absence of any conditions that might affect the brain. Researchers continued to follow-up these participants, who were given an assessment of their cognitive status by clinicians 13 years after entering the study.
Researchers undertook a statistical analysis to assess any associations between walking, brain volume and cognitive status. In their findings they took account of other factors that could have affected the results, such as age, health status, gender, education and race.
What were the basic results?
The researchers found that people who reported walking at least 72 blocks – some 6-9 miles weekly – had greater volumes of grey matter in certain regions of the brain at nine years follow-up, compared to those who walked less. This association remained significant after adjusting for factors such as age, gender, education and health status. Only people in the highest quartile, who reported walking between 72 and 300 blocks a week, showed any association with greater brain volume nine years later.
The researchers also found that greater grey matter volume in certain regions of the brain was associated with a 50% reduced risk of cognitive impairment (including both dementia and mild cognitive impairment). Overall brain volume was not associated with cognitive impairment.
How did the researchers interpret the results?
The researchers say that their study shows that walking greater distances was associated with larger volumes of grey matter nine years later, even after controlling for other health factors. Larger grey matter volume in certain areas of the brain was linked with a reduced risk of cognitive impairment 13 years on.
The strengths of this study lie in its large sample size, long follow-up period and the validated methods it used to determine both grey matter volume and diagnosis of cognitive impairment. However it has some important weaknesses:
- The researchers relied on people to report their own physical activity rather than directly measuring it. They also failed to confirm how much other types of physical activity people did. On this basis, the division of people into four groups of activity may have been inaccurate.
- They only measured brain volume once, nine years after the study started. This means they could not carry out any comparison between individual brain volume at the start of the study and nine years later. This would have allowed for them to report any changes in volume over time, which is a better measure of dementia.
- In addition, although the study enrolled 1,479 people at the start, the final sample size was 299. This large drop-out rate, can be partially explained by deaths from other causes and the fact that some people did not return for follow-up. However, excluding some people with dementia from the final analysis may have skewed the results.
As the authors note, some of these limitations mean that they cannot say for certain that greater physical activity is associated with larger volume of grey matter in later life or with a reduced risk of cognitive impairment. Given these limitations, it is still possible that ill-health led to both reduced amounts of walking and reduced brain volume.