The Fascinating Connection Between Exercise and the Ageing Brain

Between 2013 and 2015, around 100,000 volunteers in the UK Biobank study were posted a small device to wear on their wrist for a week. It recorded every movement, day and night, as they shopped, gardened, walked the dog and slept. Years later, many of the same people lay in an MRI scanner and sat a battery of computerised thinking tests.

That combination, a week of honest movement data and a detailed look at the brain, is the raw material for a new analysis by researchers at the University of Florida, Stanford University and Japan's RIKEN institute. Their question was simple to ask and hard to answer: in later life, does the amount of brisk activity you do, and even when you do it, show up in the brain?

A week on the wrist

The study, posted as a preprint on arXiv by Wasif Khan, Ruogu Fang and colleagues, focused on 45,892 participants aged 60 and over, with an average age of about 67. Rather than relying on what people remembered doing, the team used a machine-learning model to pick out minutes of moderate-to-vigorous physical activity from the raw accelerometer signal. That is activity hard enough to make you breathe faster, such as brisk walking, cycling or swimming.

Just under two-thirds of the group, 29,755 people, met the World Health Organization's guideline of at least 150 minutes of such activity a week. The researchers then compared activity levels with performance on tests of reasoning, memory, reaction time and mental flexibility, and with brain volumes measured by MRI, adjusting for a long list of factors including age, sex, education, smoking, alcohol, blood pressure, cholesterol, body weight, diabetes and deprivation.

What the scans showed

More activity went with better thinking. People who did more moderate-to-vigorous activity each week scored higher on fluid intelligence, the ability to solve unfamiliar problems, and on a paired-associate memory test, in which you learn which words belong together. Some of these links held even after the full set of adjustments. Reaction time and a test of switching between tasks also looked better among people meeting the guideline in the simpler analysis.

The brain scans told a similar story. Activity was linked with larger volumes in several deep structures, including the thalamus, a relay hub for sensory information, and the putamen, caudate and pallidum, which help control movement, habits and motivation. Across the cortex, 124 regions showed positive associations, 38 of them statistically significant. Among them were the frontal pole and medial frontal cortex, involved in planning and decision-making, the insula, and the parahippocampal gyrus, a gateway to the memory circuits of the hippocampus. For several deep structures, the benefits appeared to keep climbing among people doing more than 300 minutes a week.

Does the clock matter?

The more unusual part of the study looked at timing. The researchers sorted people into morning, afternoon, evening and mixed exercisers, based on when their brisk activity tended to happen. The paper does not spell out the exact hours used, so the details should be held lightly.

Every timing group had more grey matter than inactive participants, which is perhaps the headline that matters most: activity at any time of day seemed better than none. Beyond that, midday-to-afternoon and evening activity was linked with better scores on the paired-associate memory test. Evening activity went with larger caudate and putamen volumes, while afternoon or mixed activity went with larger pallidum and amygdala volumes.

These are intriguing patterns, not instructions. The timing analysis is new, and the authors themselves describe their work as observational. People who exercise in the evening may differ in their jobs, sleep or body clocks from those who exercise in the morning, and those differences could explain some of what was seen.

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Activity at any time of day seemed better than none.

What a randomised trial already told us

The new study fits neatly alongside some of the best experimental evidence in the field. In 2011 Kirk Erickson, then at the University of Pittsburgh, and colleagues published a randomised trial in PNAS involving 120 older adults. Half were assigned to walking three times a week, building up to 40 minutes per session; the other half did stretching and toning.

After a year, the walkers' hippocampus, a seahorse-shaped structure central to forming new memories, had grown by about 2 per cent, while it shrank by about 1.4 per cent in the stretching group. The authors calculated that this effectively reversed one to two years of age-related shrinkage. Larger gains in hippocampal volume went with bigger rises in BDNF, brain-derived neurotrophic factor, a protein that supports the growth and survival of neurons and helps new cells form in the hippocampus.

The memory results were more modest than the title suggested. Both groups improved on a spatial memory task, and the walkers did not significantly outperform the stretchers, though among walkers, bigger hippocampal gains went with bigger memory improvements. That nuance matters. Exercise reliably changes the brain's structure; the translation into everyday memory is real but gentler than headlines sometimes claim.

Reading the evidence wisely

The UK Biobank analysis has real limits. A single week of wrist data may not reflect years of habit. Because it captures one moment in time, it cannot rule out reverse causation: people whose brains are already healthier may simply find it easier to stay active. And as a preprint, it has not yet completed formal peer review.

Even so, it joins a substantial body of evidence. The 2024 Lancet Commission on dementia estimated that around 45 per cent of dementia cases could in theory be prevented or delayed by tackling 14 modifiable risk factors, and physical inactivity is on that list.

The practical message is unglamorous and encouraging. The WHO recommends 150 to 300 minutes of moderate activity a week, and in this study the people who reached that mark tended to have the sharpest results. A brisk walk to the shops, an afternoon swim or a bike ride after work all count. If you have a health condition or have been inactive for a while, a chat with your GP before stepping things up is sensible. Then go at a pace that makes you slightly out of breath, and keep going for years.

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This article is for general information and is not a substitute for personal medical advice. If you have concerns about your health, please speak to your GP or a qualified professional.