"A new study has suggested that exercising in your 40s could stop the brain shrinking," The Daily Telegraph reports.

A study found people with good fitness levels in their 40s had larger brains than their unfit peers when measured 20 years later. The concern is that people with smaller brains may be more likely to develop dementia.

The study, part of a big ongoing research project in the US (the landmark Framingham Heart Study) measured people's exercise capacity and heart and blood pressure reactions to exercise during a treadmill test, at an average age of 40.

The same people were assessed about 20 years later, with a repeat exercise test and an MRI scan to determine brain volume.

People with 20% less fitness compared to the average, had smaller brains by the equivalent of one additional year of ageing. A similar effect was seen for higher blood pressure or heart rate in response to exercise.

However, we don't know the importance of the brain size differences measured and as this was only done once, it is not clear whether the size had actually changed.

So we cannot be sure fitness levels directly caused the differences in brain size. But the research does add to the growing evidence that physical fitness and better mental capacity in older age go hand-in-hand.

What is good for the heart tends to also be good for the brain. Read more about how exercise may reduce your dementia risk.

Where did the story come from?

The study was carried out by researchers from Boston University School of Medicine, Framingham Heart Study, Harvard Medical School, Broad Institute of MIT and Harvard and the University of California. It was funded by the National Institutes for Health and the American Heart Association.

The study was published in the peer-reviewed journal Neurology.

Reports in the UK media overstated the certainty of the study. The Daily Mail's headline: "Being a couch potato shrinks the brain," makes the results sound more definite than they are. The report says that "failing to exercise" was the cause of smaller brains.

The Daily Telegraph says the study "revealed … exercising when aged between 40 and 50 could help prevent the brain shrinking". However, the study did not look at whether people exercised, how much they exercised or at what age. It only included information about their fitness levels, blood pressure and heart rate.

What kind of research was this?

This is a prospective cohort study, which tracks people over a long period of time and compares information taken at different time points. It's a good way to look for links between factors – in this case between fitness and later brain size. However, it cannot prove that one thing causes another.

What did the research involve?

Researchers took a large group of people, average age 40, and tested their fitness levels using a treadmill. They recalled them 20 years later to repeat a fitness test and have an MRI brain scan and cognitive tests. They looked for links between fitness at the first test and brain size and cognitive skills 20 years later.

The fitness tests involved people exercising on a treadmill until they reached 85% of their maximum heart rate, calculated by age and sex. Fitter people are able to exercise for longer before reaching this level. This time was used to calculate people's total exercise capacity. People's heart rate and blood pressure were also monitored before and during the test.

The researchers excluded people from their first analysis if they already had cardiovascular disease, had been taking beta blockers (drugs that slow heart rate) or if they had dementia or any condition that could affect the brain scan or cognitive tests. They were also excluded if they were unable to complete the exercise test.

In their analyses, the researchers adjusted their figures to take account of the following confounders:

age

sex

the time between examinations

whether they smoked

whether they had diabetes

whether they had a gene linked to Alzheimer's disease

whether they took medicine to treat high blood pressure

What were the basic results?

People who had 20% lower fitness levels based on the exercise capacity test had smaller brain volumes when assessed in later life. Those with a higher heart rate and diastolic blood pressure while exercising also had smaller brain volumes. Higher systolic blood pressure was also linked to smaller brain volumes, but only when the researchers looked at the subset of people with high blood pressure.

There was no link between lower exercise capacity in mid-life and any measures of cognitive function (thinking ability) in later life.

How did the researchers interpret the results?

The researchers say their findings: "provide new evidence that lower cardiovascular fitness and elevated exercise blood pressure and heart rate responses in early to midlife are associated with smaller brain volumes nearly two decades later, thereby linking fitness over the life course to brain health in later life".

They say that encouraging people to be fit in middle age could improve healthy brain ageing, especially for people with raised blood pressure.

Conclusion

We already know that high blood pressure in mid-life is linked to increased chances of getting dementia in older age. Also, taking regular exercise in middle age has been linked to a lower chance of dementia.

This study adds to what we already know about links between having a healthy heart and circulation, and a healthy brain.

The study found that people who did well in fitness tests at around 40 years of age had fewer signs of brain shrinkage at around 60. However, this did not translate into signs that the brain was working less well – perhaps because people were not old enough to have shown signs of slowed cognitive function.

We don't know from the study whether fitness levels are directly linked to brain shrinkage in a causal fashion. Therefore we can't say whether any particular amount of exercise protects against brain shrinkage. However, the researchers suggest that better cardiovascular fitness provides better blood flow and oxygen delivery to the brain, helping to keep it healthy.

The study has some limitations; importantly, brain volume was only assessed once, at the end of life, so we don’t know how much people's total brain volume had changed over time. We don't know the likely effect of the differences in brain volume measured. Also, the researchers did not calculate the possible effects of carrying out many different calculations on one set of data, which can increase the likelihood of some findings being down to chance.

Exercise has so many benefits that it can be confidently recommended, despite any questions about this particular study. However, there is no 100% guarantee that healthy lifestyles, including exercise, can prevent dementia in later life.

Analysis by Bazian

Edited by NHS Website