People who fail to exercise and are unfit in mid-life are likely to have smaller brains decades later, says a new study.

If your sofa has permanent imprints from where you’ve spent hours sitting on it, and the idea of exercise involves the walk from the couch to the fridge, you could be setting yourself up for an increased risk of cognitive decline and dementia in later life.

A study by Boston University School of Medicine found people who were unfit and did little or no exercise in their 30s and 40s were likely to have smaller brains two decades later.

Researchers studied the fitness habits of 1600 people when they were in their 30s and 40s and then carried out MRI scans on their brains 20 years later.

While a certain amount of brain shrinkage is normal as we age, researchers believe that exercise levels can determine the rate at which this happens.

On average, the study authors found that the participants’ total brain volume shrank by roughly 0.2 per cent a year. But those who were less fit when they were in their 30s and 40s saw their brains shrink faster.

For every 20 per cent reduction in fitness score below average in the initial exercise test, the participants displayed an additional 0.2 per cent brain shrinkage – the equivalent of an entire additional year of ageing.

Researchers believe this is because doing exercise helps to reduce blood pressure, which then reduces strain on the brain.

Dr Laura Phipps of Alzheimer’s Research UK said:

‘While diseases like Alzheimer’s are caused by a range of risk factors including age and genetics, damage to the blood supply in the brain also has a role to play.

‘Current evidence suggests the best way to maintain a healthy brain is to keep physically and mentally active, eat a balanced diet, not smoke, drink only within recommended limits and keep diabetes, high blood pressure and high cholesterol in check.’

Click the links for more tips on how to reduce the risk of memory loss in your 30s, 40s and 50s.