Being physically fit can lower the risk of dementia and delay someone developing it by almost 18 months by boosting brain health, research has found.

Regular exercise is so useful for maintaining cognitive function that it can even help people who are genetically more predisposed to dementia to reduce their risk by up to 35%.

The findings add to the evidence that staying fit during the course of one’s life is a key way of lowering the likelihood of developing the disease.

The study, published in the British Journal of Sports Medicine, found that people with the highest cardiorespiratory fitness (CRF) also had higher cognitive function and a lower risk of dementia.

The researchers analysed the health of 61,214 people who were aged between 39 and 70 when they enrolled in the UK Biobank study between 2009 and 2010, none of whom had dementia at the time. They were followed up for up to 12 years to see how their health progressed.

On joining they undertook a six-minute exercise test sitting on a stationary bike, to assess their fitness. They also had their cognitive function measured by neuropsychological tests and their genetic likelihood of dementia estimated using a polygenic test to assess the risk of Alzheimer’s disease.

“Our study shows that higher CRF is associated with better cognitive function and decreased dementia risk”, the researchers write in their paper.

“Moreover, high CRF may buffer the impact of genetic risk of all dementia by 35%.”

They add that higher CRF is associated with “lower risk of dementia and a delay in the onset of dementia across middle and older age” of 1.48 years.

The Swedish research team were led by Prof Weili Wu of the ageing research centre at the Karolinska Institute in Stockholm.

Dementia organisations said the findings were further proof that people could reduce their risk by maintaining a healthy lifestyle, for example by keeping fit, not smoking and not drinking to excess.

“This research emphasises that exercise is an important part of maintaining a healthy lifestyle and could reduce the risk of developing dementia later in life”, said Dr Richard Oakley, the associate director of research and innovation at the Alzheimer’s Society.

“But what is particularly promising about this study is that exercise also appears to reduce dementia risk in people who have a higher genetic risk of developing Alzheimer’s disease.”

The Lancet medical journal’s commission on dementia that reported in July identified physical inactivity as one of 14 established factors that increase the risk of dementia. Others include hearing loss, low levels of education, air pollution, social isolation and depression.

“This new research highlights how good cardiorespiratory fitness, a key measure of overall physical health, could help lower the risk of developing dementia in the future,” said Dr Jacqui Hanley, head of research at Alzheimer’s Research UK.

“However, it is important to note that we don’t know if there is a direct link between cardiorespiratory fitness and reduced dementia risk. More research is needed to find out exactly how it effects the brain.”

The researchers themselves stressed that their findings were observational and did not necessarily prove a causal relationship between physical fitness and dementia risk.

However, they nevertheless suggest that “enhancing CRF could be a strategy for the prevention of dementia, even among people with a high genetic predisposition for Alzheimer’s disease”.