Participants in the study were also grouped by their genetic risk scores. Those with moderate or high genetic risk scores for developing dementia were more 35 per cent less likely to develop the disease than those with low fitness levels, the researchers said.

People’s cardio fitness levels typically drop by around three to six per cent every 10 years starting in the 20s and 30s.

However the rate it reduces also increases over time, and by the time someone hits their 70s it can decrease by about 20 per cent per decade.

Having a low level of fitness is a strong predictor of suffering from a stroke, heart attack and other cardiovascular disease, as well as early death from all causes.

Britons without dementia aged between 39 and 70 years old were recruited for the study.

Participants had to complete a six-minute submaximal exercise test on a stationary bike and their cardiorespiratory fitness was estimated based upon their maximum heart rate, maximum oxygen consumption and body mass index, while also taking into account their age and sex.

They were then divided into three broadly equal groups: those with low, middle, or high cardiorespiratory fitness levels.

Those with higher fitness levels are typically able to exercise for longer and put their body under less stress while doing so, which means it takes longer for them to become exhausted. Such people will likely be able to run or cycle longer distances and play active sports such as tennis.

The researcher team also measured the participants cognitive function using neuropsychological tests, and their genetic predisposition for dementia was estimated using the polygenic risk score for Alzheimer’s disease.

In total those in the high fitness group had a 40 per cent lower chance of a dementia diagnosis than those in the low fitness group.

Regular exercise

The authors, who were also from the Tianjin Medical University, in China, concluded that: “Our study shows that higher [cardio fitness] is associated with better cognitive function and decreased dementia risk. Moreover, [it] may buffer the impact of genetic risk of all dementia by 35 per cent.”

They said that increasing cardiorespiratory fitness “could be a strategy for the prevention of dementia, even among people with a high genetic predisposition for Alzheimer’s disease”.

Previous research has linked regular exercise to around a 20 per cent lower chance of developing dementia.

Dr Jacqui Hanley, head of research at Alzheimer’s Research UK says: “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.

“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 affects the brain.

“Certain dementia risk factors we can’t change, like age and genetics. Interestingly, this study also looked at cardiorespiratory fitness in people who had variations in their genes that increases their risk of developing Alzheimer’s disease,” she said. 

“Understanding this interaction more could reveal new ways to help prevent dementia from developing in people who are at a higher risk. “

The study was published in the British Journal of Sports Medicine.