For women with a healthy lifestyle, just 2.6 years were lost to Alzheimer’s, and 1.4 for men. But unhealthy women suffered an average of 4.1 years of dementia, while men spent 2.1 years with the disease.

The study suggests that the approximate age for dementia onset among the healthy women would be around 86 compared to 81 for the unhealthy.

For healthy men, Alzheimer’s might be expected to kick in around 86, but unhealthy men could expect a diagnosis six years earlier, at around 80.

There are more than 850,000 people currently living with dementia in Britain, which is expected to rise to 1.5 million by 2040.

Writing in the BMJ, the researchers concluded: “This investigation suggests that a prolonged life expectancy owing to a healthy lifestyle is not accompanied by an increased number of years living with Alzheimer’s dementia.”

Commenting on the research, Dr Rosa Sancho, the head of research at Alzheimer’s Research UK, said it was clear that there were steps people could take to keep their brains healthy, and reduce the risk of dementia in later life.

‘Age, genetics, lifestyle’ affect chances of getting Alzheimer’s

“The chance of developing a disease like Alzheimer’s is affected by a complex mix of age, genetics, and other lifestyle factors,” said Dr Sancho.

“While research suggests that living a healthy lifestyle can help stave off dementia, it can also lead to people living longer, which in itself is a risk factor for the condition.

“In this study, researchers looked at untangling the association between healthy living, increasing life expectancy and Alzheimer’s. While this study cannot fully tease apart cause and effect, it hints that living longer due to a healthy lifestyle does not mean more years living with Alzheimer’s disease.

In a linked editorial in the BMJ, Dr HwaJung Choi, a researcher in the department of internal medicine at the University of Michigan’s School of Medicine, said healthier lifestyles should be promoted as a means of staving off dementia.

“Older age is strongly associated with a higher risk of dementia, so although a healthier lifestyle may delay the onset of dementia, it might also increase the number of years spent with dementia,” she said.

“A better understanding of this nuance — years spent with versus without dementia — is important for assessing the overall implications of interventions promoting a healthy lifestyle for individuals, families, and society.

“Promoting greater engagement in healthy lifestyles may increase dementia-free life years—by delaying the onset of dementia without extending life years spent with dementia.”