Experts looked at those with two or more chronic health conditions when they were aged 55, 60, 65 and 70. The study used data from more than 10,000 British men and women taking part in the Whitehall II Study.

At 55, seven per cent had two or more conditions, rising to 32 per cent at age 70.

Overall, 639 cases of dementia occurred over a typical 32-year follow-up.

Of those with no chronic conditions or only one, 3.9 per cent had dementia at the follow-up point in March 2019, compared to 8.5 per cent of those with two or more chronic health problems.

After accounting for other factors such as sex, ethnicity, education, diet and lifestyle, having two or more conditions at age 55 was associated with a 2.4 times higher risk, the authors said.

Meanwhile, developing two or more conditions between the ages of 60 and 65 was associated with a 1.5 times higher risk.

But the risk fell dramatically if people were aged 70 before they developed chronic health conditions.

The danger of dementia was also greater for those who developed severe multimorbidity – having three or more chronic conditions – earlier in life.

People with three or more conditions at the age of 55 had nearly a fivefold higher risk of dementia compared to those with no illnesses or just one condition, whereas the risk was 1.7 times higher when multimorbidity was diagnosed at age 70.

Key to prevent chronic illnesses in middle age

The authors said the study was observational and could not establish causes of dementia. However, they stressed it is the first of its kind to track a cohort of this size over a significant follow-up period, which gave them increased confidence in the findings.

All the people in the research were aged 35 to 55 at the start of the Whitehall II Study from 1985 to 1988.

The authors concluded that effective prevention and management of chronic diseases in mid-life could help to mitigate the risk of dementia in old age.

“Given the lack of effective treatment for dementia and its personal and societal implications, finding targets for prevention of dementia is imperative,” they said. 

“These findings highlight the role of prevention and management of chronic diseases over the course of adulthood to mitigate adverse outcomes in old age.

“Multimorbidity is already known to affect use of healthcare services, quality of life, and risk of mortality. Our study adds dementia to that list.”

Dr Rosa Sancho, head of research at Alzheimer’s Research UK, said the study provided “valuable insight” into the relationship between multiple health conditions in mid-life and dementia in old age.

But she said more research was needed to explore the links between individual conditions.

“This insight will enable researchers to design and deliver appropriate interventions that will reduce the number of people who go on to develop dementia,” she said.