Developing a chronic illness such as arthritis, diabetes or heart disease in middle age has been linked to a doubling in the risk of dementia.

A major, new study found that those with three or more illnesses had a five-fold risk of developing the disease compared with people who had none.

The research, which is published in the BMJ, is thought to be the first to show that 'multi-morbidity' in midlife is associated with a higher risk of dementia.

Researchers monitored the health of 10 000 people aged 35 to 55 for 30 years and found those with two or more chronic conditions had a higher risk of subsequent dementia.

This was the case at age 55, 60, 65, and 70 years but the strongest associations were seen in those with chronic illnesses in their 50s.

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Having two or more conditions at 55 raised someone's risk of dementia almost two and a half times, compared with someone who had none.

The study looked at 13 chronic illnesses; coronary heart disease, stroke, heart failure, diabetes, hight blood pressure, cancer, chronic kidney disease, chronic obstructive pulmonary disease (COPD), liver disease, depression, mental disorders, Parkinson’s disease, and arthritis.

The researchers said there was likely to be "cumulative effects of clustering of chronic diseases accelerating cognitive decline and increasing the risk of dementia." 

Inflammation in the body caused by the other conditions could also be behind the effect, while the impact of drugs taken to control the illness might also play a role in cognitive ageing.

Professor Craig Richie, Director of Brain Health Scotland, and a world-leading dementia expert, said the link with chronic illness and inflammation "had been known for decades" and said preventative strategies were a key part of why his organisation had been set up.

Of the 10,095 participants, 639 had dementia after follow-up in 31 years.

Developing two or more conditions between 60 and 65 was linked to a 1.5-fold risk. 

Every five-year decrease in age onset for the conditions - up to the age of 70 -also pushed the risk of dementia up by 18%. The more severe the illness the stronger the link appeared.

The researchers, including from University College London, said the findings highlight "the role of prevention and management of chronic diseases over the course to adulthood to mitigate adverse outcomes in old age."

Dr Lucy Stirland, of the University of Edinburgh's Centre for Clinical Brain Sciences, said the study suggested that delaying or preventing the onset of a condition, by having a healthy lifestyle, could reduce the risk of dementia.

She said: "Many of the conditions listed in the BMJ paper can be modified by lifestyle (i.e. stopping smoking, reducing alcohol, eating healthily and exercising) and seeking early care from GPs (e.g. attending screening when offered, spotting high cholesterol or high blood pressure early and getting them managed).

"I think it's important to recognise multimorbidity both as a target for improving brain health, but also as a key issue for people who already have dementia."

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At 55, 63.0% (6259/9937) of participants had none of the 13 chronic diseases considered compared with 29.8% (2322/7783) at age 70.

The Whitehall II study is observational and so cannot prove a causal relationship between having multiple conditions and developing dementia but the authors said there were a number of reasons to think a causal relationship plausible.

Speaking to The Times Professor Paul Morgan of the UK Dementia Research Institute said the list of conditions that appeared to raise the risk of the disease was "rather surprising" because it included not only those known to foreshadow dementia such as Parkinson's disease "but also other neurological and neuropsychiatric disorders linked to neuroinflammation."

Debbie Tolson, Alzheimer Scotland Professor of Dementia at University of the West of Scotland, said society had been "slow to act" on known dementia risk factors.

She said: “For too long society has seen dementia as something unavoidable, something that happens to older people.

"Dementia is an illness and not a normal part of ageing. 

"We have been slow to act on the known dementia risk factors including comorbidities in midlife, mental health and the social determinants of health. 

"There are things we can do as individuals to improve our later life health such as eating better and exercising more.

"But there are also political and public health responses that influence our exposure to environmental risks including the quality of the air we breathe and the quality of the houses we live in.  

"If we are serious about dementia prevention, we need to be serious in the actions we take to alleviate depression, social isolation and a range of health inequalities.”