Developing illnesses such as heart disease and diabetes before the age of 55 could significantly increase the chance of having dementia in later life, a new study suggests.
Strokes or the onset of mental health conditions such as anxiety and depression between 55 and 70 could also increase risk two-fold, according to research.
Experts at the University of Oxford said up to 80 per cent of dementia patients suffer from two or more chronic health conditions, but there is a "lack of understanding" when it comes to specific illnesses, age of onset and the link to dementia.
They identified "critical time windows" in which certain illnesses pose the greatest risk to patients.
For the study, published in Brain Communications, researchers analysed data from 282,712 people using the UK Biobank and looked at patterns for 46 chronic health conditions.
They found that heart conditions such as heart disease and atrial fibrillation, as well as diabetes, before the age of 55 was most strongly linked to dementia risk.
However, from 55 to 70 years of age, mental health conditions such as anxiety and depression, as well as conditions such as stroke, increased the risk of dementia two-fold.
Sana Suri, an associate professor and Oxford Brain Sciences senior fellow, said: “Although we knew that multimorbidity increased the risk of dementia, it was unclear which combinations of health conditions had the most impact and in what sequence.
“This study has identified how specific illnesses tend to co-exist with each other, and also the critical time windows in which they could pose the greatest risk.”
Elsewhere, the study suggests people who have conditions such as heart disease and diabetes in middle age, followed later by conditions such as stroke and mental health disorders were at the greatest risk of developing dementia.
Ms Suri said the presence of other illnesses should be taken into account when estimating a person’s risk of developing dementia, and could help develop strategies that aim to reduce risk at certain points in life.
She added: “This study identified associations between multimorbidity and dementia risk but we need to understand more about why this happens.
“We also need to try to replicate the study in more diverse groups of people to ensure the results are representative of the population.
“Future studies could examine whether efforts to manage or prevent cardiovascular problems in early-to-midlife, followed by mental health and neurological disorders when people are in their 50s and 60s, might reduce the risk of dementia.”
