Elderly people who avoid dementia despite having high cholesterol may hold the key to finding a cure, say scientists.

Researchers at Icahn School of Medicine at Mount Sinai, New York, found over-85s with good brain function whose cholesterol had increased since middle age were a 32 percent less likely to suffer mental decline.

Scientists admit they were left perplexed by the paradox, since cholesterol is known to be bad for cognition in old age.

Discovering the secret may lead to the development of the world's first 'dementia drug', experts claim.

Researchers found over-85s with good brain function whose cholesterol had increased since middle age were a 32 percent less likely to suffer mental decline (file image)

'We hope to identify genes or other protective factors for cogitive decline by focusing on cognitively healthy very old people who are more likely to carry protective factors,' said lead author Professor Jeremy Silverman.

'Long-lived individuals who are cognitively intact despite high risk should be targeted in research studies seeking protective factors - which could help identify future drugs and therapies to treat dementia and Alzheimer's disease.'

Medications that target the cause are desperately being sought with the number of patients set to reach one million by 2025 and two million by 2050.

Researchers used data collected across seven decades to study a link between cholesterol and a decline in cognitive function.

Professor Silverman, a psychiatrist at Icahn, said: 'Our results have important implications for researching genetic and other factors associated with successful cognitive ageing.

'The data are consistent with our protected survivor model, among individuals who survive to very old age with intact cognition, those with high risk factor levels are more likely to possess protective factors than those with lower risk factor levels.'

He underlined the results do not imply the elderly should increase their cholesterol.

'We don't think high cholesterol is good for cognition at 85 but its presence might help us identify those who are less affected by it,' Professor Silverman said.

The study found 85- to 94-year-olds with good mental faculties even though their cholesterol had increased since they were 40 had a 32 percent reduced risk for marked cognitive decline over the next decade.

This was compared with those aged 75 to 84 who were at 50 percent more risk even though they were younger.

Professor Silverman, whose findings are published in Alzheimer's & Dementia: The Journal of the Alzheimer's Association, said it suggests the older group may have a protective factor that could be identified and studied.

His team analyzed data from the original Framingham Heart Study which has been following the cardiovascular health of thousands of residents of the town in Massachusetts for 70 years.

It began in 1948 with 5,209 adult subjects and is now on its third generation of participants.

The researchers looked at rising cholesterol and substantial decline in mental function - called marked cognitive decline - at five different time points in life.

Generally there was a link. However as the age of surviving participants increased some associations were reduced or even reversed - in particular in the subgroup of cognitively healthy 85- to 94-year-olds with high cholesterol.

This contrasts with samples in other studies that have focused on elderly subjects primarily below age 75, where midlife cholesterol was associated with increased risk of cognitive decline.

Professor Silverman and colleagues are now studying other risk factors for cognitive decline - including body mass index and blood pressure.

He said: 'We hope to identify genes or other protective factors for cogitive decline by focusing on cognitively healthy very old people who are more likely to carry protective factors.'