Unhealthy cholesterol levels are linked to one of the key brain signatures of Alzheimer's disease, scientists have learned.
Relative amounts of "good" and "bad" cholesterol in the blood influence the build-up of harmful protein deposits in the brain called beta amyloid plaques, a study found.
The discovery may explain the well-known correlation between raised cholesterol and an increased risk of Alzheimer's.
Experts draw a sharp distinction between "good" cholesterol, or high density lipoprotein (HDL) and its evil twin - low density lipoprotein (LDL).
While high levels of LDL can lead to narrowed arteries and heart disease, HDL is protective.
The new US research suggests that the effects the two kinds of cholesterol have on the heart may be mirrored in the brain.
"Our study shows that both higher levels of HDL- good - and lower levels of LDL - bad - cholesterol in the bloodstream are associated with lower levels of amyloid plaque deposits in the brain," said study leader professor Bruce Reed, from the University of California at Davis (UC Davis).
"Unhealthy patterns of cholesterol could be directly causing the higher levels of amyloid known to contribute to Alzheimer's, in the same way that such patterns promote heart disease."
The scientists looked at 74 men and women aged 70 and over recruited from stroke clinics, support groups, senior citizens' facilities and the UC Davis Alzheimer's Disease Center.
Three participants were suffering from mild dementia, 38 had mild cognitive impairment - a non-serious loss of mental faculties - and 33 had no memory or thinking problems.
All had their brains scanned using a tracer chemical that binds with amyloid plaques so that they show up on the images.
Higher fasting levels of LDL and lower levels of HDL were both associated with more amyloid in the brain, according to the findings published online in the journal JAMA Neurology.
"This study provides a reason to certainly continue cholesterol treatment in people who are developing memory loss, regardless of concerns regarding their cardiovascular health," said professor Reed, associate director of the Alzheimer's Disease Center.
"It also suggests a method of lowering amyloid levels in people who are middle aged, when such build-up is just starting. If modifying cholesterol levels in the brain early in life turns out to reduce amyloid deposits late in life, we could potentially make a significant difference in reducing the prevalence of Alzheimer's, a goal of an enormous amount of research and drug development effort."
Co author Dr Charles DeCarli, director of the Alzheimer's Disease Center, said the discovery was a "wake up call" for those with a poor cholesterol balance.
Anyone having an LDL level above 100 milligrams (mg) per decilitre (dL) of blood and an HDL of less than 40 mg/dL should think about getting the two figures "into alignment".
"You have to get the HDL up and the LDL down," Dr DeCarli added.
Dr Laura Phipps, from the British charity Alzheimer's Research UK, said: "This study found an association between high cholesterol and levels of amyloid in the brain, which can be an early indicator of Alzheimer's.
"While this study did not investigate the mechanism behind the link, the findings add to existing evidence that cholesterol could play a role in the Alzheimer's disease process. Despite this, clinical trials carried out to date have not provided evidence to recommend cholesterol-lowering statin treatment as a way to treat or prevent Alzheimer's.
"This study adds to previous research suggesting that a healthy lifestyle in midlife could have benefits for our cognitive health into older age. Current evidence suggests the best way to keep our brain healthy is to eat a balanced diet, maintain a healthy weight, not smoke, exercise regularly and keep blood pressure and cholesterol in check. Anyone who has concerns about their cholesterol levels should talk to their GP."