With an aging population, the world may be facing a dementia time bomb, says a researcher at the French National Institute for Health and Medical Research at University College, London, England.

But Archana Singh-Manoux may have found something to help defuse that bomb.

She recently led a study involving 3,700 British men and women that found that low levels of high-density lipoprotein (HDL) - commonly labelled the "good" cholesterol - were linked to declining memory by age 60, a decline that often precedes development of dementias such as Alzheimer disease.

Singh-Manoux says she hopes the findings of her study will focus attention on the possible role for higher levels of HDL cholesterol in protecting memory loss.

The study measured blood cholesterol levels and conducted a simple memory test on participants at age 55 and again at age 60. At 55, those with low HDL levels had a 27-per-cent higher risk of memory loss than those with high HDL. At age 60 that percentage had risen to 53 per cent.

The study did not look at reasons why HDL cholesterol may protect memory but Singh-Manoux speculates that one possibility is that it wards off the formation of beta-amyloid plaques in the brain, a hallmark of Alzheimer's.

Cholesterol is a fatty substance made naturally by the body and found in many foods. Low-density lipoprotein (LDL) - labelled the "bad" cholesterol - is the type that builds up in artery walls, making them hard and narrow, the condition known as atherosclerosis, which can lead to heart attacks or stroke. HDL, on the other hand, which takes excess cholesterol to the liver for elimination, can lower heart-attack risk.