Past research has indicated that high cholesterol might play a role in the development of Alzheimer's disease, although results have been inconclusive. When studying this link, researchers have often viewed cholesterol as a single harmful element or focused specifically on low-density lipoproteins (LDL), a harmful form of cholesterol that puts individuals at risk for cardiovascular disease. In a new study, researchers from the University College London examined high-density lipoproteins (HDL), commonly known as good cholesterol, and their results indicate that higher levels of HDL during middle age may ward off memory loss later in life.

The study, which appears in the American Heart Association's journal Arteriosclerosis, Thrombosis and Vascular Biology, included data from 3,673 middle-aged subjects. They had received memory assessments sometime between 1995 and 1997, when they averaged 55 years of age, and were tested again five years later. Along with these cognitive evaluations, the researchers also examined blood samples, measuring total cholesterol levels and HDL levels. Because HDL removes extra cholesterol from the blood, preventing it from collecting in arteries and reducing the risk for heart attack and stroke, higher HDL levels along with low LDL levels are desirable.

When they were first evaluated, subjects with low HDL levels had a 27 percent greater chance of performing poorly on memory tests compared to those with high HDL, and this association increased as the subjects aged. By the second assessment, those with low HDL had a 53 percent greater chance of having memory deficits, and subjects whose HDL levels decreased in the time between the first and second assessment were also more likely to display a decline in their memory skills. The researchers observed no significant differences between men and women, and these results held even when they controlled for other factors, including education and occupation, history of heart disease or stroke, use of medications, and smoking or alcohol consumption.

The researchers describe multiple reasons that may be behind the connection between cholesterol and cognitive decline. In particular, HDL aids in regulating the production of beta-amyloid, the neural plaques commonly found in the brains of individuals with Alzheimer's disease. While the exact role these plaques play in Alzheimer's disease remains unclear, the researchers suggest that high levels of HDL may prevent their formation and possibly protect against or slow the progress of this devastating neurological condition. At the same time, HDL may promote cognitive health by preventing cardiovascular disease, which can have harmful effects on brain functioning.

Previous findings on the association between cholesterol and cognitive decline have been inconsistent, and this study also raises questions that cannot yet be answered. The researchers found no association between total cholesterol and memory loss, and statins, a medication commonly used to lower high cholesterol, also appeared to have no effect on cognitive performance. While this study has not clarified the mechanisms underlying the link between cholesterol and memory, it has shown that HDL and LDL both have a large impact on health, and physicians and their patients should monitor both forms of cholesterol, as this may allow many to identify and address health problems early on. Maintaining a healthy diet and active lifestyle remain integral to developing healthy cholesterol levels, and with a greater awareness of the factors that influence cognition, more individuals will be able to modify their risk factors and improve their overall health as they age.