July 16, 2008
People have long been advised to keep their cholesterol under control to limit their risk of heart disease. Now, research shows that HDL, the so-called good form of cholesterol, may also be good for the memory.
Researchers in the United Kingdom found that middle-aged people with higher levels of HDL, or high-density lipoprotein, scored higher on memory tests than people with lower levels of the health-promoting substance. The findings appeared in Arteriosclerosis, Thrombosis and Vascular Biology, a journal from the American Heart Association.
Unlike LDL, the bad form of cholesterol that has been linked to heart disease, HDL has the opposite effect of promoting heart health. So the higher your HDL, the better the protection. HDL levels above 40 mg/dL are generally considered healthy. Regular exercise boosts HDL levels . An HDL level of 60 or higher is considered heart-protective.
High levels of LDL cholesterol have long been associated with an increased risk of heart attacks, and doctors have long advised to keep cholesterol levels down. High total cholesterol levels have also been linked to an increased risk of Alzheimer's disease as well. [See the story, "High Cholesterol in Your 40s Increases Alzheimer's Risk"]
The current study focused on HDL. In the study, part of the long-running Whitehall II trials, researchers checked the cholesterol levels of 3,673 civil servants living in London. All were given memory tests around age 55, and again six years later. For the tests, researchers read a list of 20 one- and two-syllable words. Study participants then wrote down as many as they could remember over the next two minutes.
Individuals with higher HDL scored higher on the memory tests than others with lower HDL levels. In addition, among those whose HDL levels declined between tests, memory also declined.
The findings suggest that keeping levels of heart-healthy HDL high may protect against memory decline later in life.
"Memory problems are key in the diagnosis of dementia," said Archana Singh-Manoux, Ph.D., lead author of the study and Senior Research Fellow with the French National Institute for Health and Medical Research (INSERM, France) and the University College London in England. "We found that a low level of HDL may be a risk factor for memory loss in late midlife. This suggests that low HDL cholesterol might also be a risk factor for dementia."
The researchers defined low HDL as less than 40 mg/dL and high HDL as 60 mg/dL or higher. Their main findings were:
At age 55, participants with low HDL cholesterol showed a 27 percent increased risk of memory loss when compared to those with high HDL.
At age 60, participants with low HDL had a 53 percent increased risk of memory loss compared to the high HDL group.
During the five to six years between memory tests, study participants with decreasing HDL had a 61 percent increased risk of decline in their ability to remember words versus those with high HDL.
Men and women did not differ significantly in the link between lipids and memory loss, so researchers combined data from both sexes for analysis.
Total cholesterol and triglycerides did not show a link with memory decline.
Using statin drugs to lower low-density lipoprotein (LDL or "bad" cholesterol) showed no association with memory loss.
Scientists aren't sure why high HDL levels may help to keep memory sharp and protect against Alzheimer's. HDL helps clear "bad'" forms of cholesterol from the blood
"It is possible that HDL cholesterol prevents formation of beta-amyloid," Dr. Singh-Manoux said, referring to the protein that in its toxic form builds up in the brains of those with Alzheimer's, forming nerve-destroying plaques. "HDL could also affect memory through its influence on atherosclerotic disease and stroke, or vascular injury," he said."Finally, HDL cholesterol may influence memory through its anti-inflammatory and antioxidant effects."
"Many previous investigations into the association between lipids and memory in the elderly have focused on total or LDL cholesterol because of their status as proven risk factors for cardiovascular disease," Dr. Singh-Manoux addedd. "Our results show HDL cholesterol to be important for memory. Thus, physicians and patients should be encouraged to monitor levels of HDL cholesterol."
In an accompanying editorial, Anatol Kontush, Ph.D. and M. John Chapman, Ph.D., D.Sc., at INSERM and University Pierre and Marie Curie in Paris, France, noted, "It is tempting to speculate that increasing levels of "good cholesterol" might protect our memories. However, unfortunate results in large interventional trials with dietary antioxidants suggest that we should remain cautious when proposing therapeutic intervention on the basis of observational studies which do not allow causation to be inferred. Nonetheless, these studies demand that we focus more effort on research at the interface between HDL and brain function."
To raise HDL and lower LDL cholesterol, experts recommend various steps. These include lifestyle measures like:
Regular exercise, especially aerobic exercise like brisk walking, jogging, bike riding or swimming, can lower LDL and raise HDL. Exercise like yoga and tai chi, which are easier on the joints, may be a good option for the elderly.
Moderate alcohol consumption. One drink a day, especially red wine, may lower LDL.
Weight loss can have a dramatic effect on lowering LDL.
A heart-healthy diet, including eliminating unhealthy trans fats from the diet and consuming monounsaturated fats like olive oil.
If lifestyle measures do not produce dramatic improvements in cholesterol, statin drugs can also improve LDL cholesterol levels. Some studies suggest that statins may help to lower the risk of Alzheimer's, while others show no proven benefits. Additional research is needed to resolve this issue.
Finally, high dose niacin is known to be one of the most effective ways of increasing good, HDL cholesterol. Consult your doctor before considering taking niacin.
David Gimeno, Ph.D.; Mika Kivimaki, Ph.D.; Eric Brunner, Ph.D.; and Michael G. Marmot, M.D., Ph.D.: Whitehall II study. American Heart Association, July 1, 2008