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By Anne Harding

WEDNESDAY, November 10 ( — Controlling your cholesterol in middle age is good for your heart and blood vessels, but according to a new study, it may not lower your risk of Alzheimer’s disease, as some research has suggested.

High cholesterol levels have been linked to the buildup of brain plaques associated with Alzheimer's. However, studies exploring the relationship between cholesterol levels in midlife and dementia down the road have had mixed results. The new study, which appears in the journal Neurology, is the latest to question the long-term link between cholesterol and dementia.

Researchers in Sweden and the U.S. followed 1,462 Swedish women between the ages of 38 and 60 for more than three decades. All of the women were dementia-free when the study began; by the end, 11% had developed Alzheimer's disease or another form of dementia.

The women who had higher-than-average cholesterol levels at the start of the study did not appear to be at greater risk for dementia than women with low cholesterol, the study found. “On neither an individual nor population level can we determine whether a person will develop dementia based on a midlife cholesterol level,” the authors concluded.

By contrast, women whose cholesterol levels declined over the course of the study did appear to be at greater risk. More than 17% of the women whose cholesterol dropped the most went on to develop dementia, compared to just 9% of the women whose cholesterol levels held steady, the study found.

Drops in cholesterol aren't thought to cause dementia. Instead, the appetite changes and weight loss that often accompany the onset of dementia can cause cholesterol levels to fall, says Mary Haan, a professor of epidemiology at the University of California, San Francisco.

“That’s why a low cholesterol level is associated with a lower risk when you would expect that to be the other way around,” says Haan, who wrote an editorial on the study but wasn’t involved in the research.

Cholesterol-lowering statin drugs did not play a significant role in the results, since the vast majority of the study, which ended in 2000, took place before statins became widely used.

Haan cautions that the relationship between cholesterol levels and dementia risk could be different in men, who tend to have higher cholesterol for a longer period of time in middle age than women. “Midlife cholesterol may have a stronger effect on dementia in men than in women, and late-life cholesterol might affect dementia in women more than men,” she says.

And it's possible that the study underestimated the link between cholesterol and dementia, Haan adds. Long-term studies in older people are often inexact because of the high proportion of participants who die or drop out, she explains.

The study results do not give middle-aged people license to let their cholesterol levels run wild, of course. The authors, led by Michelle Mielke, PhD, a psychiatrist at Johns Hopkins University, in Baltimore, Md., emphasize that people should continue to eat a healthy diet, exercise, and, if necessary, take medication to control their cholesterol levels and minimize their risk of heart disease.