The study by researchers at Kaiser Permanente's Division of Research and the University of Kuopio in Finland appears in the journal Dementia and Geriatric Cognitive Disorders.
The four-decade study of 9,844 men and women found that having high cholesterol in midlife (240 or higher milligrams per deciliter of blood) increases, by 66 percent, the risk for Alzheimer's disease later in life.
Even borderline cholesterol levels (200 - 239 mg/dL) in midlife raised risk for late-life vascular dementia by nearly the same amount: 52 percent.
Vascular dementia, the second most common form of dementia after Alzheimer's disease, is a group of dementia syndromes caused by conditions affecting the blood supply to the brain.
Scientists are still trying to pinpoint the genetic factors and lifestyle causes for Alzheimer's disease.
By measuring cholesterol levels in 1964 to 1973 based on the 2002 Adult Treatment Panel III guidelines (the current practice standard) when the Kaiser Permanente Northern California members were 40 to 45 years old, then following the participants for 40 years, this study is the largest long-term study with the most diverse population to examine the midlife cholesterol levels and late-life dementia.
"Our study shows that even moderately high cholesterol levels in your 40s puts people at greater risk for Alzheimer's disease and vascular dementia decades later," said the study's senior author.
Rachel Whitmer, Ph.D., a research scientist and epidemiologist at the Kaiser Permanente Division of Research in Oakland, Calif.
"Considering that nearly 100 million Americans have either high or borderline cholesterol levels, this is a disturbing finding. The good news here is that what is good for the heart is also good for the mind, and this is an early risk factor for dementia that can be modified and managed by lowering cholesterol through healthy lifestyle changes," the expert added.