Researchers then took a look at their files some three decades later. Almost 600 of the people who originally participated in the study went on to be diagnosed with Alzheimer's disease or vascular dementia, which is the second-most common form of dementia after Alzheimer's. The patients were diagnosed between 1994 and 2007.
The new study found that adults in their 40s with high cholesterol levels were 66 percent more likely to develop Alzheimer's disease when they got older. A high level is 240 or higher milligrams per deciliter.
Those with borderline cholesterol levels, between 200 and 239 milligrams per deciliter of blood, were at 52 percent greater risk of developing vascular dementia.
The study is published online in the journal Dementia & Geriatric Cognitive Disorders.
The Centers for Disease Control in Atlanta says an estimated five million Americans have Alzheimer's disease. The disease usually begins after age 60.
Known risk factors include family history, high cholesterol, high blood pressure, and diabetes.
According to the CDC, there are several treatable diseases and conditions that can mimic symptoms of Alzheimer's disease. Those include depression, thyroid problems and dehydration.
There is no known cure for Alzheimer's disease.