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EPIDEMIOLOGY OF ALZHEIMER’S
Markers on the path (1999)

Only about one person in 1,000 has Alzheimer’s disease at age 65, but that risk increases to about three in 100 by age 80. By age 90, about 18 in 100 have some symptoms.
The source of these data, as well as other important facts about Alzheimer’s risk, is the branch of health science known as epidemiology. Using past and current medical data, epidemiologists investigate the occurrence of diseases such as Alzheimer’s in specific populations. Their findings relate to such things as percentages, prevalence (frequency), incidence (new cases) and risk factors. All are crucial to understanding the evolution of a disease, as well as guiding the focus of research and evolving treatments.

The process
“Epidemiology involves a dynamic process of the generation of hypotheses, collection of data, and analysis, which requires constant collaboration among physicians, epidemiologists, biostatisticians and other researchers,” say some doctors. “It takes a speculative approach to develop a hypothesis or an idea, and then to collect and analyze data to prove or disprove the hypothesis.”
Doctors say that the proving and disproving aspects of epidemiology are essential. He cites scientific papers he published in the early 1980s that advanced the hypothesis that women were at greater risk for Alzheimer’s disease than men because of such a pattern found in prevalence studies. “A subsequent study conducted here at Mayo Clinic showed that the risk is the same for men and women,” Doctors say. “The new study measured the incidence of Alzheimer’s disease — that is, the risk of developing new disease over time in the population. The studies in the early 1980s had instead measured prevalence — that is, the number of people with Alzheimer’s at a given point in time in the population. There was a difference in prevalence between men and women because women live longer than men after developing Alzheimer’s disease. My initial hypothesis had been disproved.”
It’s this process that many times leads to better understanding of the causes of disease and may lead to treatments.
Steps toward prevention and treatment
Epidemiologic studies of Alzheimer’s try to identify ways to prevent the disease before it occurs, and to develop new treatments. This is especially important for families with a hereditary link to Alzheimer’s. By studying the patterns of the disease within families, researchers uncover genetic factors that predispose a person to the disease.
For example, everyone has two apolipoprotein genes, one inherited from each parent. Mayo Clinic researchers are interested in a particular form of this gene, known as APOE E4, that is known to predispose individuals to Alzheimer’s disease.
“We studied whether the risk of Alzheimer’s developing in individuals who have one or two copies of the APOE E4 gene was increased,” says Dr. Rocca. “This was especially important to people with a family history of Alzheimer’s. We were able to confirm that people who carry one or two copies of APOE E4 are more likely to get Alzheimer’s than is a person without the APOE E4 gene.”
Physicians and scientists also are studying the hypothesis that postmenopausal women who take estrogen lower their risk of Alzheimer’s disease. It already has been shown that estrogen helps prevent or slow the effects of osteoporosis and has some protective effect on the aging of the brain, including slowing cognitive decline.
Mayo Clinic researchers studied approximately 200 women with Alzheimer’s disease and 200 women of the same age who had no evidence of the disease. “We found that the women with Alzheimer’s disease had taken estrogen less frequently than those without the disease,” says Dr. Rocca.” So far, we’ve proven that estrogen and Alzheimer’s disease are linked in some way. But there are several possible ways to interpret our findings, and it’s premature to say that post-menopausal women should take estrogen to protect themselves against Alzheimer’s.”
More studies are needed to prove estrogen’s role in the prevention and treatment of the disease. “We’re on the path to resolving this question,” Dr. Rocca says. “But the final proof requires an experiment, such as a clinical trial, in which people are given estrogen and then followed to confirm a protective, beneficial effect of estrogen.”

When scientific advances apply to you
How do epidemiologic data find its way to consumers? Research findings of all kinds are published in medical or scientific publications that have undergone rigorous peer review. After results appear in journals, the general media often publishes and broadcasts major findings, raising obvious questions for consumers, such as how does this new finding apply to me and when will it be available as a treatment?
“I encourage people to stay abreast of new findings and to ask their physicians when they have a question,” Dr. Rocca says. “However, they also must realize that there’s a difference between a step forward in research and translation of that step into a new treatment. One study does not translate into an immediate treatment for which they would be candidates.”
Fortunately, there are an increasing number of Internet sites, books and other resources to aid consumers who want to know how research affects medical treatment. The Alzheimer’s Association, which reports major findings and advances to local chapters, is a reliable and accurate source.