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Globally, about 42 million people now have dementia, with Alzheimer's disease as the most common type of dementia. Rates of Alzheimer's disease are rising worldwide. The most important risk factors seem to be linked to diet, especially the consumption of meat, sweets, and high-fat dairy products that characterize a Western Diet. For example, when Japan made the nutrition transition from the traditional Japanese diet to the Western diet, Alzheimer's disease rates rose from 1% in 1985 to 7% in 2008, with rates lagging the nutrition transition by 20-25 years. The evidence of these risk factors, which come from ecological and observational studies, also shows that fruits, vegetables, grains, low-fat dairy products, legumes, and fish are associated with reduced risk. "Using Multicountry Ecological and Observational Studies to Determine Dietary Risk Factors for Alzheimer's Disease," a review article from the Journal of the American College of Nutrition presents the data.

In addition to reviewing the journal literature, a new ecological study was conducted using Alzheimer's disease prevalence from 10 countries (Brazil, Chile, Cuba, Egypt, India, Mongolia, Nigeria, Republic of Korea, Sri Lanka, and the United States) along with dietary supply data 5, 10, and 15 years before the prevalence data. Dietary supply of meat or animal products (minus milk) 5 years before Alzheimer's disease prevalence had the highest correlations with Alzheimer's disease prevalence in this study. The study discussed the specific risk each country and region faces for developing Alzheimer's disease based on their associated dietary habits.

Residents of the United States seem to be at particular risk, with each person in the U.S. having about a 4% chance of developing Alzheimer's disease, likely due in part to the Western dietary pattern, which tends to include a large amount of meat consumption. The author, William B. Grant, states, "reducing meat consumption could significantly reduce the risk of Alzheimer's disease as well as of several cancers, diabetes mellitus type 2, stroke, and, likely, chronic kidney disease."

He concludes, "Mounting evidence from ecological and observational studies, as well as studies of mechanisms, indicates that the Western dietary pattern -- especially the large amount of meat in that diet -- is strongly associated with risk of developing Alzheimer's disease and several other chronic diseases. Although the traditional Mediterranean diet is associated with about half the risk for Alzheimer's disease of the Western diet, the traditional diets of countries such as India, Japan, and Nigeria, with very low meat consumption, are associated with an additional 50% reduction in risk of Alzheimer's disease."