Using Multicountry Ecological and Observational Studies to Determine Dietary Risk Factors for Alzheimer's Disease.

UNLABELLED: Rates of Alzheimer's disease (AD) are rising worldwide. The most important risk factors seem to be linked to diet. For example, when Japan made the nutrition transition from the traditional Japanese diet to the Western diet, AD rates rose from 1% in 1985 to 7% in 2008. Foods protective against AD include fruits, vegetables, grains, low-fat dairy products, legumes, and fish, whereas risk factors include meat, sweets, and high-fat dairy products. The evidence comes from ecological and observational studies as well as investigations of the mechanisms whereby dietary factors affect risk. The mechanisms linking dietary risk factors to AD are fairly well known and include increased oxidative stress from metal ions such as copper as well as from advanced glycation end products associated with high-temperature cooking, increased homocysteine concentrations, and cholesterol and its effects on amyloid beta, insulin resistance, and obesity. Lower 25-hydroxyvitamin D concentrations also are associated with increased risk of AD. In addition to reviewing the journal literature, a new ecological study was conducted using AD 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 less milk 5 years before AD prevalence had the highest correlations with AD prevalence in this study. Thus, reducing meat consumption could significantly reduce the risk of AD as well as of several cancers, diabetes mellitus type 2, stroke, and, likely, chronic kidney disease. TEACHING POINTS: • Single-country ecological data can be used to find links between diet and AD because the national diet changes, such as during the nutrition transition to a Western diet. • Multicountry ecological studies can be used to find links between dietary factors and risk of AD. • Prospective observational studies are useful in linking dietary components and patterns to risk of AD. • The most important dietary link to AD appears to be meat consumption, with eggs and high-fat dairy also contributing. • Diets high in grains, fruits, vegetables, and fish are associated with reduced risk of AD, but these factors cannot counter the effects of meat, eggs, and high-fat dairy. • Higher vitamin D status is associated with reduced risk of AD.