Different Dietary Patterns and their Effects on Alzheimer’s Disease

Alzheimer's disease (AD) is among the most common neurodegenerative diseases worldwide, yet the factors that drive it remain misunderstood. Growing evidence links diet, obesity, and the gut-brain axis to neurodegeneration, and three genes involved in metabolism and amyloid biology (SIRT1, APP, and LEP) correlated with diet. This review examines what is known about how four dietary patterns affect brain health: the US Western diet, the Japanese diet, the Mediterranean (including MIND) diet, and intermittent fasting. The United States and Japan, two high-income countries with very different diets, are compared directly using food-group intake, ultra-processed food intake (USA 55.0%, Japan 38.2% of calories), obesity rate, life expectancy, and early-onset dementia prevalence. Japan has a comparatively older population and therefore a higher all-age dementia prevalence than the United States, despite having a lower early-onset dementia prevalence. The Mediterranean diet and intermittent fasting are discussed through clinical, observational, and laboratory research rather than population data. These findings are hypothesis-generating and motivate gene-expression and longitudinal studies of diet, obesity, and brain health.