While there are many examples of people who live for 100 years or more with little evidence of a decline in brain function, many others are not so fortunate and experience a neurodegenerative disorder, such as Alzheimer disease or Parkinson disease. Although an increasing number of genetic factors that may affect the risk for neurodegenerative disorders are being identified, emerging findings suggest that dietary factors play major roles in determining whether the brain ages successfully or experiences a neurodegenerative disorder. Dietary factors may interact with disease-causing or predisposing genes in molecular cascades that either promote or prevent the degeneration of neurons. Epidemiologic findings suggest that high-calorie diets and folic acid deficiency increase the risk for Alzheimer disease and Parkinson disease; studies of animal models of these disorders have shown that dietary restriction (reduced calorie intake or intermittent fasting) and dietary supplementation with folic acid can reduce neuronal damage and improve behavioral outcome. Animal studies have shown that the beneficial effects of dietary restriction on the brain result in part from increased production of neurotrophic factors and cytoprotective protein chaperones in neurons. By keeping homocysteine levels low, folic acid can protect cerebral vessels and prevent the accumulation of DNA damage in neurons caused by oxidative stress and facilitated by homocysteine. Although additional studies are required in humans, the emerging data suggest that high-calorie diets and elevated homocysteine levels may render the brain vulnerable to age-related neurodegenerative disorders, particularly in persons with a genetic predisposition to such disorders.