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.