What do we know about diet and prevention of Alzheimer's?

Can eating certain foods or following a certain diet help prevent or delay the development of dementia caused by Alzheimer's disease? Many studies show that what we eat affects the aging brain's ability to think and remember. These findings have led to a study of common dietary patterns and whether they can make a difference.

The Mediterranean diet, the related MIND diet (which includes elements aimed at lowering blood pressure) and other models of healthy eating have been associated with cognitive benefits in studies, although the evidence is not as strong as for other interventions such as physical activity, blood pressure reduction and cognitive training. Researchers are now testing these diets more closely to see if they can prevent or delay Alzheimer's disease or age-related cognitive decline.

Diet and risk of dementia

Changes in the brain can occur years before the first symptoms of Alzheimer's disease appear. These early changes in the brain point to the possibility of preventing or delaying the symptoms of dementia. Scientists are looking at many possible ways to do this, including medications, lifestyle changes, and combinations of these measures. Unlike other risk factors for Alzheimer's disease that we cannot change, such as age and genetics, people can control lifestyle choices such as diet, exercise and cognitive training.

How can what we eat affect our brains? It is possible that a certain diet affects biological mechanisms, such as oxidative stress and inflammation, that underlie Alzheimer's disease. Or perhaps diet acts indirectly by influencing other risk factors for Alzheimer's disease, such as diabetes, obesity and heart disease. A new line of research is examining the relationship between gut microbes -- tiny organisms in the digestive system -- and the aging processes that lead to Alzheimer's disease.

The Mediterranean Diet and MIND Diet and Alzheimer's

The Mediterranean diet is a diet that emphasizes fruits, vegetables, whole grains, legumes, fish and other seafood, unsaturated fats such as olive oil, and low amounts of red meat, eggs, and sweets. A variation of this diet called MIND (Mediterranean-DASH Intervention for Neurodegenerative Delay) includes the DASH (Dietary Approaches to Stop Hypertension) diet, which has been shown to reduce high blood pressure, a risk factor for Alzheimer's disease.

Some, but not all, observational studies - those that observe people or measure certain outcomes without treatment - have shown that the Mediterranean diet is associated with a lower risk of developing dementia. These studies compared cognitively normal people who ate a Mediterranean diet with those who ate a Western-style diet, which contains more red meat, saturated fat and sugar.

Evidence in favor of the MIND diet comes from observational studies of more than 900 older adults without dementia, which have shown that adherence to the MIND diet is associated with a reduced risk of developing Alzheimer's disease and a slower rate of cognitive decline.

Not all studies have shown an association between good nutrition and improved cognition. In general, the evidence suggests, but does not prove, that following a Mediterranean or similar diet can help reduce the risk of developing Alzheimer's dementia or slow cognitive decline. To learn more, scientists, backed by the NIA and other organizations, are conducting clinical trials, which are considered the gold standard of medical evidence, to shed light on all causes and effects. (A list of trials currently recruiting participants is at the end of this article.)

Although scientists do not yet know why the Mediterranean diet may help the brain, its effects on improving cardiovascular health may in turn reduce the risk of dementia. Two recent studies suggest that within this diet, fish consumption may be the strongest factor in improving cognitive function and slowing cognitive decline. In contrast, the typical Western diet increases the risk of cardiovascular disease, possibly contributing to more rapid brain aging.

In addition, a Mediterranean diet may increase certain nutrients that can protect the brain through anti-inflammatory and antioxidant properties. It can also inhibit beta-amyloid deposits, which are found in the brains of people with Alzheimer's disease, or improve cellular metabolism, protecting against the disease.

What do we know about individual foods?

Many foods - blueberries, leafy greens, curcumin (found in turmeric) and others - have been studied for their potential cognitive benefits. These foods have been thought to have anti-inflammatory, antioxidant, or other properties that may help protect the brain. So far, there is no evidence that eating or not eating a particular product can prevent Alzheimer's disease or age-related cognitive decline.

But scientists continue to look for a clue. One study, based on reports from older adults about their eating habits, found that eating a daily serving of green leafy vegetables, such as spinach and kale, was associated with slower age-related cognitive decline, possibly due to the neuroprotective effects of some nutrients. Studies have also shown that a diet that includes regular fish consumption is associated with higher cognitive function and slower cognitive decline with age. Another recent study on mice found that eating large amounts of salt increased levels of the tau protein found in the brains of people with Alzheimer's disease and caused cognitive decline.

What about vitamins and supplements?

Many over-the-counter vitamins and dietary supplements, including vitamins B and E and ginkgo biloba, have been studied in observational studies and clinical trials in terms of preventing Alzheimer's disease or cognitive decline. The idea is that these dietary supplements may fight oxidative damage or inflammation, protect nerve cells, or affect other biological processes associated with Alzheimer's disease.

Take DHA (docosahexaenoic acid), for example. Studies in mice have shown that this omega-3 fatty acid, found in salmon and some other fish, reduces beta-amyloid plaques, a hallmark of Alzheimer's disease. However, human clinical trials have yielded mixed results. In a study of 485 elderly people with age-related cognitive decline, those who took DHA supplementation daily for 24 weeks showed improved learning and memory compared to those who took a placebo. Another study involving 4,000 elderly people, conducted primarily to study eye disease, found that taking omega-3 supplements, alone or in combination with other supplements, did not slow cognitive decline.

No vitamin or supplement is currently recommended for the prevention of Alzheimer's disease or cognitive decline. Although many are widely available in pharmacies and online, they have not been tested for their effects on thinking. Their safety and effectiveness are virtually unknown, and they may interact with other medications. (Note: vitamin B12 or folate deficiency can cause memory problems, which may be reversible with proper treatment).

For more information, visit the National Center for Complementary and Integrative Health and the U.S. Food and Drug Administration.

Researchers continue to search for answers

The idea of Alzheimer's disease as a metabolic disease affecting the brain and markers of Alzheimer's disease, such as glucose metabolism, have prompted scientists to move in different directions. In addition to the Mediterranean diet and its variations, they are studying other diets as well as individual foods and nutrients.

For example, a ketogenic diet is a high-fat, low-carb diet that stimulates the production of ketones, chemicals that help brain cells function. Studies show that this diet can have different effects on gut bacteria in people with and without cognitive impairment, and can help brain cells make better use of energy, improving their overall functioning.

Researchers are looking for answers to these questions:

• What foods are critical to brain health and should be included in dietary interventions?
• Which groups of people are most likely to benefit from dietary interventions to prevent dementia and cognitive decline?
• Can dietary interventions introduced in mid-life lead to better results?

These clinical trials are recruiting participants to test dietary interventions:

• Enhanced Mediterranean Diet to Prevent Alzheimer's Disease - Cognitively normal adults age 65 and older in Kansas City, Kansas, were randomly assigned to a Mediterranean or low-fat diet to evaluate the effects on cognitive function, brain volume and other indicators.
• Mediterranean Diet, Weight Loss and Cognitive Function in Obese Elderly - This Chicago study will test the effect of a Mediterranean diet with and without caloric restriction on weight loss and cognitive function in obese older adults.
• Multicultural Healthy Diet to Reduce Cognitive Decline - This 18-month study will examine whether an anti-inflammatory diet designed for a multicultural population in the Bronx, New York, can improve cognitive function.
• Brain Energy for Amyloid Transformation in Alzheimer's Disease - Older adults with MCI in Winston-Salem, North Carolina, were randomized to follow a modified Mediterranean ketogenic diet (low carb, high fat) or an American Heart Association high carb, low fat diet for 16 weeks, with follow-up to assess effects on cognition and Alzheimer's disease biomarkers.

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