Does the Mediterranean diet improve brain health?

DEAR DOCTOR K:

I know that the Mediterranean diet is supposed to improve heart health. Recently I heard it also improves brain health. Is that pretty well established? Of all the organs I want to protect, my brain is “numero uno.”

DEAR READER:

I agree with your priorities regarding organs: My brain is “numero uno,” too. And I do think the evidence is strong that the Mediterranean diet does protect the brain. This diet emphasizes fruits, vegetables, grains, beans, nuts, fish, poultry, olive oil and wine.

Most of the evidence about the health benefits of the Mediterranean diet comes from observational studies. In such studies, large numbers of people are followed for many years. Information about their lifestyle and any diseases they may have developed is collected. Most such studies have found that people who follow the Mediterranean diet have a lowered risk of developing Alzheimer’s disease.

However, observational studies that link a lifestyle behavior to a disease (or to protection against a disease) can’t prove causation. That is, it may be true that people who follow a Mediterranean diet do have a lower risk of Alzheimer’s, but that doesn’t mean that the diet is the reason for the lower risk. Something else about people who follow the Mediterranean diet may protect them from Alzheimer’s.

To prove causation, scientists must conduct randomized controlled trials. Such studies are not often done to test the effect of lifestyle on health; they are very expensive and impractical. To prove that the Mediterranean diet reduced the risk of Alzheimer’s, scientists would need to have thousands of people assigned at random to either follow the Mediterranean diet or not — for 20 to 30 years. And the scientists would need to ensure that those tens of thousands of people really were eating the way the study told them to eat.

Smaller and shorter randomized trials are more practical, but less likely to provide definite answers. One such study was reported in 2013. About 300 people in Spain were assigned to follow either the Mediterranean diet or a low-fat diet for about four years. Intensive tests of thinking were performed at the start of the study and after four years. The people who followed the Mediterranean diet had a modest improvement in several measures of thinking. Those who followed the low-fat diet had a slight deterioration.

In 2015, a novel observational study was reported. About 700 people, with an age range of 65 to 90, had magnetic resonance imaging (MRI) performed to examine their brains. Those who followed the Mediterranean diet had larger brains, or brains that had shrunk less since they were young. They had brains that were the size of people five years younger, compared to people who did not follow the diet. The specific factors in the diet that seemed most closely tied to larger brains were eating lots of fish and little red meat.

I’m not arguing that the Mediterranean diet has been proven to protect the brain. But the evidence is strong enough that I have long since adopted the diet myself.