You may know that a Mediterranean diet — rich in fruits, vegetables, olive oil, legumes, whole grains and fish — offers many heart-healthy benefits. But a Mediterranean diet may also benefit your brain.

Studies show people who closely follow a Mediterranean diet are less likely to have Alzheimer's disease than people who don't follow the diet.

Research suggests a Mediterranean diet may:

• Slow cognitive decline in older adults
• Reduce the risk of mild cognitive impairment (MCI) — a transitional stage between the cognitive decline of normal aging and the more-serious memory problems caused by dementia or Alzheimer's disease
• Reduce the risk of MCI progressing into Alzheimer's disease

It's unclear which parts of the Mediterranean diet might protect brain function.

Researchers speculate that making healthy food choices may improve cholesterol and blood sugar levels and overall blood vessel health, which may in turn reduce the risk of MCI or Alzheimer's disease.

Another theory suggests that following a Mediterranean diet may help prevent brain tissue loss associated with Alzheimer's.

But for now it's difficult to say what exactly explains the relationship between following a Mediterranean diet and reducing the risk of Alzheimer's disease. Some research shows that individuals with moderate seafood consumption had fewer Alzheimer's-related changes in their brains among people carrying the apolipoprotein E (APOE e4) gene, which is thought to increase Alzheimer's risk.

But overall, evidence isn't strong enough to show that the Mediterranean diet reduces Alzheimer's disease risk. One issue is that most studies on the effects of diet on dementia are based on dietary questionnaires completed by participants who may have trouble recalling what they ate or have memory problems.

So, one study used a modified food questionnaire developed for use in older adults to address this issue. The study looked at whether following a Mediterranean diet, the Dietary Approaches to Stop Hypertension (DASH) diet designed to treat high blood pressure or a hybrid diet that combined aspects of both diets known as the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet could reduce the risk of Alzheimer's disease.

The results showed that people who strictly followed any of the three diets had a lower risk of Alzheimer's disease. Moreover, even modest adoption of the MIND diet approach, such as eating two vegetable servings a day, two berry servings a week and one fish meal a week, appeared to lower the risk of Alzheimer's disease.

More research and clinical trials are needed to know to what degree a Mediterranean diet prevents Alzheimer's or slows the progression of cognitive decline. Nonetheless, eating a healthy diet is important to stay physically and mentally fit.