A "Mediterranean diet" does not lower the odds of developing dementia, a 20-year study from Sweden suggests.

Previous studies of the potential cognitive benefits of the so-called Mediterranean diet – broadly defined as a diet rich in vegetables, legumes, fruits, fish, and unsaturated fats, such as olive oil, and low in dairy, red meats, and saturated fats – have turned up mixed results, according to the National Institutes of Health's National Institute on Aging (NIA).

However, two 2019 studies in the journal JAMA that included thousands of people and decades of follow-up found no evidence that the Mediterranean diet reduces dementia risk, or that that diet quality affects dementia risk, broadly.

The new Swedish study casts further doubt on the diet's brain-bolstering benefits.

"We did not find any association between either conventional dietary habits or adherence to a Mediterranean diet and subsequent incidence of dementia," first author Dr. Isabelle Glans, a member of the Clinical Memory Research unit at Lund University in Sweden, told Live Science in an email.

These findings, which align with those found in prior studies of similar size and length, were published 12 October in the journal Neurology.

That said, similar to many previous studies, the research relied on self-reported dietary data from participants, which may not be wholly accurate and can somewhat skew the interpretation of the results.

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The effect of diet on dementia

Physiologist Ancel Keys and biochemist Margaret Keys, a husband-wife duo, derived the Mediterranean diet from Ancel's influential research on the link between men's diets and their risk of heart attack and stroke.

The research suggested that diets low in saturated fat protect against cardiovascular disease, and Ancel and Margaret drew loose inspiration from Greek, Italian, and other Mediterranean cuisines to write their popular diet books, according to The Conversation.

In theory, by guarding against cardiovascular disease, the Mediterranean diet could indirectly reduce the risk of dementia, according to the NIA.

That's because plaque build-up in arteries (atherosclerosis), strokes, high blood pressure, high blood sugar, and diabetes may all raise the risk of dementia, and maintaining a healthy diet can help lower the risk of these conditions.

The Swedish study does not completely overturn this idea, but it suggests that diet alone does not exert a notable influence on the course of cognitive function in later life.

"Diet as a singular factor may not have a strong enough effect on cognition, but is more likely to be considered as one factor embedded with various others, the sum of which may influence the course of cognitive function," Dr. Nils Peters, a neurology specialist at the Stroke Center Klinik Hirslanden in Zurich, Switzerland, and Benedetta Nacmias, an associate professor of neurology at the University of Florence, wrote in a 12 October commentary published in Neurology.

These other factors include exercising regularly; avoiding smoking; drinking only in moderation; and keeping one's blood pressure in check, they wrote. In particular, evidence suggests that regular physical activity and consistent blood pressure control are protective against cognitive decline and that these factors are likely more influential than diet, according to the NIA.

The new research included data from about 28,000 people who took part in the Malmö Diet and Cancer Study, a study launched in the Swedish city of Malmö in the 1990s.

At the study's start, the participants were 58 years old, on average; at that time, they provided dietary data in the form of a week-long food diary; a detailed questionnaire about the frequency and quantity at which they consumed various foods; and an interview about their dietary habits.

Based on this information, the research team "scored" each participant on how strictly they adhered to standard Swedish dietary recommendations or to a specific version of the Mediterranean diet.

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Over the next 20 years, 1,943 people, or 6.9 percent of the participants, were diagnosed with some form of dementia. These diagnoses included the two most common forms of dementia: dementia related to Alzheimer's disease (AD) and vascular dementia, which arises from poor blood flow to the brain.

Participants who stuck to either a conventional diet or to the Mediterranean diet did not have a lower incidence of any type of dementia than participants who did not adhere well to either, the researchers found. They also found no link between diet and a specific marker of Alzheimer's disease, which they screened for in about 740 of the participants with cognitive decline.

Overall, the study "does not indicate a specific effect of diet on the course of cognitive function," Peters and Nacmias wrote. But like similar studies conducted in the past, the work does have its limitations, they noted.

For example, the baseline dietary information gathered from each participant may not reflect how their diet has changed through time. What's more, study participants may have somewhat misreported their true dietary habits.

The best way to test the long-term effect of the Mediterranean diet on cognition would be to run a long-term randomized controlled trial. In such a trial, groups of participants would be asked to follow specific diet plans, or even given all their food, for a long period of time and they'd be monitored for signs of dementia throughout.

"However, it is probably not feasible to design a 20-year randomized controlled trial with strict dietary habits to adhere to," the study authors wrote in their report.

Some shorter term trials of this type can be found on the NIA website and the Alzheimers.gov Clinical Trials Finder. However, for now, the available evidence suggests that the Mediterranean diet is not a silver bullet for dementia prevention.

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