Mediterranean Diet Linked to Less Cognitive Decline

May 01, 2013

Adherence to a Mediterranean diet is linked to a reduced incidence of cognitive decline, a new population study has found. The association was seen in both blacks and whites but was not evident in patients with diabetes.

The study, which is the largest to look at the association between the Mediterranean diet and memory, is published in the April 30 issue of Neurology.

Lead author Georgios Tsivgoulis, MD, University of Athens, Greece, commented to Medscape Medical News, "My take-home message is that diet is important not just for a person's weight and appearance. It is also important for mental health in the long term. What you eat affects your cardiovascular health, your looks and your brain cells."

He recommended a few simple changes that can be made to anyone's diet to include more Mediterranean influence. These are to increase the consumption of fresh fruit, use raw olive oil (in salads), and eat more oily fish, such as salmon. He added that fried foods should be avoided, even if frying with olive oil, as when heated, the oil loses some of its beneficial properties.

 
… a very important contribution Dr. Melissa Armstrong
 

Commenting on the study for Medscape Medical News, Melissa Armstrong, MD, University of Maryland School of Medicine, Baltimore, said, "This report is a very important contribution to our understanding of how diet may affect risks of cognitive impairment and suggests that further investigation into diet as a way to possibly prevent cognitive decline is warranted." She added, "particularly given limited treatment options, finding ways to prevent cognitive impairment is critical."

She noted that strengths of the study included the ability to study a large cohort of ethnically diverse individuals, information on many possible confounders, and a thorough statistical analysis looking for patterns in the data.

"One must be cautious not to overemphasize the results, though, given limitations inherent in population-based studies (such as using cognitive screening not routinely used in clinical practice), the number of statistical analyses done (suggesting that some of the positive results may be due to chance alone), and the fact that the main finding just barely met statistical significance," she added.

REGARDS Study

For the current analysis, Dr. Tsivgoulis and colleagues examined data from the REGARDS (Reasons for Geographic and Racial Differences in Stroke) study, a large population-based study with oversampling of black participants and persons from the Stroke Belt region of the United States. Dr. Tsivgoulis noted that the Stroke Belt includes southern states, such as Alabama and North and South Carolina, parts of the United States with an increased risk for stroke. "This is a lower socioeconomic area of the US and includes a higher proportion of African Americans in the population," he added.

The study included more than 30,000 individuals age 45 years or older, self-identified as non-Hispanic black or white between 2003 and 2007. The main aim was to look at reasons for geographic and racial differences in stroke. Cognitive status was evaluated at baseline and annually during a mean follow-up period of 4 years by using the Six-Item-Screener (SIS) test. Average food consumption information was obtained by using a self-administered food-frequency questionnaire.

 
What you eat affects your cardiovascular health, your looks, and your brain cells. Dr. Georgios Tsivgoulis
 

The current analysis excluded participants with a history of stroke, impaired cognitive status at baseline, or missing data on food-frequency questionnaires, leaving a total of 17,478 individuals in whom adherence to a Mediterranean diet (scored as 0 to 9) was computed from the food questionnaires.

Results showed that 1248 (7%) of individuals developed incident cognitive impairment. Higher adherence to the Mediterranean diet (Mediterranean diet score, 5 to 9) was associated with a lower likelihood of incident cognitive impairment before and after adjustment for potential confounders, including demographic characteristics, environmental factors, vascular risk factors, depressive symptoms, and self-reported health status.

Table 1. Risk for Cognitive Impairment With Mediterranean Diet

Endpoint Odds Ratio (95% Confidence Interval)
Before adjustment 0.89 (0.79 - 1.00)
After adjustment 0.87 (0.76 - 1.00)

 

"The Mediterranean diet was first evaluated in Mediterranean countries," Dr. Tsivgoulis said. "But now we have shown a clear benefit of this diet in inhabitants of the stroke belt in the US, an entirely different population."

There was no interaction between race and the association of Mediterranean diet with cognitive status. But there was a strong interaction of diabetes mellitus on the relationship. High adherence to the diet was associated with a lower likelihood of cognitive impairment in nondiabetic participants but not in patients with diabetes.

Table 2. Risk for Cognitive Impairment With Mediterranean Diet in Participants Without and With Diabetes

Diabetic Status Odds Ratio (95% Confidence Interval)
No diabetes 0.81 ( 0.70 - 0.94)
Diabetes 1.27 (0.95 -1.71)

 

Trying to explain the diabetes observations, Dr. Tsivgoulis noted that patients with diabetes have different diets compared with the rest of the population, and those diets can be substantially different from the Mediterranean diet.

For example, the Mediterranean diet requires a large fruit consumption, but people with diabetes are not supposed to eat much fruit because of its high sugar content. "It might also be that diabetics have a stricter diet than the general population and that it is hard to show a benefit of the Mediterranean diet in this situation," he added.

Dr. Tsivgoulis said he had anticipated that the diet may be more protective in blacks than in whites because blacks tend to be more overweight, but this wasn't the case. But he added, "Diabetes is also more prevalent in blacks and that might be why we didn't show a greater effect."

Multifactorial Mechanism?

Noting that previous studies have suggested benefits of the Mediterranean diet on stroke and on cardiovascular and cerebrovascular risk factors, Dr. Tsivgoulis speculated on 2 possible mechanisms that may account for the association between the diet and lower incidence of cognitive impairment.

"The Mediterranean diet contains food components that are rich in antioxidant properties, which could slow the progression of cognitive decline and cell death," he said. "Also the diet lowers vascular risk factors, which would reduce the incidence of vascular dementia."

The researchers were not able to look at what types of cognitive impairment were being reduced because the SIS test used does not allow this. "Because there were 30,000 people included in the original study, we had to use a relatively simple test for cognition, so the SIS test was chosen. This is a reliable test for cognition but it is also quite a crude test," Dr. Tsivgoulis said.

However, he pointed out that after adjustment for incident stroke, the benefit of the Mediterranean diet was attenuated, which suggests that the benefit could be due to vascular factors. "It looks as though it is reducing vascular dementia. This may be the major mechanism, but I also think there are other mechanisms involved too. The diet is anti-inflammatory and antioxidant. These will be contributory factors. We are probably seeing just the tip of the iceberg."

Dr. Tsivgoulis and his colleagues are planning next to look at the effect of the Mediterranean diet on incident stroke in this same population. "This has been studied before and an association has been found but not in this population or in a study of this size. Our dataset is the one of the largest population-based studies worldwide."

The study was funded by the National Institute of Neurological Disorders and Stroke, National Institutes of Health, and Department of Health and Human Services. The authors have disclosed no relevant financial relationships.

Neurology 2013;80:1684-1692. Abstract

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