Can a Mediterranean diet reduce the risk of Alzheimer’s? Original paper

Mild cognitive impairment (MCI) is a neurocognitive disorder involving memory problems more severe that what is expected based on a person’s age but less severe than in Alzheimer’s disease (AD). MCI, however, frequently leads to AD, which itself worsens with time and often results in death — AD is currently (in 2021) the fifth leading cause of death for people older than 65 in the United States.^[1]

No drugs have been shown to slow or prevent AD: nearly all of the existing drugs approved to treat AD (e.g., acetylcholinesterase inhibitors) merely improve symptoms, and these improvements are often fairly minor. Therefore, nonpharmaceutical strategies to prevent MCI and AD need to be explored. One such strategy may be a Mediterranean diet, an eating pattern typically characterized by higher intakes of fish, fruit, vegetables, legumes, nuts, whole grains, and olive oil; a limited intake of meat and dairy; and a moderate intake of alcohol (often wine specifically).

This systematic review and meta-analysis assessed 22 studies (11 cross-sectional studies, 8 cohort studies, and 1 case-control study) for an association between adherence to a Mediterranean diet and the risks of MCI and AD. In 17 of the 22 studies, the mean age of the participants was over 60.

For the meta-analysis, adherence to a Mediterranean diet was based on a 9-point system, with higher scores indicating greater adherence. Of the 11 included studies, 2 looked at MCI risk and 9 at AD risk. The risk of bias was assessed using the Newcastle-Ottowa scale.

The investigators also reviewed studies on the association between adherence to a Mediterranean diet and potential mediators of cognitive decline. These studies assessed various outcomes, including brain volume (assessed via MRI), brain levels of amyloid beta (assessed via various methods), and brain glucose metabolism (assessed via FDG-PET).

Adherence to a Mediterranean diet was associated with lower risks of MCI and AD. More precisely, every 1 point increase in adherence score was associated with a 9% lower risk of MCI and an 11% lower risk of AD. The results were similar when restricting the analysis to cohort studies. The risk of bias was rated as low for 9 studies and medium for 2 studies.

With regard to potential mediators of this relationship, clues were found in studies on cognitively normal people:

• Of the 7 studies assessing brain volume, 4 linked Mediterranean-diet adherence to higher cortical thickness, while 3 found no association.

• Of the 7 studies assessing brain levels of amyloid beta, 3 linked Mediterranean-diet adherence to lower levels of amyloid beta plaques, while 4 found no association. One additional study linked Mediterranean-diet adherence to lower levels of a combination of beta amyloid plaques and tau tangles.

• The 3 studies assessing brain glucose metabolism linked lower Mediterranean-diet adherence to lower brain glucose metabolism.

The findings of this study point to an association between adherence to a Mediterranean dietary pattern and lower risks of MCI and AD. Unfortunately, this evidence is observational; it can’t determine if following a Mediterranean diet will reduce the risks of MCI and AD.

Thankfully, several ongoing trials are testing a Mediterranean diet (often in combination with other diets or interventions) for its effects on cognitive decline and AD. One of these trials, lasting 3 years, is testing the MIND diet. This diet incorporates elements of the Mediterranean and DASH diets: it favors berries, beans, vegetables, leafy greens, whole grains, and olive oil; it includes moderate fish, poultry, and wine (1 glass per day); and it limits sweets, cheese, butter, red meat, and fried food.