Research Lends More Support for Mediterranean Diet to Protect Brain Health

Does adherence to a Mediterranean diet and any resulting changes to the gut microbiota have neuroprotective effects against Alzheimer disease (AD) and Parkinson disease (PD)?

Both neurodegenerative diseases lack a cure and both are projected to increase worldwide in the coming decades due to the aging of the population. A recent review sought to determine if the Mediterranean diet, by modulating gut microbiota, is protective by assessing the effects of diet adherence in relation to AD or PD risk.

The Mediterranean diet is plentiful in fiber and other nutrients, while the Western diet, low in fiber, may be linked with AD or PD, said the authors, who conducted a systematic literature review to examine knowledge gaps in the relationship between Mediterranean diet, gut microbiota, and AD or PD.

The Mediterranean diet is linked with improved cardiovascular and neurologic health. The theory is that dysbiosis, an imbalance in the gut microbial environment, allows neuroinflammation to thrive, leading to AD or PD. This might be moderated by a diet high in fiber and polyphenols, increasing beneficial gut bacteria.

The review sought to add to an earlier meta-analysis, which found that persons with high adherence to the Mediterranean diet had a 40% risk reduction of developing AD.

“As fiber is a main energy substrate to gut microbiota, the beneficial effects of adhering to the MeDi [Mediterranean diet] may be mediated by the gut microbial composition,” they wrote.

The review used 7 databases (PubMed, CINAHL, EMBASE, Web of Science, Global Health, Biological Abstracts, and Grey Literature Report databases) for 2 searches, one on the Mediterranean diet or microbiota and PD or AD and the other on Mediterranean diet and microbiota.

Of 4672 studies identified, 64 were included in the analysis. Both observational and interventional study designs were included.

To qualify for inclusion, studies must have examined Mediterranean diet and AD or PD risk, microbial or metabolomic composition and AD or PD, or Mediterranean diet and microbial or metabolomic composition. Studies were excluded if patients were already diagnosed with AD or PD, and studies assessing Mediterranean diet adherence in patients with diagnosed AD or PD were not included because diagnosis of these diseases can change dietary habits.

None of the studies examined a combined link among all 3 factors (Mediterranean diet, gut microbiota, and AD or PD). Therefore, the retrieved studies were divided into 5 groups: Mediterranean diet and AD risk (n = 4), Mediterranean diet and PD risk (n = 2), Mediterranean diet and microbial composition or metabolomics ( n = 21), AD and microbial composition or metabolomics (n = 7), and PD and microbial composition or metabolomics (n = 30).

There was an overall 32% reduction in risk of AD development when adhering to a Mediterranean diet compared with low adherence, with a low heterogeneity between studies (HR, 0.68; 95% CI, 0.60-0.70; I² = 12%).

In the subgroup analysis, there was a 41% (HR, 0.59; 95% CI, 0.48–0.74) and 25% (HR, 0.71; 95% CI, 0.63-0.74) reduction in risk for AD for high and medium adherence, respectively.

However, the Mediterranean diet was linked with a lower risk of PD in 1 study of women only (HR, 0.55; 95% CI, 0.31-0.99). In another study, adjusted models including men and women showed a trend toward reduced risk. Because PD is more common in men, additional research is needed in more representative samples to expand on the association of Mediterranean diet and PD.

The study also analyzed the taxonomy of microbiota changes when adhering to the diet, finding:

• 8 genera and 2 species had an inverse relationship AD
• 1 family, 8 genera and 3 species had an inverse relationship with PD

One weakness of the review, the authors noted, as that they could not find 1 study that examined all 3 factors. In addition, the sample sizes of the reviewed studies varied, and adherence to the diet relied on self-report. More studies are needed to investigate if Mediterranean diet adherence, gut microbiota, and neurodegeneration are causally related.

Reference

Solch RJ, Aigbogun JO, Voyiadjis AG, et al. Mediterranean diet adherence, gut microbiota, and Alzheimer's or Parkinson's disease risk: a systematic review. J Neurol Sci. Published online Jan 25, 2022. doi:10.1016/j.jns.2022.120166