22 September 2017
Solfrizzi et al.  recently published a systematic review of observational studies published between 2014 to 2016 on dietary factors and late-life cognitive disorders. While the authors relayed the growing promise and potential of diet to change the course of late-onset Alzheimer’s disease and related dementia, their conclusion—that certain foods that have been thought to increase health risk are not as damaging as previously described—is not consistent with the emerging unifying concepts on protective dietary factors for Alzheimer’s disease [2-4]. To draw such strong conclusions based on only observational studies may not be warranted.
Using a single cross-sectional study based on lifestyle questionnaires in subjects with and without mild cognitive impairment (MCI) in China , the authors drew the conclusion that there may be a beneficial role for eggs in dementia prevention, albeit the reduction in odds ratio was very small (0.025). Similarly, they cited another cross-sectional study in China in which vegetarians were observed to have higher risk for dementia , which they attributed to a protective effect of red meat for dementia. However, this study may be confounded by the fact that the vegetarians may be deficient in vitamins D and B12, which are also known risk factors for dementia as recognized by Solfrizzi et al. as well. Given that both eggs and red meat contain significant amounts of saturated fat and cholesterol, their beneficial roles for dementia risk would be inconsistent with the well-established dementia-promoting roles of saturated fat and cholesterol, recognized in this study and others [7, 8]. Moreover, the emerging dietary patterns demonstrated to be protective for dementia are all more heavily plant-based and limited in animal products, particularly red meat [3, 4].
One important question for this emerging field of nutritional interventions for Alzheimer's disease is: what is the optimal dietary pattern for maintenance of cognitive function in aging and dementia for the general population? Solfrizzi and colleagues pointed out the importance of focusing on specific foods and dietary patterns rather than specific isolated nutrients. Now that some beneficial dietary patterns have been identified, the next big question is: How they can be improved? For example, we should assess the various food components of the Mediterranean diet and the MIND diet for their role in brain protection. For instance, what is the rationale for including poultry in the diet? Although there are studies that support fish consumption as protective for the brain, there are also studies that disprove that when looking at the effects on consumption over a lifetime . As most of the fish benefits stem from omega-3 fatty acids, we need to assess whether fish consumption is superior to an algae-derived docosahexaenoic acid/eicosapentaenoic acid (DHA/EPA) supplement. Given that fish is a source of saturated fat and many toxins, it would be better to replace fish with a safe DHA/EPA source. Likewise, it is likely the beneficial effects of the polyphenols from the wine in the Mediterranean diet can also be gained by drinking grape juice or eating whole grapes instead to limit the potential harm from the alcohol .
Altogether, we strongly concur with the authors that dietary patterns are a highly promising area of Alzheimer's disease prevention research. Nevertheless, in order for the field to accelerate its development, the research needs to be scrutinized carefully to identify emerging consistent conceptual ideas and critical questions to drive the field forward. Increasing the number of randomized controlled trials testing either dietary patterns or specific food components of dietary patterns for cognitive health or delay of dementia progression would help clarify some of the inconsistencies found in the observational studies. Solfrizzi and colleague have done an admirable job of demonstrating that there are ample indicators for dietary pattern interventions for Alzheimer's disease. Now as a field, we must move rapidly to understand the detailed interactions using carefully designed clinical studies.
Feng-Yen Li, PhD
Medical Research Specialist
Physicians Committee for Responsible Medicine
5100 Wisconsin Ave. NW, Suite 400
Washington, DC 20016, USA
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 Morris MC, Tangney CC (2014) Dietary fat composition and dementia risk. Neurobiol Aging 35 Suppl 2, S59-64.
 Danthiir V, Hosking D, Burns NR, Wilson C, Nettelbeck T, Calvaresi E, Clifton P, Wittert GA (2014) Cognitive performance in older adults is inversely associated with fish consumption but not erythrocyte membrane n-3 fatty acids. J Nutr 144, 311-320.
 Lee J, Torosyan N, Silverman DH (2017) Examining the impact of grape consumption on brain metabolism and cognitive function in patients with mild decline in cognition: A double-blinded placebo controlled pilot study. Exp Gerontol 87, 121-128.