Reference: BMC Med. 2023 Mar 14;21(1):81
Practice Point: Limited evidence supports the Mediterranean diet for the prevention of dementia.
EBM Pearl: Data fishing is looking at a large data set through various lenses until you find the result you are seeking.
Think fast: what did you eat in the last 24 hours? How well does that align with the Mediterranean diet? And how does that affect your dementia risk? These are the questions a group of investigators asked in a recent study that mined data from more than 60,000 white adults enrolled in the UK Biobank. Their analyses found an association between better “adherence” to the Mediterranean diet and lower risk of dementia, but we aren’t all-in just yet.
The Mediterranean diet emphasizes whole foods and includes vegetables, fruits, grains, nuts, seeds, fish, poultry, dairy, olive oil, and importantly, red wine. It has been hypothesized to reduce the risk of dementia, and as such, we agree it makes sense to study it. Study it well, maybe we should have specified. In this prospective cohort study, participants completed up to five 24-hour dietary recalls through a validated online assessment tool. The authors then averaged the data and used 3 different scoring systems to assess how well that input adhered to the Mediterranean diet. While the 3 assessment tools used in this study report “adherence”, really what they are assessing is how well what participants happened to eat aligned with the Mediterranean diet — no one was assigned to a Mediterranean diet or told to eat that way. When the data were grouped according to low, medium, and high adherence, high adherence was associated with a statistically significant reduction in dementia risk, but only based on one scoring system and not the other. With all these different assessment tools, we can’t help but wonder if there is some component of fishing here — researchers using multiple scoring systems in the hopes of finding one which supports their hypothesis.
While the reported outcomes suggest that a diet more closely aligned with the Mediterranean diet may reduce dementia risk, we wonder if whatever it was that made people more likely to eat a diet most like the Mediterranean diet is also what is responsible for their lower dementia risk. The results are also subject to recall bias. Moreover, several 24-hour snapshots of diet information may not accurately reflect years of dietary habits. In addition, reverse causality may be at play as declining cognition could lead to poorer dietary choices (as opposed to the other way around).
If we are saying that data mining isn’t the best way to study this association, the obvious next question is, what is? A randomized trial, right? But studying the effects of nutrition on clinical outcomes is hard. If we were to study this association with a trial, we would expect a lower magnitude of effect as we reduce systematic bias. So would a trial be worth it? Maybe not. All that being said, recommending the Mediterranean diet to patients as a way to decrease the risk of dementia (when we have little else to offer them) is a low-risk intervention. We are of the opinion that if you want to recommend this to your patients, go for it. If you don’t, fine. Having a conversation about what’s healthy and what’s not is probably just as beneficial.
For more information, see the topic Mild Cognitive Impairment (MCI) in DynaMed.
DynaMed EBM Focus Editorial Team
This EBM Focus was written by Nicole Jensen, MD, Family Physician at WholeHealth Medical. This EBM Focus was written by Nicole Jensen, MD, Family Physician at WholeHealth Medical. Edited by Alan Ehrlich, MD, Executive Editor at DynaMed and Associate Professor in Family Medicine at the University of Massachusetts Medical School; Katharine DeGeorge, MD, MS, Deputy Editor at DynaMed and Associate Professor of Family Medicine at the University of Virginia; Dan Randall, MD, Deputy Editor at DynaMed; Vincent Lemaitre, PhD, Senior Medical Writer at DynaMed; Elham Razmpoosh, PhD, Postdoctoral fellow at McMaster University; and Sarah Hill, MSc, Associate Editor at DynaMed.