Why you should be skeptical of recent news headlines about the Mediterranean diet and brain health

Joy Victory is Deputy Managing Editor of HealthNewsReview.org. She tweets as @thejoyvictory.

Another week, another study about the Mediterranean diet in the news.

This time, it’s an observational study finding an association between the diet and a reduced rate of brain shrinkage among people in their early 70s. At least a half-dozen news sites jumped on this one, likely hoping to capitalize on resolution-minded readers hungry for ways to eat better in the new year.

As is typical for the buzz around this kind of study, most of the news stories we read framed the findings as a cause-and-effect pattern–that eating foods that fall under the broad umbrella of “Mediterranean” will help your brain stay plumper (and presumably healthier) as you age. 

Or as, NBCNews.com reported, the diet could “help preserve your brain into old age.”

Without explaining any of the study limitations for this or any of the other findings discussed in their story, NBCNews.com even goes on to explain–erroneously–that “other studies have shown that good diets can prevent Alzheimer’s and other types of dementia.”

Red flag alert: As much as we wish otherwise, there is currently no proven way to prevent Alzheimer’s, whether via “good” diets, exercise, brain training, or any other healthy behavior. Those things may be associated with reduced risk of the disease, but there has never been a large randomized trial proving that such things reduce risk. 

‘The key to helping you live longer’

Fortunately, readers of The Los Angeles Times’s story will come away a bit more informed. The story avoids the cause-and-effect language that permeated the NBCNews.com coverage, stating that “brain shrinkage is less pronounced in older folks whose diets hew closely to the traditional diet of Mediterranean peoples” — not that the diet “could save the brain,” as NBCNews.com incorrectly put it. The LA Times story also discusses the complexities of researching the links between diet and health–how, for example, it’s challenging to tease out what’s really causing the reduced brain shrinkage among older adults. Is it the diet, the story points out, or something else?

This note of caution was missing from CNN.com’s story, which reads almost like an advertisement. We’re told the study “seems to confirm” that the diet has “lasting benefits for brain health.” It then quickly transitions away from the cognitive benefits, and sells the diet as the “key to helping you live longer.” 

“It helps you manage your weight better, and can lower your risk for cancer and cardiovascular diseases” the story says. It ends on the definitive note that “bottom line: you’ll likely be physically and mentally healthier long into old age if you stick with this diet.”

This emphatic tone may leave readers with the impression that by eating a certain way, they have way more control over their longevity than they really do. In reality, a complicated mix of factors contribute to physical and mental health (or lack thereof) as we get older, and the story should have noted that.

What did the researchers find, in precise terms?

Also, CNN.com–and several other news sites–didn’t explain exactly what the researchers found. Their story says that the Mediterranean diet group was “less likely to lose brain volume as they aged.” A HealthDay story was similarly vague, saying the Mediterranean group “retained more brain volume.”

Only the New York Times explained that the strictest Mediterranean eaters in the study measured “10 milliliters greater total brain volume” than the least adherent. But at just four paragraphs long, the story left out any nuances or commentary from outside experts.

But does this 10 milliliters really mean anything? It’s not clear: This story–and many others–made the irresponsible leap that a bigger brain directly translates to increased cognitive ability, better memory, and somehow provides a buffer against dementia, as we point out in today’s “Five-Star Friday (and a few frustrating flops).

And why no discussion of recall bias?

One other elephant in the research lab is that this study hinges on what people reported one-time, via an 168-item “food frequency questionnaire” filled out at home and mailed to the researchers.

This type of data gathering is highly prone to “recall bias,” explains this excellent article from Slate.com. A large analysis looking the validity of the U.S. National Health and Nutrition Examination Survey discovered that people aren’t very accurate when reporting what they eat, even when they’re asked fairly frequently to fill out a dietary survey.

“If people had eaten as little as they reported,” Slate said of the analysis’s findings, “they would be starving.”

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Comments (2)

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Charles Carter

January 8, 2017 at 12:17 am

Another excellent article!
Every article about association and causation needs a link to Spurious Correlations http://www.tylervigen.com/spurious-correlations
It makes your point in a very humorous way

Stephen Cox, MD

January 9, 2017 at 10:50 am

I agree with your criticism of the statements alleging such CNS protection from the Mediterranean diet. None-the-less, evidence does show improved cardiovascular benefits from this diet and not others, and studies/observations also show prolonged longevity and less disease among those small societies that live and exercise with Mediterranean style nutrition, called the Blue Zones. This important fact should not be discarded even though it MAY be a leap to claim such brain benefits(even if true). Please educate me more if evidence disagrees with this. Thank you.