Does the Mediterranean Diet Reduce Cardiovascular Risk

by Stephan Guyenet 

By now, most of you have probably heard about the recent study on the “Mediterranean diet” (1), a diet that was designed by diet-heart researchers and is based loosely on the traditional diet of Crete and certain other Mediterranean regions.  The popular press has been enthusiastically reporting this trial as long-awaited proof that the Mediterranean diet reduces the risk of cardiovascular events– by a full 30 percent over a 4.8-year period.  I wish I could share their enthusiasm for the study.

Let me start off by saying that I think the Mediterranean diet is a relatively healthy diet pattern, certainly a major improvement over the typical diet.  In this particular study, participants were encouraged to eat more olive oil, nuts, fish, fruit, vegetables, beans, white meat and wine; and eat fewer baked goods, spread fats, red meats and soda.  The Mediterranean diet group was split into two, with one half receiving extra-virgin olive oil and the other half receiving nuts.  Here is how they were instructed and followed up:

For participants in the two Mediterranean diet groups, dietitians ran individual and group dietary-training sessions at the baseline visit and quarterly thereafter. In each session, a 14-item dietary screener was used to assess adherence to the Mediterranean diet (Table S1 in the Supplementary Appendix) so that personalized advice could be provided to the study participants in these groups.

The comparison group was a “low-fat diet” group.  Diet patterns scarcely changed in this group over time, with dietary fat only decreasing from 39 to 37% of calories over the course of the trial.  The reason becomes clear when you read the description of how they were instructed and followed up:

Participants in the control group also received dietary training at the baseline visit and completed the 14-item dietary screener used to assess baseline adherence to the Mediterranean diet. Thereafter, during the first 3 years of the trial, they received a leaflet explaining the lowfat diet (Table S2 in the Supplementary Appendix) on a yearly basis.

So basically, they got training at baseline and a pamphlet in the mail once a year, while the Mediterranean diet group got quarterly visits with a dietitian, diet screening, and “personalized advice”.  Anyone see a problem here?  The investigators did, halfway through the study:

However, the realization that the more infrequent visit schedule and less intense support for the control group might be limitations of the trial prompted us to amend the protocol in October 2006 [3 years after the beginning of the trial -SG]. Thereafter, participants assigned to the control diet received personalized advice and were invited to group sessions with the same frequency and intensity as those in the Mediterranean-diet groups, with the use of a separate 9-item dietary screener (Table S3 in the Supplementary Appendix).

Why did they have a low-fat diet control group to begin with?  The point of a control group is to eliminate all variables from your comparison except the factor you’re interested in, in this case, diet.  In this study, they presumably chose a low-fat diet for the control group because multiple trials have shown it to be ineffective at preventing cardiovascular events*.  If you want to see a treatment effect, you need to use a relatively ineffective comparison group, or else there will be no difference between groups.  Since people tend to become healthier when they adopt any sort of health-oriented diet change, and/or meet with dietitians and doctors regularly, the low-fat diet intervention should havecontrolled for this potentially important variable.

However, since the diet of the control group remained mostly unchanged, and it received a less intensive intervention than the Mediterranean diet group, it was not a proper control, and there’s no way to know how much of the 30 percent reduction in events was due to the Mediterranean diet itself, and how much was due to increased diet vigilance and visiting regularly with dietitians.  Dr. Peter Attia recently made the same point (2).  So while we can say that an intensive Mediterranean diet intervention lowers cardiovascular event risk more than a low-intensity intervention that has little effect on diet patterns, strictly speaking we can’t say that the Mediterranean diet itself lowers cardiovascular risk.  This point will be lost on most of the popular press and many researchers, because this study seemed to provide the answer everyone wanted to hear.

That being said, if I had to guess, I’d say the Mediterranean diet probably does lower risk.  That’s why I find this study so frustrating– it came so close to conclusively demonstrating what other evidence has been hinting at for some time.

* At least in studies lasting a few years done on people who already have cardiovascular disease.  That doesn’t necessarily mean a low-fat diet wouldn’t prevent atherosclerosis if administered before the development of disease– but these types of trials are virtually impossible in humans.

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