Linking Mediterranean Diet scores with test results: important research

Much of what we know about a Mediterranean-style diet comes from what are known as observational studies: these enroll participants in the study, usually survey them in some way, and then follow those participants over time (as long as possible), count how many times a pre-selected thing happens to the participants, and then compares the results of the initial survey of participants who experienced that pre-selected thing with the surveys of those who did not experience that pre-selected thing.

When you describe observational studies in those terms, it's pretty clear that any link we might draw between the results of a survey and something that happens to the participants later is tenuous at best and sometimes downright ludicrous. Just look at these graphs: who knew that the number of people who drowned by falling into a swimming pool correlates with the number of films Nicolas Cage appeared in?

This isn't to say that all observational studies are junk, but certainly some are better than others, and today's article is one of them. The authors of this study, which appeared in JAMA Network Open (2018;1(8):e185708) sought to explain any correlation between Mediterranean Diet scores of women enrolled in the Women's Health Study and cardiovascular disease events those women experienced by also looking at metabolic scores.

The Women's Health Study enrolled over 25,000 women of at least 45 years of age between 1993 and 1996. Upon enrollment the participants responded to a detailed dietary questionnaire and provided demographic and health history information. They provided blood for testing and their blood pressures were tested.

The authors analyzed the participants' dietary questionnaires and were able to assign each woman a Mediterranean Diet score based on her responses (take a quiz to find out your Mediterranean Diet score). A 'perfect' Mediterranean Diet received 9 points, with 0-3 being considered a low score, 4-5 a middle score, and 6-9 a high score.

After an average of 12 years had passed (depending on when the participant had enrolled in the study), the authors looked at the Mediterranean Diet scores of those who experienced cardiovascular disease events like heart attack, stroke, coronary arterial revascularization (bypass), or death from heart disease related events, and compared them with those who did not after taking into account variables like age, Body Mass Index (BMI) and menopausal status.

But they didn't stop there. In addition to assessing diet alone and comparing those with a low Mediterranean Diet score with those who had a high Mediterranean Diet score, the authors also repeated these calculations multiple times, each time including an additional, single variable of concern. For example, in addition to controlling for age, BMI, and menopausal status, they also controlled for blood pressures. Then they took out blood pressures and instead controlled for cholesterol scores, then the same with inflammatory markers, glucose scores, and insulin scores.

These separate calculations allowed the authors to see how much influence each metabolic score had on the outcome of the comparisons between high and low Mediterranean Diet scores. Compared to those with low mediterranean Diet scores, those with high Mediterranean Diet scores were 23% less likely to experience heart attack, stroke, coronary arterial revascularization (bypass), or death from heart disease related events. Of this, almost 30% of these effects were due to lower markers of inflammation, glucose metabolism and insulin resistance were responsible for 28% of the effects, Body Mass Index was responsible for 27% of the effects, and blood pressure was responsible for just over 26% of the effects (effects are not cumulative so the percentages will not add up to 100%).

What this means for you

Despite being an observational study, this does shed some light on just what it is that the Mediterranean Diet does that has such a profound effect on health: it reduces inflammation, improves glucose metabolism and insulin resistance, and helps maintain healthy blood pressures and a healthy weight. Here's more on what makes a Mediterranean Diet.

First posted: January 2, 2019