Health and eating well – the Mediterranean Diet

Last week there was much reporting on the benefits of he Mediterranean Diet based on a paper in the New England Journal of Medicine. CNN described it as “particularly large and well done study.” They went on to say that the Mediterranean Diet could reduce the incidence of heart attack and stroke by nearly 30%.

That sounds great.

But what did the study actually show? A copy of the paper is readily available online. The abstract for the paper claims:

We designed a randomized trial to test the efficacy of two Mediterranean diets (one supplemented with extra-virgin olive oil and another with nuts), as compared with a control diet (advice on a low-fat diet), on primary cardiovascular prevention.

The paper concludes with the claim:

The results support the benefits of the Mediterranean diet for the primary prevention of cardiovascular disease.

Is it any wonder that the media is out telling us all the benefits of the Mediterranean Diet?

But it turns out that is not quite what the paper actually says. The people studied were a bit older and not as healthy as many. The authors are very much up front with that. The study include woman ages 60 to 80 and men ages 55 to 80 who:

had either type 2 diabetes mellitus or at least three of the following major risk factors: smoking, hypertension, elevated low-density lipoprotein cholesterol levels, low high-density lipoprotein cholesterol levels, overweight or obesity, or a family history of premature coronary heart disease.

In other word they were not the young and healthy. They were not at all representative of the general population.

A key tenet of data analysis is to be vary careful applying the results of any study beyond the class of subjects in the study. Any claims of the universal benefits of the Mediterranean diet to the general public is not justified based on this study. The authors have looked only at an older segment of the population with known health risks. So if you have many of the well known risks of heart disease it may well be worth following the diet. If you don’t have those risks the study is not very informative.

Another point to consider is what the diet was compared to. The control group for the study was given some basic instruction on healthful diets. But in general the study compares those following the Mediterranean Diet to the diet of the general population. There are any number of “healthy diets.” Heart specialists for years have been saying reduce consumption of red meats and eat plenty of fruits and vegetables. These are all components of the Mediterranean diet. So perhaps the study is just one more in the long list of those that say eat well if you want to remain healthy. But even that is giving it too much credit. A better statement would be: if you have well know risks for heart disease and stroke then eat well.

How good is the claimed 30% reduction in the risk of heart attack and strokes? We are used to hearing the results of the political polls with estimates of 52% plus or minus 2%. In this “particularly large study” the authors claim something like an improvement of 30% plus or minus about 25% for the type of people they looked at. That is a difference, but covers a very large range. With that range better to just claim an improvement without highlighting the 30% estimate.

And for those who like to think that one study tells us a lot, even if the results seem significant, I must recommend the video clip of a talk given by Rebecca Goldin of George Mason University in September 2012 posted on youtube. Start at about the 48 minute mark.