More on Mediterranean diet – what it means in real life

I’ve spoken to a few friends about the recent New England Journal of Medicine study highlighting the benefits of a Mediterranean diet. (I wrote about it here.) I think the media have misrepresented or misunderstood the real-life implications.

As a reminder, the study highlighted that a Mediterranean diet rich in fish, olive oil, nuts, vegetables and wine can produce health benefits. Indeed, patients at risk of having a heart attack were better off than patients on a regular diet (the control group was counseled to be on a low-fat diet, but they didn’t actually change their diet much). The Mediterranean diet group, to help them remember and adhere were given free olive oil or nuts at regular intervals.

With results in hand, the important questions are ‘how impactful is this Mediterranean diet?’ and ‘what should this all mean in real life?’

As to how much better . . . . this is where the media have focused on statistics that are misleading. The actual benefit was 0.3% per year. The relative benefit was a 30% reduction in heart attacks. This latter number, which seems impressive, is what was highlighted in the press. The NY Times, for example, wrote an article only mentioning this 30% relative reduction in heart attacks.

Let’s take a look at the math to understand the real world implications. Before doing so, it is important to understand three concepts:

  • absolute risk reduction
  • relative risk reduction
  • “number needed to treat” — This is the number of people that need to be on a therapy to prevent one bad outcome.

To illustrate these concepts, imagine a population with a heart attack rate of two events per million people per year. If you could cut that in half (to one heart attack per million people), have you done much good? On the one hand, you decreased the rate of heart attack by 50% (50% relative risk reduction)! On the other hand, the ‘number needed to treat’ is one million people to prevent one heart attack. This reduction of one event across one million people is an absolute risk reduction of 0.0001%. This is not impressive.

In the case of the Mediterranean diet, while the relative risk reduction is 30%, the absolute risk reduction is only 0.3%. Mathematically, the “number needed to treat” is 333. Stated another way, for every 333 people who change their diet to be low in fat and rich in vegetables, fish, olive oil, wine, and nuts, one heart attack per year will be prevented. That is a lot of people that need to change in order to prevent one heart attack. For the avoidance of doubt, it’s not going to happen. People are resistant to change. This particular intervention requires effort, behavior modification, and money.

Further, any intervention must be compared against the alternatives. In this case, alternate interventions include smoking cessation, weight loss, blood pressure control, cholesterol reduction, and exercise. These therapies have a great benefit and a lower ‘number needed to treat.’ Accordingly, these are the interventions that should be emphasized if a person genuinely wants to lower his or her risk of heart attack.

This NEJM study on the benefits of a Mediterranean diet is most noteworthy for those already conscious about their health. If you enjoy such things as fish, wine and olive oil, take heart. Not only are they pleasing, they’re also good for you. Enjoy. While your heart attack risk is probably already low, all the small things you do to preserve your health probably are synergistic in aggregate and will work to keep you healthy for a long, long time. The French have the right idea when they raise a glass of wine to make the toast: “A votre sante” . . . “To your health.”

sources: Mediterranean Diet Shown to Ward Off Heart Attack and Stroke (NY Times); Primary Prevention of Cardiovascular Disease with a Mediterranean Diet (NEJM)

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