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Diet quality matters

I talk to my own patients about their diet, so I hear about (and debunk) dietary myths pretty frequently. One of the most persistent myths is that being a vegetarian means your diet is always more healthy than that of those who eat animal-based products. Unfortunately, as with so many things in the realm of nutrition, the truth is more complicated than "plants=good, animals=bad" (or "carbs=bad" or "fat=bad" or even "don't eat anything green/white").

A study just published in the Journal of the American College of Cardiology pretty convincingly shows that a vegetarian diet can indeed be bad for you, using data gathered through three long-term, large scale studies: The Nurses' Health Study, the Nurses' Health Study 2, and the Health Professionals Follow-up Study, which include nearly 210,000 men and women and gathered information for 22 years or more.

The authors analyzed the participants' diets from the food frequency questionnaires administered every 2 to 4 years. For the purposes of their study, they created three different dietary scoring systems:

• first, an overall plant-based diet index (PDI), which emphasizes more plants and less animal products;
• a healthful plant-based diet index (hPDI), which emphasizes healthier plant-based foods like whole grains (as opposed to refined grains), legumes, fruits and vegetables, and coffee or tea instead of sugar-sweetened beverages; then finally
• an unhealthy plant-based diet (uPDI) which includes unhealthy but plant-based foods like more refined grains, processed foods, and sugar-sweetened beverages.

For all three of the scoring systems the researchers assigned higher scores for closer adherence to the type of diet: for example, for the overall plant-based diet those who ate the fewest plant-based items received a lower score than those who ate the most. For the unhealthy plant-based diet, however, those who consumed more of the less-healthy plant-based foods received a higher score than those who consumed more of the healthier plant-based foods.

For their results the authors focused on cardiovascular disease: specifically, myocardial infarction (heart attack), regardless of whether the participant died of the heart attack or not. The dietary scores of those who experienced a heart attack were compared with those who did not.

After taking into account Body Mass Index (and its changes over time), gender, smoking status, personal and family healthy history, and chronic conditions like high blood pressure or diabetes, the authors found that those who followed a more strictly plant-based diet, regardless of whether it was healthy or not, and consuming less than 3 or 4 servings of animal-based foods per day, were indeed less likely to experience a heart attack than those who consumed the highest level of animal-based foods per day (which was 5-6 servings per day). That said, the difference was only about 8%: a more plant-based diet, regardless of whether it was healthy or not, meant a person was only 8% less likely to experience a heart attack than those who ate more animal-based foods.

On the other hand, those who received the highest score for a healthy plant-based diet were 25% less likely to experience a heart attack than those with the lowest score for a healthy plant-based diet. Inversely, those with the highest score for an unhealthy plant-based diet were 32% more likely to have a heart attack than those with the lowest unhealthy diet score.

There's an important caveat here: these diets are plant-based, not plant-exclusive: again, the lowest level of animal intake was up to 3-4 servings of animal-based foods per day. That might mean red meat, or it might mean using cow milk in your coffee. A Mediterranean-style diet is just as plant-based: over the course of a week you might average 9-10 servings of animal protein (including meat, fish, and poultry) plus a total of 2-3 servings of other animal-based foods (dairy items like butter, milk, or cheese). That's well within the authors' cutoff of 3-4 servings of animal-based products per day.

What this means for you

The take-home here should not be that everyone should go vegan, nor that a vegetarian diet is bad for you (or good for you). The real take-home is that it's the quality of the calories you consume that matters. And not just a little bit, but a lot: improving your Mediterranean Diet score by just two points - for example, by switching your daily lunch sandwich from white bread to whole wheat bread and from pastrami and cheese to peanut butter and jelly - can reduce your risk of death from all causes by 25%. Find out more about how these small changes in your diet can have a big impact in our Mediterranean Diet section.

First posted: October 11, 2017