Vegetarian required? The evidence isn’t in

When people talk about 'plant-based diets', most people assume that the speaker is referring to a vegetarian diet (no land or water-based animal protein, often including dairy and eggs) at the very least - while often the speaker means not just a vegetarian diet, but a vegan diet (no animal-sourced products at all, including no dairy products or eggs whatsoever).

What we mean by a plant-based diet is quite different: we mean a diet that is mostly plants, but may also include lean meats, with mostly fish or shellfish but also incorporating some land animal protein. In today's diet wars, often those touting a vegetarian or vegan plant-based diet will assert that such diets are always better for you than an omnivorous diet (one that includes some land animal protein). Of course, the question is, "what does 'better for you'" mean?

One way to look at the question is to look at how that style of eating affects people's risk of Metabolic Syndrome, a term for a combination of metabolic abnormalities that include high waist measurement, high blood pressure, high fasting blood sugars, high triglycerides, and low HDL cholesterol scores (that's the good cholesterol). Those with diagnosed Metabolic Syndrome are at higher risk of diabetes, heart disease, and stroke.

An international team of researchers noted that there are a fair number of studies that look at the effects of a vegetarian diet on Metabolic Syndrome or its various components, but the results have been inconsistent. They sought to bring some clarity to the research by performing a review and meta-analysis of relevant studies that compared the effects of a vegetarian diet with the effects of an omnivorous diet on either Metabolic Syndrome as a cluster of conditions or on the specific components of the condition (Clin Nutr 2019;38:1117-1132).

Accordingly, the authors searched the literature and identified randomized controlled trials, cohort studies, or cross-sectional studies that specifically compared those following vegetarian diets (either lacto-ovo-vegetarian, vegetarians who eat fish, vegetarians who strictly limit land animal protein, or vegans) with those following omnivorous diets. Studies must have included only one type of intervention diet, have a control group, report sufficient outcomes data, and be published in either English or German, among other criteria.

The authors found 6 randomized controlled trials, 2 cohort studies, and 63 cross-sectional studies that met their standards to include in their review and meta-analysis.

Unfortunately, the results are inconclusive - not just because many results tended toward clinically insignificant, but because the quality of the studies themselves were poor: among other problems, the authors judged that the randomized controlled trials did not treat the two groups similarly enough (performance bias), or more people dropped out of one arm of a study than another (attrition bias), while other studies did not account for common variables and yet others didn't adequately blind those assessing outcomes (people doing the analyses are not supposed to know which treatment people are receiving).

What this means for you

Until there is solid research to prove otherwise, there is no reason to believe that avoiding animal proteins will protect you from Metabolic Syndrome or its components any more than a healthy diet that includes moderate amounts of lean land-animal and water-based-animal proteins. Indeed, the research regarding a Mediterranean-Style Diet and its effects on Metabolic Syndrome is quite clear.

First posted: July 24, 2019