Nutritional Pearls: Is a Vegetarian Diet Always Better?

Your patient is a 33-year-old man who tells you that he has been trying to eat healthier for several years, but with little success. Recently, he has decided to switch from following several popular diets over to a vegan diet, and asks if you think this will help him to lower his risk of heart disease.

How would you advise your patient?

(Answer and discussion on next page)

Dr. Gourmet is the definitive health and nutrition web resource for both physicians and patients with evidence-based resources including special diets for coumadin users, patients with GERD/acid reflux, celiac disease, type 2 diabetes, low sodium diets (1500 mg/d), and lactose intolerance. 

Timothy S. Harlan, MD, is a board-certified internist and professional chef who translates the Mediterranean diet for the American kitchen with familiar, healthy recipes. He is an assistant dean for clinical services, executive director of The Goldring Center for Culinary Medicine, associate professor of medicine at Tulane University in New Orleans, and faculty chair of the Certified Culinary Medicine Specialist program.

Answer: Currently, there is no reason to believe that avoiding animal proteins will protect you from metabolic syndrome.

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, yet 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 I mean by a plant-based diet is quite different: I 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 that exclude animal proteins of any kind 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, who are at higher risk of diabetes, heart disease, and stroke.

The Research

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.

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.

The Results

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 2 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.

What’s the Take Home?

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.

Reference:

Picasso MC, Lo-Tayraco JA, Ramos-Villanueva JM, et al. Effect of vegetarian diets on the presentation of metabolic syndrome or its components: A systematic review and meta-analysis. Clin Nutr. 2019. 38(3):1117-1132.