Are Vegetarian and Vegan Diets Best for Preventing and Treating Diabetes?

Boris Hansel, MD


December 26, 2018

Editor's Note: The following is an edited, translated transcript of a video commentary by endocrinologist-nutritionist Boris Hansel, MD, an obesity management specialist who practices in Paris, France.

I'm going to try to answer a question asked by both patients and care providers: Is a vegetarian or vegan diet the ideal diet for preventing and treating diabetes?

A quick Internet search would yield plenty of popular articles that advocate a vegetarian diet. According to certain websites, such a diet prevents the onset of diabetes or, in the case of confirmed diabetes, enables one to stop treatments.

Of course, such claims are completely false. What's more challenging, however, is to determine whether a vegetarian diet is the one that should be recommended on a first-line basis in patients with diabetes in the hopes of achieving diabetic control and preventing complications.

Interpret Studies With Caution

Reviews and meta-analyses on this subject conclude rather strongly that diets that are low in or that contain no foods of animal origin are beneficial. We can cite a review that was recently published in Current Diabetes Reports.[1]

It discusses a lower incidence of diabetes in longitudinal studies and decreased glycated hemoglobin levels and diabetes treatments required in randomized studies where a vegetarian diet was compared with low-fat diets. For my part, I think these results and claims—which seem a bit exaggerated to me—need to be seen in relative terms.

In regard to cohort studies, a lot of caution is warranted. Although the incidence of diabetes in certain studies was lower by up to one half in the followers of a vegetarian diet, it must be borne in mind that this is a typical situation where it is impossible to establish a causal link between a way of eating and a disease risk, given the importance of confounding factors. Being a vegetarian is associated, on average, with a largely healthier lifestyle. This is common knowledge.

As for randomized trials, it must be said that they are of short duration. For the most part, when we look at the meta-analyses, we find greater weight loss with vegetarian diets than with the diets tested in the control groups.

The main hypothesis for explaining this difference in weight loss between the groups is that in the open-label studies, the participants in the intervention group were more or less consciously influenced to lose weight, even if, in principle, weight loss was not an objective. Therefore, weight loss would have been what was responsible for the improvement in glycemic control rather than the quality of the diet, per se.

Also, it is well known that high-protein or high-fat diets have a dramatic effect on glycemic control in patients with diabetes, in addition to bringing about rapid weight loss.

What About the Mediterranean Diet?

Last, to the best of my knowledge, no randomized trial has ever compared a vegetarian diet with a Mediterranean-type diet, which contains foods of animal origin. This is a major shortcoming.

Ideally, these diets should be compared if one really wants to conclude that vegetarianism is superior. It is a shortcoming all the more so because there is abundant literature in favor of the Mediterranean diet for preventing and treating cardiometabolic risk factors, and especially for improving glycemic control in diabetics.

Although no clinical trial has compared the vegetarian diet with the Mediterranean diet, there is a recently published network meta-analysis[2] from which one can make an indirect comparison between the vegetarian and Mediterranean diets and, more generally, other types of diets—namely, the Paleo diet, high-protein diet, low-carb diet, and a diet with low glycemic index and load.

The overall finding of this network meta-analysis is in favor of the Mediterranean diet when it comes to glycemic control. The Mediterranean diet seems to be at least as effective or even superior to a vegetarian diet, which does not fare so badly either and which is associated with better diabetes control.

Think Long-term

The answer to the question "Is a vegetarian diet the one to recommend on a first-line basis in patients with diabetes?" is, at least in my opinion, no. My message is that one should recommend a diet that can be followed over the long term.

If the patient chooses a vegetarian diet, one can respect this choice entirely. The same goes for an animal fat–free diet, on the condition that a dietetic follow-up is provided. If, on the other hand, a patient wants to eat a diet containing foods of animal origin, one can very well recommend another balanced and health-friendly diet, the model being the Mediterranean diet.


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