Type 2 Diabetes Meta-Analysis: What Happened to the Vegan Diet?

On the same day that the study showing vegetarians to have a 30% reduced risk of heart disease was released on PubMed, a meta-analysis of diets used to treat type-2 diabetes was also released (1).

The abstract mentions vegan diets:

“We included randomized controlled trials (RCTs) with interventions that lasted ≥ 6 mo that compared low-carbohydrate, vegetarian, vegan, low-glycemic index (GI), high-fiber, Mediterranean, and high-protein diets with control diets including low-fat, high-GI, American Diabetes Association, European Association for the Study of Diabetes, and low-protein diets.”

And their conclusion is:

“Low-carbohydrate, low-GI, Mediterranean, and high-protein diets are effective in improving various markers of cardiovascular risk in people with diabetes and should be considered in the overall strategy of diabetes management.”

Knowing that PCRM had conducted a clinical trial using a whole-foods vegan diet to treat type-2 diabetes with some decent success (more info), I was surprised that the vegan diet didn’t get mentioned in the conclusion. It turns out that they weeded out any diets that were not tested in at least two different trials. They did have some good things to say about the vegan diet in the paper and suggested more studies were needed. Ditto for the one trial using a vegetarian diet (2).

In terms of the other diets, they found that low-carbohydrate, low-GI, Mediterranean, and high-protein diets all reduced Hb A1c (a marker of blood sugar levels; lower is better), and all but the high-protein diets also improved lipid profiles.

What is the difference between a low-carbohydrate diet and a high-protein diet? A “low-carbohydrate diet” is defined as limiting carbohydrate to 20 to 60 g/day, which is very low. A “high-protein” diet is more than 20% of daily calories as protein, and generally has a higher amount of carbohydrate than the low-carbohydrate diets.

They defined a Mediterranean Diet as, ” rich in olive oil, legumes, unrefined cereals, fruit, and vegetables, low in meat and meat products, and with moderate contents of dairy products (mostly cheese and yogurt), fish, and wine.” They attributed the benefits of a Mediterranean diet to the higher monounsaturated fat (MUFA) content which has been shown “to have an impact on the lipid profile, insulin sensitivity, and postprandial glucose concentrations [blood sugar after a meal].”

I can’t vouch for the analytical methods used in this meta-analysis – there were an enormous amount of details comparing all sorts of different metrics – but the above are the author’s main points. They stated that no funding was received for the study.

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1. Ajala O, English P, Pinkney J. Systematic review and meta-analysis of different dietary approaches to the management of type 2 diabetes. Am J Clin Nutr. 2013 Jan 30. [Epub ahead of print] | link

2. Kahleova H, Matoulek M, Malinska H, Oliyarnik O, Kazdova L, Neskudla T, Skoch A, Hajek M, Hill M, Kahle M, Pelikanova T. Vegetarian diet improves insulin resistance and oxidative stress markers more than conventional diet in subjects with Type 2 diabetes. Diabet Med. 2011 May;28(5):549-59. (Abstract only) | link

2 Responses to “Type 2 Diabetes Meta-Analysis: What Happened to the Vegan Diet?”

  1. Healthy Longevity Says:

    I think the findings of this meta-analysis should be interpreted with caution due to the lack of considering of the overall diet quality of the control group. The researchers made a good point about changes in cardiovascular risk factors as perhaps being falsely attributed to changes in macronutrients instead of weight loss, but unfortunately did not mention this in their conclusions.

    “Another major confounder was the independent effect of weight change on the other measured variables (glycemic control and lipid profile). It is difficult to isolate the effect of weight change on these markers of cardiovascular risk, and thus, these benefits could be falsely attributed to the change in quantity of a macronutrient when the change was due to the impact of weight loss alone. This possibility might be of particular relevance when the effect of low-carbohydrate diets is interpreted.”

  2. mollyjade Says:

    Having been active in the diabetes community for years, this doesn’t really surprise me. Changing your diet in any way, especially if it results in weight loss, seems to help lower bg initially. And most of these diets emphasize non-starchy vegetables and sometimes beans, both of which have been shown to lower diabetes risk.

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