Clinical Question

What is the usefulness of low-carbohydrate or very low-carbohydrate diets for people with type 2 diabetes mellitus?

Bottom Line

Patients with type 2 diabetes can try cutting out bread and pasta, leaving the skin on chicken, or eating nuts and avocados to lower their blood glucose levels. Six months of a low-carbohydrate diet (less than 26% of calories from carbohydrates) was more likely than other diets or usual diet to reduce A1C level to less than 6.5% or a fasting blood glucose level to less than 126 mg per dL (7.0 mmol per L), with or without continued medication therapy. A very low-carbohydrate diet (less than 10% of calories from carbohydrates) worked for those who were able to stay on it. Body weight and serum triglyceride levels decreased over six months, but the difference was not sustained. The studies combined in this analysis were not great, and the numbers of studies and study participants were small. (Level of Evidence = 1a–)

Synopsis

The researchers searched Cochrane CENTRAL, four other databases, and three trial registries for published and unpublished randomized trials in any language that compared low-carbohydrate diets with low-fat diets or no specific diet (wait-listing) in patients with type 2 diabetes. Two authors independently selected articles for inclusion, abstracted the data, and assessed the research for risk of bias. They identified 23 small studies. Compared with control diets in eight studies of 264 patients, some of whom were receiving diabetes medication, low-carbohydrate diets achieved diabetes remission after six months in an additional 32 participants for every 100 treated (number needed to treat = 4; 95% CI, 2 to 6). Longer-term data are not available. Patients not receiving medication did not have a greater likelihood of remission than patients in the control group. Body weight and serum triglyceride levels (but not other lipid levels) decreased over six months, but the difference disappeared at 12 months. Participants who were able to follow a very low-carbohydrate diet—50% to 80% of participants enrolled in those studies, even with behavioral support—had similar weight loss as patients on a low-carbohydrate diet. Most of the research had moderate to high risk of bias, and there was moderate heterogeneity among the studies. There is evidence of publication bias regarding weight loss.

Study design: Meta-analysis (randomized controlled trials)

Funding source: Foundation

Setting: Various (meta-analysis)

Reference: Goldenberg JZ, Day A, Brinkworth GD, et al. Efficacy and safety of low and very low carbohydrate diets for type 2 diabetes remission: systematic review and meta-analysis of published and unpublished randomized trial data. BMJ. 2021;372:m4743.

Editor's Note: Dr. Shaughnessy is an assistant medical editor for AFP.