Carbohydrate restriction for glycaemic control in Type 2 diabetes: a systematic review and meta-analysis

Aim: To conduct a systematic review and meta-analysis to evaluate the effect of carbohydrate restriction on glycaemic control in Type 2 diabetes.

Methods: We searched Medline, EMBASE and CINAHL for the period between 1976 and April 2018. We included randomized controlled trials comparing carbohydrate restriction with a control diet which aimed to maintain or increase carbohydrate intake, and that reported HbA_1c as an outcome and reported the amount of carbohydrate consumed during or at the end of the study, with outcomes reported at ≥3 months.

Results: We identified 1402 randomized controlled trials, 25 of which met the inclusion criteria, incorporating 2132 participants for the main outcome. Definitions of low carbohydrate varied among the studies. The pooled effect estimate from meta-analysis was a weighted mean difference of -0.09% [95% CI -0.27, 0.08 (P = 0.30); I² 72% (P <0.001)], suggesting no effect on HbA_1c of restricting the quantity of carbohydrate. A subgroup analysis of diets containing 50-130 g carbohydrate resulted in a pooled effect estimate of -0.49% [95% CI -0.75, -0.23 (P <0.001); I² 0% (P = 0.56)], suggesting a clinically and statistically significant effect on HbA_1c in favour of low-carbohydrate diets in studies of ≤6 months' duration.

Conclusions: There was no overall pooled effect on HbA_1c in favour of restricting carbohydrate; however, restriction of carbohydrate to 50-130 g per day had beneficial effects on HbA_1c in trials up to 6 months. Future randomized controlled trials should be of >12 months' duration, assess pre-study carbohydrate intake, use recognized definitions of low-carbohydrate diets and examine reasons for non-adherence to prescribed diets in greater detail.