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low and very low carbohydrate diets for type 2 diabetes remission: systematic review and meta-analysis of published and unpublished randomized trial data.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 (Clinical research ed.). 2021;372:m4743
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Diet modification has long been recognised as a component for the treatment of diabetes. Diets low in carbohydrates have been extensively researched, as a diet for those with Type 2 Diabetes (T2D). This systematic review and meta-analysis aimed to determine the effect of low carbohydrate diets on T2D. The systematic review found 23 studies, including 1357 individuals, investigating the role of low carbohydrate diets on T2D outcomes. Low carbohydrate diet was defined as less than 130g of carbohydrate (less than 26% of calories from carbohydrate) for at least 12 weeks. Results reported at 6 months, found low carbohydrate diets were more effective than a normal diet at achieving diabetes remission. However, this effect did not persist until 12 months, although longer term improvements were seen in blood lipids, weight loss and measures of prediabetes. It was concluded that individuals with T2D, eating a low carbohydrate diet for 6 months may reverse the disease. This study could be used by healthcare professionals to recommend a short-term low carbohydrate diet to individuals with T2D, to improve their chance of going into remission.

Abstract

OBJECTIVE To determine the efficacy and safety of low carbohydrate diets (LCDs) and very low carbohydrate diets (VLCDs) for people with type 2 diabetes. DESIGN Systematic review and meta-analysis. DATA SOURCES Searches of CENTRAL, Medline, Embase, CINAHL, CAB, and grey literature sources from inception to 25 August 2020. STUDY SELECTION Randomized clinical trials evaluating LCDs (<130 g/day or <26% of a 2000 kcal/day diet) and VLCDs (<10% calories from carbohydrates) for at least 12 weeks in adults with type 2 diabetes were eligible. DATA EXTRACTION Primary outcomes were remission of diabetes (HbA1c <6.5% or fasting glucose <7.0 mmol/L, with or without the use of diabetes medication), weight loss, HbA1c, fasting glucose, and adverse events. Secondary outcomes included health related quality of life and biochemical laboratory data. All articles and outcomes were independently screened, extracted, and assessed for risk of bias and GRADE certainty of evidence at six and 12 month follow-up. Risk estimates and 95% confidence intervals were calculated using random effects meta-analysis. Outcomes were assessed according to a priori determined minimal important differences to determine clinical importance, and heterogeneity was investigated on the basis of risk of bias and seven a priori subgroups. Any subgroup effects with a statistically significant test of interaction were subjected to a five point credibility checklist. RESULTS Searches identified 14 759 citations yielding 23 trials (1357 participants), and 40.6% of outcomes were judged to be at low risk of bias. At six months, compared with control diets, LCDs achieved higher rates of diabetes remission (defined as HbA1c <6.5%) (76/133 (57%) v 41/131 (31%); risk difference 0.32, 95% confidence interval 0.17 to 0.47; 8 studies, n=264, I2=58%). Conversely, smaller, non-significant effect sizes occurred when a remission definition of HbA1c <6.5% without medication was used. Subgroup assessments determined as meeting credibility criteria indicated that remission with LCDs markedly decreased in studies that included patients using insulin. At 12 months, data on remission were sparse, ranging from a small effect to a trivial increased risk of diabetes. Large clinically important improvements were seen in weight loss, triglycerides, and insulin sensitivity at six months, which diminished at 12 months. On the basis of subgroup assessments deemed credible, VLCDs were less effective than less restrictive LCDs for weight loss at six months. However, this effect was explained by diet adherence. That is, among highly adherent patients on VLCDs, a clinically important reduction in weight was seen compared with studies with less adherent patients on VLCDs. Participants experienced no significant difference in quality of life at six months but did experience clinically important, but not statistically significant, worsening of quality of life and low density lipoprotein cholesterol at 12 months. Otherwise, no significant or clinically important between group differences were found in terms of adverse events or blood lipids at six and 12 months. CONCLUSIONS On the basis of moderate to low certainty evidence, patients adhering to an LCD for six months may experience remission of diabetes without adverse consequences. Limitations include continued debate around what constitutes remission of diabetes, as well as the efficacy, safety, and dietary satisfaction of longer term LCDs. SYSTEMATIC REVIEW REGISTRATION PROSPERO CRD42020161795.

Expert Review

Conflicts of interest: None
Take Home Message:
  • Type 2 diabetes remains a significant and worsening problem worldwide, despite many pharmaceutical developments and a global emphasis on glycemic control.
  • This review highlights structured LCDs as a worthwhile option for the management and treatment of diabetes, providing an opportunity for Nutritional Therapy Practitioners to support clients in adopting evidence-informed, modifiable dietary and lifestyle changes for Type Two Diabetes.
Evidence Category:
  • A: Meta-analyses, position-stands, randomized-controlled trials (RCTs)
  • X B: Systematic reviews including RCTs of limited number
  • C: Non-randomized trials, observational studies, narrative reviews
  • D: Case-reports, evidence-based clinical findings
  • E: Opinion piece, other
Summary Review:
  • Type 2 diabetes is the most common form of diabetes, accounting for 90-95% of cases.
  • Previous randomised trials assessed low carbohydrate diets (LCDs) (<26-45% of daily calories from carbohydrate) as encouraging to improve blood glucose control and outcomes of type 2 diabetes but did not systematically assessed remission of diabetes using low carbohydrate diets (LCDs) and very low carbohydrate diets (VLCDs) for people with type 2 diabetes.
  • Systematic reviews (SR) and meta-analyses represent the most valuable, reliable, and objective tool to summarise evidence from primary studies.
  • This SR assessed 23 randomised controlled trials comparing LCDs with mostly low fat control diets in individuals / subjects / participants with type 2 diabetes. LCDs were defined as diets with less than 130 g/day or less than 26% of calories from carbohydrates, based on 2000 kcal/day. The authors used the Cochrane Risk of Bias tool 2.0 (RoB 2) to assess methodological quality of evidence, GRADE to assess the certainty of evidence
  • On the basis of assessment of moderate to low certainty evidence, individuals / subjects / participants adhering to a LCD for six months may experience remission of type 2 diabetes without adverse consequences.
  • Primary outcomes of interest were remission of type 2 diabetes (dichotomously defined as HbA1c <6.5% or fasting glucose <7.0 mmol/L), with or without the use of diabetes medication.
  • Eight studies reported on remission of diabetes at six months. Pooled analysis showed that when remission was defined by an HbA1c level below 6.5% independent of medication use, LCDs increased remissions by an additional 32 per 100 patients followed (risk difference 0.32, 95% confidence interval 0.17 to 0.47; 8 studies, n=264; GRADE=moderate)
  • When remission was defined by an HbA1c level below 6.5% and the absence of diabetes medication, LCDs increased remissions at a lower rate (risk difference 0.05, –0.05 to 0.14; 5 studies, n=199; GRADE=low)
  • Additional primary outcomes were weight loss, HbA1c:
  • 18 studies reported on Weight loss results (mean difference –3.46, 95% confidence interval –5.25 to –1.67; n=882 (note that positive results not sustained at 12 mo)
  • Seventeen studies reported on HbA1c levels at six months, LCDs achieved greater reductions in HbA1c than did control diets (mean difference –0.47%, –0.60 to –0.34; n=747
  • Limitations of study: 1) the definition of remission of diabetes, 2) Self-reported dietary intake data are prone to measurement error, particularly in dietary trials in which participants are not blinded
  • This SR was funded in part by Texas A&M University.
  • The authors declared no conflicts of interest.
Clinical practice applications: The Authors highlight LCD diets incorporating carbohydrate of less than 130 g/day or less than 26% of calories (based on 2000 kcal/day) may be a safe strategy to help individuals with type 2 diabetes achieve weight loss and better blood glucose control over a six-month period. Results may not be sustained at 12 months.
Considerations for future research:
  • The definition of diabetes remission needs clarification, especially with regard to threshold concentrations of Hb1Ac or fasting glucose and the use of diabetes medication.
  • Safety concerns have been raised with LCDs. Although no significant or clinically important increase in total or serious adverse events was identified in this SR, these outcomes should be reported in future trials to confirm the certainty of evidence for safety.
  • The Authors suggest long term, well designed, calorie controlled randomised trials are needed to determine the effects of LCD on sustained weight loss and remission of diabetes.
  • Larger treatment effects for LCDs in shorter term trials (3 to <6 months), may be trialed as an effect modifier

Lifestyle medicine

Fundamental Clinical Imbalances : Hormonal ; Immune and inflammation
Environmental Inputs : Diet
Personal Lifestyle Factors : Nutrition
Functional Laboratory Testing : Not applicable

Methodological quality

Jadad score : Not applicable
Allocation concealment : Not applicable

Metadata

Nutrition Evidence keywords : Type 2 diabetes ; Low carbohydrate diet