Glycated haemoglobin (HbA1c)

Method

Whole-blood glycated haemoglobin by HPLC, capillary electrophoresis or immunoassay, NGSP/DCCT-aligned. Where a study reports on the IFCC scale in mmol/mol the scale is recorded on the edge, because the two scales are related by a linear transform and not by a factor.

The method is part of the endpoint identity. Studies measuring the same quantity by a different method are held as separate endpoints.

Notes

Fasting blood glucose is the same construct over a different integration window — see that entry. The merge is tempting because both fall with benefit and both are called 'glycaemic control', but this endpoint cannot move in a four-week trial and the merge therefore imports spurious nulls. Also confounded by anything shortening erythrocyte lifespan (haemolysis, iron deficiency, recent transfusion), which lowers this number without any change in glycaemia; note that haematocrit and blood haemoglobin concentration are separate endpoints in this vocabulary and their movement is a warning flag on an edge here.

Findings

4 findings from 3 compounds, strongest first.

• melatonin — decreases

  BLikelyGrade B, Likely. Human evidence, limited replication or design limits.Finding

  “Melatonin supplementation was associated with a statistically significant reduction in glycated hemoglobin (HbA1c) levels compared to placebo [mean difference [MD]: -0.65; 95% CI: -1.28, -0.02; P = 0.04]”

  Study

  meta-analysis

  Participants

  325

  Population

  adults with type 2 diabetes mellitus

  The effect of melatonin supplementation on glycemic control in patients with type 2 diabetes.2025PMID:40698248

• L-glutamine — no detected effect on

  BLikelyGrade B, Likely. Human evidence, limited replication or design limits.FindingNull result

  “Fasting glycemia and glycated hemoglobin A1c values before and after 60 days of supplementation with EGln or maltodextrin indicated prediabetes, but no significant differences were observed between the EGln and placebo groups.”

  Study

  RCT

  Participants

  30

  Duration

  2 months

  Dose

  12.4 g oral effervescent glutamine daily

  Population

  community-dwelling older adults aged over 60 from three daycare centres in Brazil; safety/tolerability endpoint, not the trial's primary outcome

  Tolerability of glutamine supplementation in older adults: a double-blind placebo-controlled randomized clinical trial.2024PMID:38808890

• curcumin — no detected effect on

  BLikelyGrade B, Likely. Human evidence, limited replication or design limits.FindingNull resultreplicated ×1

  “Nanocurcumin supplementation showed no significant effects on fasting blood glucose (SMD: -0.44; 95% CI: -2.16 to 1.28), HbA1c (SMD: 0.19; 95% CI: -1.26 to 1.65), or lipid profile parameters, including total cholesterol, LDL-C, HDL-C and triglycerides (all p > 0.05).”

  Study

  meta-analysis

  Participants

  336

  Population

  adults with type 2 diabetes mellitus (T2DM)

  Evaluating the Effects of Nanocurcumin Supplementation in Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.2026PMID:42171191

• curcumin — no detected effect on

  BLikelyGrade B, Likely. Human evidence, limited replication or design limits.FindingNull resultreplicated ×1

  “However, curcumin was not significantly associated with HbA1c based on pooling three included studies with MD (95% CI) of −0.24 (−0.66, 0.18) %, I2 = 85.8%, see Supplementary Appendix P.”

  Study

  meta-analysis

  Dose

  80–3000 mg/day pooled across bioavailability-enhanced forms (80–1000 mg/day), curcumin extracts (1500 mg/day) and turmeric powder (2000–3000 mg/day)

  Population

  Adults with MAFLD (NAFLD/NASH); 3 RCTs pooled; glycaemic indices were an additional (secondary) outcome of this review

  Effect -0.24 mean difference, 95% CI -0.66 to 0.18

  An updated meta-analysis of effects of curcumin on metabolic dysfunction-associated fatty liver disease based on available evidence from Iran and Thailand.2023PMID:37037891