A recent and comprehensive review has suggested that turmeric supplementation can play a positive role in reducing glycaemic levels and other metabolic parameters in type 2 diabetes mellitus, and mitigate related conditions, including prediabetes and metabolic syndrome (MetS).
Type 2 diabetes mellitus (TSDM) has become a massive global health burden, with more than 437.9m people affected by it in 2019, and an increase in cases being particularly prominent in developing countries.
Treatment is critical for disease control, but in less developed countries, access to advanced therapies is limited. As such, attempts are under way to harness the properties of indigenous plants long used for medicinal purposes.
Turmeric (Curcuma longa), used in food and medicine, has been studied for diabetes, and numerous systematic reviews and meta-analyses (SRMAs) of randomised controlled trials (RCTs) provide mixed results, due to heterogeneous populations, uses, preparation forms, durations, and outcomes.
In the present study researchers from Bangkok’s Mahidol University, published in PLOS One, researchers reviewed the available evidence on the effects of supplementing turmeric on metabolic and glycaemic parameters in MetS, prediabetes, and T2DM.
News Medical reports that The Cochrane Central Register of Control Trials, Medline, Scopus, and Embase databases were searched for relevant SRMAs published until September 2022.
SRMAs were included if they included adults with prediabetes, MetS, or T2DM, assessed the effects of turmeric supplementation relative to placebo or standard therapy, and compared glycaemic parameters.
Turmeric supplementation included whole preparation (whole rhizome and standard powder), curcuminoid extracts, preparation with low-dose of piperine, or bioavailability-enhanced preparations.
The study’s primary outcomes included fasting blood glucose (FBG) and haemoglobin A1C (HbA1c) levels, and other outcomes included lipid profiles, blood pressure, C-reactive protein (CRP), uric acid and insulin resistance.
Additionally, the team performed an updated meta-analysis of RCTs from previously-published SRMAs.
AMSTAR 2 and Cochrane risk-of-bias tools were used to assess the methodological quality of SRMAs and their included RCTs. The quality of evidence was determined based on publication bias, indirectness, imprecision, inconsistency, and risk of bias.
Publication bias and heterogeneity of studies were evaluated using Egger's test and I-squared statistic, respectively.
Assessment
The researchers included 14 SRMAs (out of more than 3 500 hits) for the current review, which had been published between 2015 and 2022, including 5-26 RCTs with samples ranging from 290 to 1 790 participants.
Curcuminoid extracts, bioavailability-enhanced preparations, and whole preparation were assessed in 10, 13, and nine studies, respectively.
SRMAs were rated as having critically low-quality of evidence. Further, most SRMAs had a high degree of overlap, i.e., 31 RCTs were included in several SRMAs. Nine SRMAs specified changes in FBG or HbA1c after turmeric supplementation.
Of these, eight revealed significant reductions in FBG or HbA1c levels. Three SRMAs reported a significant decline in insulin resistance.
Significant reductions in total (TC) and low-density lipoprotein (LDL-C) cholesterol levels were reported in four SRMAs. None of the SRMAs found increased high-density lipoprotein cholesterol (HDL-C) levels.
Six SRMAs revealed decreases in triglyceride (TG) levels. Several SRMAs reported changes in blood pressure, body mass index (BMI), and CRP, but with limited statistical significance.
Overall, 28 RCTs comprising 2 362 patients aged 34-70 were included in the updated meta-analysis. These RCTs compared the effects of whole turmeric powder, extracts, and bioavailability-enhanced preparations relative to placebo. The risk of bias was rated as high in seven RCTs, with some concern in 14 and low in seven.
Turmeric supplementation significantly decreased HbA1c and FBG levels relative to placebo/standard treatment. Further, it also significantly reduced LDL-C, HDL-C, diastolic blood pressure, and insulin resistance; there were no changes in TG or TC levels.
In sub-group analyses by supplement type, reductions in FBG were noted with bioavailability-enhanced preparations and curcuminoid extracts; however, all supplement forms resulted in substantial decreases in HbA1c.
The reductions in HbA1c or FBG levels increased with higher supplement doses. Decreases in FBG were evident in MetS and T2DM patients, while only T2DM patients showed reductions in HbA1c.
Sensitivity analysis produced consistent results. Egger’s test and funnel plots ruled out publication bias, except for BMI or FBG outcomes. The quality of evidence was rated moderate based on inconsistency due to heterogeneity.
What they found
The findings suggest significant reductions in HbA1c and FBG levels by about 0.13% to 0.51% and 8 mg/dL, respectively, after turmeric supplementation, especially with bioavailability-enhanced and curcuminoid extract preparations.
Sub-group analysis revealed beneficial effects on glycaemic management in MetS and T2DM. Future investigations are necessary to assess if these effects are sustained over the long term and whether supplementation can alleviate the risks of diabetic complications.
Study details
Effects of turmeric (Curcuma longa) supplementation on glucose metabolism in diabetes mellitus and metabolic syndrome: An umbrella review and updated meta-analysis.
Thanika Pathomwichaiwat, Peerawat Jinatongthai ,Napattaoon Prommasut, Kanyarat Ampornwong, Wipharak Rattanavipanon, Surakit Nathisuwan, Ammarin Thakkinstian.
Published in PLOS ONE on 20 July 2023
Abstract
Aims
This study aims to comprehensively review the existing evidence and conduct analysis of updated randomised controlled trials (RCTs) of turmeric (Curcuma longa, CL) and its related bioactive compounds on glycaemic and metabolic parameters in patients with type 2 diabetes (T2DM), prediabetes, and metabolic syndrome (MetS) with a sub-group analysis of different CL preparation forms.
Methods
An umbrella review (UR) and updated systematic reviews and meta-analyses (SRMAs) were conducted to evaluate the effects of CL compared with a placebo/standard treatment in adult T2DM, prediabetes, and MetS. The MEDLINE, Embase, The Cochrane Central Register of Control Trials, and Scopus databases were searched from inception to September 2022. The primary efficacy outcomes were haemoglobin A1C (HbA1C) and fasting blood glucose (FBG). The corrected covered area (CCA) was used to assess overlap. Mean differences were pooled across individual RCTs using a random-effects model. Subgroup and sensitivity analyses were performed for various CL preparation forms.
Results
Fourteen SRMAs of 61 individual RCTs were included in the UR. The updated SRMA included 28 studies. The CCA was 11.54%, indicating high overlap across SRMAs. The updated SRMA revealed significant reduction in FBG and HbA1C with CL supplementation, obtaining a mean difference (95% confidence interval [CI]) of –8.129 (–12.175, –4.084) mg/dL and –0.134 (–0.304, –0.037) %, respectively. FBG and HbA1C levels decreased with all CL preparation forms as did other metabolic parameters levels.
The results of the sensitivity and subgroup analyses were consistent with those of the main analysis.
Conclusion
CL supplementation can significantly reduce FBG and HbA1C levels and other metabolic parameters in T2DM and mitigate related conditions, including prediabetes and MetS.
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