A recent study published in the PLOS One Journal evaluated the effects of turmeric supplementation on glucose metabolism in patients with metabolic syndrome (MetS), type 2 diabetes mellitus (T2DM), or prediabetes.
Study: Effects of turmeric (Curcuma longa) supplementation on glucose metabolism in diabetes mellitus and metabolic syndrome: An umbrella review and updated meta-analysis. Image Credit: SOMMAI/Shutterstock.com
Background
T2DM has become a significant global health burden, with over 437.9 million individuals affected in 2019. The increase in T2DM cases has been more prominent in developing countries. While T2DM management requires multiple interventions, access to treatment is critical for disease control.
Access to advanced therapies is limited in less developed countries. As such, attempts are underway 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. Besides, numerous systematic reviews and meta-analyses (SRMAs) of randomized controlled trials (RCTs) provide mixed results due to heterogeneous populations, uses, preparation forms, durations, and outcomes.
About the study
In the present study, researchers reviewed the available evidence on the effects of supplementing turmeric on metabolic and glycemic parameters in MetS, prediabetes, and T2DM.
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 glycemic 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 hemoglobin A1C (HbA1c) levels. Besides, 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 recent SRMAs published in March-September 2022.
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.
Findings
The researchers included 14 SRMAs (out of more than 3,500 hits) for the current review. They were 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. Further, 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.
Conclusions
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 glycemic 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.
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