Effect of Curcumin on Diabetic Kidney Disease: A Systematic Review and Meta-Analysis of Randomized, Double-Blind, Placebo-Controlled Clinical Trials

Background Curcumin, a polyphenolic constituent from Curcuma longa, possesses antioxidant, hypolipidemic, and antidiabetic properties and has been reported to protect against diabetic kidney disease (DKD); however, the effect is inconsistent. Objective This systematic review and meta-analysis aimed to investigate the effect of curcumin supplementation on renal function, lipid profile, blood pressure, and glycemic control in DKD. Methods A systematic and comprehensive literature search of interrelated randomized controlled trials (RCTs) was conducted in PubMed, Embase, Cochrane Library, Web of Science, Scopus, and ClinicalTrials.gov from inception to July 30, 2021. Two investigators independently extracted data and assessed the risk of bias. Weighted mean differences (WMDs) with 95% confidence intervals (CIs) were calculated to describe the effect sizes using a fixed-effect model. Statistical analysis was performed using STATA 14.0 and RevMan 5.3. Results Five RCTs involving 290 participants with DKD were included. Curcumin supplementation significantly improved the serum creatinine (WMD: −0.16 mg/dL, 95% CI: −0.3 to −0.02, P = 0.029, I2 = 0%, moderate certainty), total cholesterol (WMD: −10.13 mg/dL, 95% CI: −17.84 to −2.14, P = 0.01, I2 = 0%, moderate certainty), systolic blood pressure (WMD: 3.94 mmHg, 95% CI: 1.86 to 6.01, P < 0.01, I2 = 33.5%, moderate certainty), and fasting blood glucose (WMD: −8.29 mg/dL, 95% CI: −15.19 to −1.39, P = 0.019, I2 = 43.7%, moderate certainty) levels; however, it had no significant effects on blood urea nitrogen, proteinuria, triglyceride, low-density lipoprotein-cholesterol, high-density lipoprotein-cholesterol, and diastolic blood pressure levels. Conclusions Curcumin may provide great potential effects against DKD. More large-scale and high-quality RCTs are required to confirm these findings.


Introduction
Diabetic kidney disease (DKD), a common microvascular complication of diabetes, is characterized by increased albuminuria level or urinary albumin-to-creatinine ratio (UACR), decreased glomerular filtration rate (GFR), or both [1]. According to the International Diabetes Federation data, approximately 463 million people were living with diabetes mellitus in 2019, and this number is projected to increase to 700 million by 2045 [2]. Up to 40% of patients with type 2 diabetes develop DKD, and DKD is currently the leading cause of end-stage renal disease (ESRD) worldwide [3]. Patients with DKD have a higher risk for cardiovascular morbidity and mortality, which are closely associated with risk factors such as hyperglycemia, dyslipidemia, and hypertension, than those without DKD [4]. DKD has become a considerable and growing challenge for health systems. e introduction of lifestyle changes, for example, exercise and diet, optimal control of blood pressure (BP), glucose, and lipids, and treatment with pharmacological agents or pharmaceuticals, such as angiotensin-converting enzyme inhibitors and angiotensin receptor blockers, are recommended by clinical practice guidelines for the treatment of patients with DKD [5,6]. Sodium-glucose cotransporter 2 inhibitors, a new class of antidiabetic agents, have now been shown to prevent major kidney outcomes in individuals with diabetes [3,7]; however, close attention needs to be paid to their adverse effects [8]. Although these managements are effective, they do not seem to substantially decrease the morbidity of DKD and patients with this condition still have a high risk of disease progression [9], indicating the urgent need for complementary and alternative therapies.
Turmeric, a powder of the rhizomes of Curcuma longa L. (Zingiberaceae), is a well-known spice additive that is ubiquitous in Asian cuisine [10]. Turmeric is also a staple component of Ayurveda and traditional Chinese medicine and has been used as a remedy for various chronic diseases, such as cardiovascular disease and diabetes [11] and Alzheimer's disease [12]. Curcumin is a naturally occurring polyphenolic compound extracted from turmeric and has various pharmacological activities, including antioxidant, anti-inflammatory, immunomodulatory, hypolipidemic, glycemic control, and improving hepatic function as well as serum cortisol levels [13][14][15]. Glucose and lipid metabolism disorder, oxidative stress, and inflammation are among the major contributors to the pathogenesis of DKD [5], indicating that curcumin has properties that might improve the pathological changes in DKD. Animal experiments have revealed that curcumin administration in rats or mice with DKD could alleviate kidney damage by increasing antioxidant mediators [16], suppressing inflammatory factors [17], and decreasing proteinuria (PRO) [18]. Clinical trials have shown that curcumin supplementation is beneficial in improving renal function, lipid profile, BP, and fasting blood glucose (FBG) in patients with DKD [19][20][21][22][23]; however, contradictory results have been reported. Some studies reported that curcumin administration can significantly improve renal function [21,22], lipid profile [19], BP [19], and FBG levels [19,20], whereas other trials reported contrasting results [20,23]. erefore, this study aimed to perform a comprehensive review and metaanalysis of previous pilot, double blind, placebo-controlled, randomized controlled trials (RCTs) on the effect of curcumin supplementation on DKD.

Materials and Methods
is meta-analysis was performed according to the guidelines of Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) [24] and was registered at INPLASY (https://inplasy.com/) with a registration ID INPLASY202180001. e checklist of PRISMA is shown in Supplementary Table S1.

Search Strategy.
e literature was systematically searched using electronic databases, including PubMed, Embase, Cochrane Library, and Web of Science and a clinical trial registry (ClinicalTrials.gov), from inception to July 30, 2021. Medical Subject Heading terms and text words were used as search terms, including "curcumin," "turmeric," "diabetic," "diabetic nephropathies," "diabetic kidney disease," and "randomized controlled trial." All searches were limited to English-language publications. We also manually screened the reference list of eligible studies in case of missing appropriate studies. e detailed search strategies for databases are presented in Supplementary  Table S2.

Study Selection.
A trial was included if it met the following criteria: (i) designed as an RCT; (ii) included patients diagnosed with DKD, defined as urinary albumin-to-creatinine ratio ≥30 mg/g or albumin excretion rate ≥30 mg/ 24 h or estimated GFR (eGFR) <60 mL/min/1.73 m 2 for ≥3 months caused by diabetes mellitus [25] (no restrictions on the patients' sex, region, race, and disease course were implemented); (iii) administered curcumin to participants in the intervention group and placebo to the control group (with no limitation in dosage or follow-up period); and (iv) reported at least one of the following outcomes: renal function parameters: including serum creatinine (SCr), blood urea nitrogen (BUN), and PRO; lipid profile, including total cholesterol (TC), triglycerides (TG), lowdensity lipoprotein-cholesterol (LDL-C), high-density lipoprotein-cholesterol (HDL-C); BP, including systolic blood pressure (SBP) and diastolic blood pressure (DBP); and FBG levels. A study was excluded if it met the following criteria: (i) included participants not diagnosed with DKD; (ii) performed the trial in animals; (iii) published as an observational study, case report, meta-analysis, or review; and (iv) included participants who received drugs or supplements other than curcumin.

Data Extraction.
Two investigators (J.Z. and C.M.) independently screened eligible studies according to predetermined criteria. One investigator (J.Z.) extracted the following data from each study: study characteristics, including first author, publication year, country, and study design; participant characteristics, including sample size, mean age, and sex ratio; interventions and comparison, including the use and dosage of turmeric/curcumin and the follow-up period; and available outcomes. e other investigator (C.M.) rechecked the data for consistency. Any discrepancy between the two investigators (J.Z. and C.M.) was resolved through a discussion with a third investigator (J.A.). For missing data, we tried our best to contact corresponding authors via e-mail.

Risk of Bias Assessment.
Two investigators (J.Z. and C.M.) independently assessed the risk of bias of the included studies using the Cochrane Risk of Bias tool [26], including six items: selection bias, performance bias, detection bias, attrition bias, reporting bias, and bias from other sources. ese items were judged as "low risk," "high risk," or "unclear." Any disagreements were resolved through a discussion with a third investigator (J.A.). Evidence-Based Complementary and Alternative Medicine

Data Synthesis and Analysis.
e effects of outcomes in the intervention and placebo groups were estimated by comparing the changes in mean differences before and after therapy (i.e., end values minus baseline values). e standard deviation (SD) changes in outcomes were calculated according to the following formula: SD � sqrt [(SD before treatment) 2 + (SD after treatment) 2-(2R × SD before treatment × SD after treatment)], where R, a correlation coefficient, was equal to 0.5 [27]. Weighted mean differences (WMDs) with corresponding 95% confidence intervals (CIs) for continuous outcomes were calculated to describe the effect sizes. Cochran's Q and I2 statistics were employed to evaluate statistical heterogeneity. I 2 values of <25%, >50%, and >75% were considered to indicate low, moderate, and high heterogeneity, respectively [28]. Participants (≤60 versus > 60 years), dosage of curcumin supplementation (≥1500 versus < 1500 mg/day), and follow-up period (≤2 versus > 2 months) or sensitivity analysis was conducted to identify the sources of heterogeneity. Furthermore, sensitivity analysis was performed to evaluate the robustness of outcomes. For outcomes with small heterogeneity, sensitivity analyses were performed using the conversion effect model. In contrast, if outcomes had considerable heterogeneity, sensitivity analysis was performed by omitting one study at a time. Begg's and Egger's tests were used to examine potential publication bias [29]. Statistical analysis was performed using STATA 14.0 (StataCorp, College Station, TX, USA) and RevMan 5.3. P values <0.05 were considered statistically significant.
2.6. GRADE Assessment. Two investigators (J.Z. and C.M., W.S., or J.A.) independently evaluated the certainty of the evidence for outcomes according to the GRADE system, an approach for rating the quality of evidence and strength of recommendations, graded as high, moderate, low, or very low certainty [30]. GRADE assessments were downgraded according to the following criteria: (i) risk of bias was downgraded if one of the items used to evaluate the risk of bias was considered to be at a high risk of bias and most of the included studies were identified as having a high risk of bias; (ii) imprecision was downgraded if the 95% CIs for effect estimates overlap zero for continuous outcomes; (iii) inconsistency was downgraded if there was substantial heterogeneity (I 2 > 50% and P < 0.1) that was unexplained by any subgroup or sensitivity analysis; (iv) indirectness was downgraded if there were any influencing factors that limited the interpretation of outcomes; and (v) publication bias was downgraded if there was a significant difference in the evidence of publication bias according to either Begg's test or Egger's test (P < 0.05).
According to predefined inclusion criteria, 100 records were excluded after screening the titles and abstracts. Consequently, after full-text assessment and detailed examination, seven records were excluded because of having an inappropriate study design, using an inappropriate intervention, or not reporting the data of interest. Ultimately, five RCTs [19][20][21][22][23] were included. e PRISMA flow diagram is shown in Figure 1.

Characteristics of Included Studies.
Overall, five trials involving 290 patients were included. ese trials were conducted in Iran and Mexico and were published between 2011 and 2020. Two trials included patients with overt diabetic nephropathy [21,22]; two trials were performed in patients with diabetic proteinuric chronic kidney disease [20,23]; and one trial enrolled patients with diabetes requiring hemodialysis [19]. All included studies were randomized, double-blind, placebo-controlled trials. e sample size varied from 20 to 54. e mean age of patients was 60 years. ree trials used curcumin capsules at doses ranging from 320 to 1670 mg/day [20,22,23]. One study used turmeric capsules that included curcumin at a dose of 66.3 mg/day [21], whereas another trial used nanocurcumin capsules at a dose of 80 mg/day [19]. e follow-up periods ranged from 2 to 6 months. e main characteristics of included trials are summarized in Table 1.

Methodological Quality Assessment.
All studies had a randomized parallel design. Two trials (40%) showed a low risk of bias and used permuted blocks [22] and computergenerated random numbers [19], respectively. e remaining three RCTs were classified as having an unclear risk of bias owing to inadequate descriptions of randomization methods [20,21,23]. With respect to allocation concealment, blinding of participants and researchers, incomplete outcome data, selective reporting, and other biases, all included trials were classified as having a low risk of bias. Notably, four trials (80%) that did not blind outcomes were assessed as having a high risk of bias [20][21][22][23]. e risk of bias is presented in Figure 2.
3.4.1.2 BUN. ree trials with 139 participants studied the effect of curcumin on BUN. e pooled effect of curcumin administration showed no significant difference in BUN (WMD: −0.80 mg/dL, 95% CI: −5.62 to 4.02, P � 0.933), with high heterogeneity (I 2 � 60% and P heterogeneity � 0.082). Subgroup analysis showed that the effect of curcumin supplementation on BUN also had Evidence-Based Complementary and Alternative Medicine substantial heterogeneity in all the analyzed subgroups (Table 3). Further sensitivity analysis revealed that the heterogeneity of I 2 � 60% changed to I 2 � 0% after removing the study by Shafabakhsh et al. [19], which was probably the major source of the heterogeneity. When the other two trials with 86 participants were pooled, the revised result showed that the effect of curcumin on BUN still had no significant difference from that of placebo (WMD: 1.10 mg/dL, 95% CI: −1.72 to 3.92, P � 0.446; I 2 � 0%, P heterogeneity � 0.601; Figure 3(b) and Table 2 Table 2). Subgroup analysis revealed that the statistical heterogeneity decreased in participants with age ≤60 years (I 2 � 41.4% and P heterogeneity � 0.181), dosage of curcumin intake ≥1500 mg/day (I 2 � 47.4% and P heterogeneity � 0.168), and follow-up period >2 months (I 2 � 47.4% and P heterogeneity � 0.168), suggesting that these factors may be the sources of the heterogeneity (Table 3).  Table 2).
3.4.2.4 LDL-C. ree trials with 139 participants assessed the effects of curcumin on LDL-C. e meta-analysis showed that curcumin supplementation did not affect changes in LDL-C (WMD: −1.06 mg/dL, 95% CI: −17.89 to 15.77, P � 0.902; I 2 � 77.2%, P heterogeneity � 0.012; Figure 4(d) and Table 2). e considerable heterogeneity between studies remained after subgroup analysis according to the mean age of participants, dosage of curcumin, and follow-up period (Table 3). e sensitivity analysis also did not explain the heterogeneity. After carefully reading the original articles, we found that the participants in the three studies were different, although they were all diagnosed with DKD. Participants with overt type 2 diabetic nephropathy were   Figure 5(a) and Table 2). 3.4.3.2 DBP. Four trials with 244 participants investigated the effect of curcumin supplementation on DBP. e differences in DBP were not statistically significant (WMD: 0.21 mmHg, 95% CI: −5.64 to 6.05, P � 0.944; I 2 � 92.3%, P heterogeneity ＜0.001) between the curcumin group and the placebo group ( Figure 5(b) and Table 2). Heterogeneity between studies was insignificant (I 2 � 21.4%, P heterogeneity � 0.259) after subgroup analysis according to follow-up period (>2 months), which was probably the source of the heterogeneity (Table 3).

Effects of Curcumin Supplementation on FBG.
Four trials with 190 participants analyzed the impact of curcumin supplementation on FBG. e overall effect size revealed a significant decrease in FBG after curcumin   Table 2).

Sensitivity Analysis.
For outcomes with insignificant heterogeneity (SCr, TC, TG, and HDL-C), no significant impact of the conversion effect model on the overall effect sizes was observed. With respect to outcomes with substantial heterogeneity (BUN, PRO, LDL-C, and DBP), the overall effect sizes were not significantly affected when any single study was omitted. ese indicated the robustness of the combined result. However, the overall effect sizes for outcomes with insignificant heterogeneity (SBP and FBG) were influenced by the conversion effect model, indicating the poor robustness of SBP and FBG as outcomes. erefore, the results on SBP and FBG should be interpreted with caution.

Publication Bias.
No evidence of publication bias was observed for the effects of curcumin supplementation on SCr, BUN, PRO, TC, TG, HDL-C, LDL-C, SBP, DBP, and FBG based on Begg's test and Egger's test (Supplementary Table 2S3).

GRADE Assessment.
e overall certainty of the evidence for the effects of curcumin supplementation on renal function, lipid profile, BP, and glycemic control is presented in Table 4. e quality of the evidence for SCr, TC, SBP, and FBG was graded as "moderate" after being downgraded for risk of bias. Meanwhile, the quality of the evidence for BUN, PRO, TG, HDL-C, and DBP was graded as "low" after being downgraded for risk of bias and imprecision. e quality of the evidence for LDL-C was downgraded for risk of bias, inconsistency, and imprecision and was graded as "very low."

Discussion
To our knowledge, this is the first meta-analysis of RCTs to analyze the effects of curcumin supplementation on DKD. Our findings showed that supplemental curcumin, in comparison with placebo, significantly improved the SCr, TC, SBP, and FBG levels in patients with DKD, with moderate certainty of evidence; however, it had no significant effects on the BUN, PRO, TG, HDL-C, LDL-C, and DBP levels.      Evidence-Based Complementary and Alternative Medicine e presence of PRO indicates renal parenchymal injury, which has been considered a clinical hallmark and a sign of renal dysfunction [31]. As PRO increases, eGFR decreases, resulting in end-stage renal disease and an increased risk of cardiovascular events [32,33]. It is generally known that elevated levels of SCr and BUN in serum also indicate renal impairment. In the current meta-analysis, curcumin supplementation significantly decreased the SCr levels. Nevertheless, no effects of curcumin on BUN and PRO were observed in patients with DKD, consistent with the findings of a previous systematic review that investigated the role of curcumin in the treatment of renal disorders [34]. However, evidence from animal studies showed that curcumin significantly reduced the levels of SCr, BUN, and PRO [18], indicating that the differences in the effects of curcumin between animal experiments and clinical trials need further verification.

Evidence-Based Complementary and Alternative Medicine
Lipid accumulation in podocytes results in podocyte injury and renal impairment and has been considered a driving factor for the development and progression of DKD [35]. Dyslipidemia is positively related to atherosclerosis and increases the risk of cardiovascular disease [36], indicating the urgent need for the management of dyslipidemia. Curcumin has been widely investigated owing to its obvious hypolipidemic effects. A study showed that curcumin significantly reduced the elevated levels of TC and TG and increased the level of HDL-C in rats fed a high-fat diet by increasing hepatic fatty acid oxidation activity and improving the apoptotic status of liver tissue [37]. In addition, the hypolipidemic effects of curcumin are also attributed to the suppression of the rate-limiting enzyme in the TC synthesis pathway; upregulation of the expressions of ABCA1, Apo-A1, and SR-BI, which are associated with reverse TC transport through HDL-C particles; induction of LDL-C-receptor expression; and inhibition of ApoB100 expression [38]. Meta-analyses of RCTs reported that curcumin supplementation showed benefits on lipid metabolism in patients with type 2 diabetes [39], polycystic ovary syndrome [14], and nonalcoholic fatty liver disease [40]. Our findings demonstrated a remarkable reduction in the TC level after curcumin administration in patients with DKD. However, the results should be interpreted with caution owing to the limited number of studies, and larger-scale trials are required to verify the results.
Hypertension is also closely associated with CKD development and progression [1]. It has been demonstrated that a 10-20 mmHg increase in SBP increases the risk of DKD by 21% [41]. erefore, reducing BP, particularly SBP, can help decrease the risk of DKD. e effect of curcumin supplementation on BP is not fully conclusive. Damoon et al. reported that the hypotensive effect of nanocurcumin on cardiovascular disease is mainly focused on decreasing SBP [42]. Amir et al. observed favourable effects of curcumin administration only on SBP levels in different participants with a ≥12-week follow-up [43]. In contrast, Maryam et al. observed that curcumin supplementation improved DBP in participants with metabolic syndrome; however, it was not associated with a change in SBP [44]. e result of the present meta-analysis indicated that curcumin administration had a prominent effect on SBP in patients with DKD. We speculate that the discrepancy in the effects of curcumin supplementation on BP may be associated with differences in participants and follow-up periods across studies. A few potential mechanisms have been proposed to explain the  beneficial effects of curcumin on BP. Curcumin prevents the development of hypertension by downregulating angiotensin II type-1 receptor (AT1R) expression to alleviate AT1R-mediated vasoconstriction [45]. In addition, curcumin inhibits vascular smooth muscle cell (VSMC) migration by inhibiting NF-κB-mediated NLRP3 expression in angiotensin II-treated VSMCs [46]. Hyperglycemia is also identified as an important factor for CKD development beyond its links to lipid accumulation and hypertension [1]. Intensive glycemic therapy, such as controlling the haemoglobin A1c levels within 6.5-7.0%, contributes to reducing the risk of DKD [47]. Evidence from both in vivo and vitro studies indicated a strong potential effect of curcumin against insulin resistance and diabetes [48,49]. Previous meta-analyses showed that curcumin significantly improved FBG levels in individuals with polycystic ovary syndrome [14], some degree of dysglycemia [50], and metabolic syndrome [44]. Consistent with these findings, we also observed conspicuous effects of curcumin supplementation on FBG levels in patients with DKD, adding to the existing evidence on the benefits of curcumin in improving FBG. However, there is no evidence about the impact of curcumin supplementation on other glycemic parameters, such as haemoglobin A1c, serum insulin, and homeostasis model assessment-insulin resistance index in the included trials. Largescale studies focusing on these topics are needed in the future.

Strengths and Limitations.
To our knowledge, this is the first meta-analysis of RCTs to assess the effects of curcumin supplementation on DKD and to provide evidence for curcumin as a promising agent in the treatment of this condition. We conducted the current study strictly according to the PRISMA guidelines and observed significant effects of curcumin on SCr, TC, SBP, and FBG levels. Furthermore, we evaluated the certainty of the evidence for outcomes using the GRADE method. e evidence for SCr, TC, SBP, and FBG was graded as having moderate certainty.
Several limitations should be considered when interpreting our findings. First, the sample sizes and the number of trials that were eligible for inclusion were small. Second, various dosages and durations of treatment were applied for the intervention. ird, the adverse effects of curcumin require attention; however, it is difficult to assess the safety of curcumin owing to the limited number of reports on its potential adverse effects. Finally, three (60%) trials did not provide adequate information on randomization methods and four (80%) trials did not perform blinding of outcomes, which may have reduced the quality of the evidence of the studies.

Implications for Practice.
According to the available evidence from this meta-analysis, curcumin supplementation has beneficial effects on SCr, TC, SBP, and FBG levels, 444◯ moderate SCr, serum creatinine; BUN, blood urea nitrogen; PRO, proteinuria; TC, total cholesterol; TG, triglycerides; LDL-C, low-density lipoprotein-cholesterol; HDL-C, high-density lipoprotein-cholesterol; SBP, systolic blood pressure; DBP, diastolic blood pressure; FBG, fasting blood glucose; WMD, weight mean difference; CI, confidence interval; RCT, randomized controlled trial. a For risk of bias, the majority of included studies were considered to be at high risk of bias due to the fact that outcomes assessment was not blinded, so it was downgraded. b Although there was substantial heterogeneity for the effect of curcumin on BUN, it was explained when the study conducted by Shafabakhsh et al. was removed according to the sensitivity analysis (original: I 2 � 60%, P heterogeneity � 0.082; after study was removed: I 2 � 0%, P heterogeneity � 0.601). c e 95% CIs for effect estimates overlap the zero. d Although there was considerable heterogeneity for the effect of curcumin on PRO, it was associated with mean age (≤60 years, I 2 � 41.4%), curcumin intake (≥1500 mg/day, I 2 � 47.4%), and follow-up period (＞2 months, I 2 � 47.4%). e ere was substantial heterogeneity (I 2 ≥ 77.2%, P < 0.1) that was unexplained by any subgroup or sensitivity analysis for the effect of curcumin on LDL-C. f Although there was substantial heterogeneity, it was explained for the effect of curcumin on DBP with follow-up periods (＞2 months, I 2 � 21.4%).
with moderate certainty, in patients with DKD and seems to be a promising agent against DKD. Nevertheless, our results should be interpreted with caution considering the abovementioned limitations.

Implications for Research.
As the results of meta-analyses are closely related to evidence-based treatment decisions in the clinic, rigorously designed studies are highly warranted. Large-sample, multicentre, high-quality, and well-designed clinical trials should be registered to ensure the transparency of the process and improve the methodological quality. Moreover, future studies should also focus on adverse effects, apart from efficacy, to evaluate the safety of curcumin.

Conclusions
In conclusion, the current study shows that curcumin supplementation provides evident improvements in the SCr, TC, DBP, and FBG levels in patients with DKD, with moderate certainty of evidence. However, although curcumin supplementation has great potential effects on DBP and FBG, the results should be interpreted with caution because of the poor robustness of the evidence. e effect of curcumin supplementation on DKD should be confirmed by more large-scale and high-quality RCTs.

Data Availability
e data used to support the findings of this study are included within the article and in the Supplementary Materials.

Disclosure
Zhao Jie and Mo Chao are co-first authors.

Conflicts of Interest
e authors declare that there are no conflicts of interest.