werfwqf

1. Accepted Manuscript
Meta-analysis of randomized controlled trials of 4weeks or longer
suggest that curcumin may afford some protection against
oxidative stress
Si Qin, Lifan Huang, Jiaojiao Gong, Shasha Shen, Juan Huang,
Yao Tang, Hong Ren, Huaidong Hu
PII: S0271-5317(18)30138-6
DOI: doi:10.1016/j.nutres.2018.08.003
Reference: NTR 7931
To appear in: Nutrition Research
Received date: 1 February 2018
Revised date: 4 August 2018
Accepted date: 17 August 2018
Please cite this article as: Si Qin, Lifan Huang, Jiaojiao Gong, Shasha Shen, Juan Huang,
Yao Tang, Hong Ren, Huaidong Hu , Meta-analysis of randomized controlled trials of
4weeks or longer suggest that curcumin may afford some protection against oxidative
stress. Ntr (2018), doi:10.1016/j.nutres.2018.08.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. Please note that during the production process errors may
be discovered which could affect the content, and all legal disclaimers that apply to the
journal pertain.

2. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
1
Meta-analysis of randomized controlled trials of 4 weeks or longer suggest that curcumin may afford
some protection against oxidative stress
Si Qin1, 2
, Lifan Huang2
, Jiaojiao Gong2,3
, Shasha Shen2,3
, Juan Huang2,3
, Yao Tang2,3
, Hong Ren3
and
Huaidong Hu2,3#
1
Center for Endocrine Diseases, The Third Affiliated Hospital of Chongqing Medical University, Chongqing,
P.R. China.
2
Department of Clinical Nutrition, The Second Affiliated Hospital of Chongqing Medical University,
Chongqing, P.R. China.
3
Institute for Viral Hepatitis, Key Laboratory of Molecular Biology for Infectious Diseases (Ministry of
Education), Institute for Viral Hepatitis, Department of Infectious Diseases, The Second Affiliated Hospital of
Chongqing Medical University, Chongqing, P.R. China.
Dr. Si Qin, E-mail: qinsi315@163.com
Dr. Lifan Huang, E-mail: huanglifan10@163.com
Dr. Jiaojiao Gong, E-mail: gongjiaojiaogo@163.com
Dr. Shasha Shen, E-mail: shashashensss@163.com
Dr. Juan Huang, E-mail: huangjuan_108@163.com
Dr. Yao Tang, E-mail: ty542610@126.com
Dr. Hong Ren, E-mail: renhong0531@vip.sina.com
# Corresponding author: Dr. Huaidong Hu. E-mail address: huhuaidong@sina.com.
Department of Clinical Nutrition, The Second Affiliated Hospital of Chongqing Medical University, No.76,
Linjiang Road, Chongqing 400010, P.R. China.
ACCEPTED MANUSCRIPT

3. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
2
Abbreviations
OS, Oxidative stress;
GPX, Glutathione peroxidase;
RBC, Red blood cells
MDA, Malondialdehyde;
SOD, Superoxide dismutase;
SMD, Standardized mean difference;
CI, Confidence interval;
ROS, Reactive oxygen species;
PRISMA, Preferred reporting items for systematic reviews and meta-analyses;
SD, Standard deviation;
SE, Standard error;
T2DM, Type 2 diabetes mellitus;
MetS, Metabolic syndrome;
TBARS, Thiobarbituric acid reactive substances;
Nrf2, Nuclear factor erythroid 2-related factor 2;
ARE, Antioxidant response elements;
GST, Glutathione-S-transferase;
HO-1, Hemeoxygenase-1;
NQO1, Quinone oxidoreductase;
ACCEPTED MANUSCRIPT

4. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
3
Abstract
Oxidative stress (OS) is associated with aging and multiple diseases, yet the effects of curcumin in humans
are not definite. We undertook a meta-analysis of the effects of curcumin on OS biomarkers. In January 2018,
we searched PubMed, Books@Ovid, Journals@Ovid, EMBASE, MEDLINE(R), and Web of Science to identify
randomized controlled trials conducted ≥ 4 weeks and investigating the effects of curcumin on OS biomarkers,
including glutathione peroxidase (GPX) activity in red blood cells (RBC), serum malondialdehyde (MDA)
concentrations, and superoxide dismutase (SOD) activity. The standardized mean difference (SMD) with a 95%
confidence interval (CI) was used to present the results. The meta-analysis included eight clinical studies (626
patients). There was a significant reduction in circulating MDA concentrations (SMD = -0.769, 95% CI: -1.059
to -0.478) and a significant increase in SOD activity (SMD = 1.084, 95% CI: 0.487 to 1.680) following
curcumin supplementation. There was no change in the GPX activity in RBC. There was no significant
association between the MDA-lowering effect of curcumin with underlying diseases or treatment duration.
However, curcumin showed the MDA-lowering effect at curcuminoids doses ≥ 600 mg/d (P < 0.0001). This
effect was greater when combined with piperine than curcuminoids alone (SMD = -1.085, 95% CI: -1.357 to
-0.813; SMD = -0.850, 95% CI: -1.158 to -0.542). Curcumin may play an anti-oxidative role by reducing
circulating MDA concentrations and increasing SOD activity. Further research of curcumin in different
populations with multiple biomarkers of redox status are required.
Keywords: Curcumin; Oxidative Stress; Malondialdehyde; Superoxide dismutase; Glutathione peroxidase;
Meta-analysis.
ACCEPTED MANUSCRIPT

5. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
4
1. Introduction
Curcumin (chemical name: diferuloylmethane), a bioactive dietary polyphenol, is the main curcuminoid present
in turmeric (Curcuma longa), which is a popular spice in Indian cuisine that is widely used in Middle Eastern
and South Asian countries [1, 2]. As curcumin is known to act on various molecular targets, clinical studies have
indicated that curcumin possesses various biological activities in the treatment of chronic diseases, including
pulmonary disorders, various cancers, and autoimmune diseases [3]. Moreover, a recent meta-analysis showed
that overexpression of inflammatory biomarkers, such as cytokines, played a key role in the development of
major depression, and curcumin was shown to have anti-depressive effects in patients [4].The anti-oxidative
property of curcumin has been considered to be crucial for the beneficial role it plays in these disease conditions
[5].
Oxidative Stress (OS) is characterized by an imbalance between the production and elimination of reactive
oxygen species (ROS) [6]. OS has been associated with aging as well as multiple chronic diseases including
cancer, pulmonary disorders, diabetes mellitus, neurodegenerative and cardiovascular diseases [7, 8]. ROS are
by-products of reduction-oxidation reactions. The production of ROS can lead to protein carboxylation, DNA
damage, cytoskeletal disruption and lipid peroxidation [9]. As ROS have a very short half-life, it is impossible
to directly measure their levels. Changes in the serum concentrations of malondialdehyde (MDA; as an
end-product of lipid peroxidation induced by ROS), glutathione peroxidase (GPX; as an antioxidant enzyme )
and superoxide dismutase (SOD; as an additional antioxidant enzyme ) activity can be considered to be major
manifestations of OS [10]. Moreover, a large number of studies, from both in vitro and animal models, have
indicated that curcumin can reduce serum MDA concentrations and increase SOD and GPX activity [11-17].
ACCEPTED MANUSCRIPT

6. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
5
However, clinical trials regarding this effect of curcumin among humans have reached conflicting conclusions
[18-25]. Some studies have reported promising effects [18-21, 23-25], whereas one study showed that the MDA
concentrations reduced in both the experimental and control groups [22]. Thus, we conducted a meta-analysis of
published clinical trials to assess the effect of curcumin on biomarkers of oxidative stress.
2.Approach
2.1.Literature search
In January 2018, we searched PubMed, EMBASE, Web of Science, Medline Ovid, Books@Ovid,
Journals@Ovid, and The Cochrane Library databases up to January 2018 using the search terms: (curcuminoids
OR curcuminoid OR curcumin OR curcuma OR turmeric OR turmeric extracts OR curcuma longa OR curcuma
longas OR longa, curcuma OR longas, curcuma OR diferuloylmethane OR curcuma zedoarias OR zedoaria,
curcuma) AND (malondialdehyde OR propanedial OR malonyldialdehyde OR malonaldehyde OR
malonylaldehyde OR sodium malondialdehyde OR malondialdehyde, sodium OR MDA OR superoxide
dismutase OR SOD OR glutathione peroxidase OR GPX). The online search was limited to published clinical
studies in the English language.
2.2.Inclusion and exclusion criteria
The inclusion criteria for clinical studies included: (1) drug or placebo-controlled parallel randomized trials or
crossover trials; (2) studies that used curcuminoids or turmeric preparations with specified content of curcumin
ACCEPTED MANUSCRIPT

7. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
6
or curcuminoids, regardless of the dosage and frequency of administration (this inclusion criterion was
established according to a review about the efficacy of turmeric/curcumin in lowering blood lipid levels in
patients with cardiovascular risk factors [26]); (3) treatment duration ≥ 4 weeks (this inclusion criterion was
established in accordance with two review articles [26, 27]) and (4) ≥ 30 subjects. The exclusion criteria were (1)
non-comparative data; (2) study duration < 4 weeks; and (3) incomplete data. We examined the reference lists of
all the studies included using the above methods.
2.3.Efficacy measures
The primary outcome measures included GPX activity in red blood cells (RBC), and serum MDA
concentrations and SOD activity. Relevant adverse effects were defined as a secondary outcome.
2.4.Data extraction
Two authors (SQ and LFH) independently screened the abstracts, the titles or both sections of the studies found
in the initial search. All the potential articles were investigated as full articles. Differences in opinion were
resolved by a third party (Author HDH). An adapted Preferred Reporting Items for Systematic Reviews and
Meta-Analyses (PRISMA) flow chart for study selection was followed [28]. Eligible full-text articles were
reviewed, and the following essential data were independently abstracted by two authors (SQ and LFH): (1)
first author's name; (2) year of publication; (3) treatment regimen; (4) study design; (5) duration of treatment; (6)
patient characteristics; (7) type and dose of curcuminoid or turmeric powder used; (8) number of patients; (9)
ACCEPTED MANUSCRIPT

8. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
7
pre- and post-intervention oxidative stress parameters. Disagreements were resolved by discussion or by the
third party (Author HDH). Authors were contacted for the acquisition of missing data.
2.5.Study quality
Two authors (SQ and LFH) independently assessed the risk of bias in each trial using the Cochrane Handbook
for Systematic Reviews of Interventions [29]. Disagreements were resolved by consultation or the involvement
of a third party (Author HDH). The items used for assessing the quality of each study were: random sequence
generation, allocation concealment, blinding, incomplete outcome data (the acknowledgement of dropouts),
selective reporting, and other potential sources of bias. According to this tool, the risk of bias was marked as
low, high or unclear.
2.6.Quantitative Data Synthesis and Analyses
Meta-analysis was conducted using Stata version 12.0 (Stata Corporation, College Station, TX, USA). Only
continuous variables were extracted in this study. Plasma MDA concentrations were collated in nmol/ml. If
mean values and standard deviation (SD) had to be acquired indirectly, they were calculated from the mean
values and standard error (SE) using the following formulas: SD = SE × square root n (n: number of
participants); mean = mean post-treatment - mean pre-treatment and SD = square root [(SD pre-treatment)2
+ (SD post-treatment)2
- (2R × SD pre-treatment × SD post-treatment)], assuming a correlation coefficient (R) = 0.5 [30]. The standardized mean
difference (SMD) with a 95% confidence interval (CI) was used to present the results because the means of
ACCEPTED MANUSCRIPT

9. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
8
individual studies were variable, and the units of the measurements were different. Statistical heterogeneity
between the eligible trials was detected using the I-square (I2
) and Chi-square tests. If heterogeneity was not
significant (P > 0.1 and I2
< 50%), the fixed-effect model was applied for analyses; in other cases, a
random-effect model was used. Subgroup analyses were used to explore the heterogeneity of studies in terms of
the underlying disease, administered dose, the differences in the form of intervention (turmeric or
curcuminoids), and duration of treatment. In addition, the biological sample preparation, storage, and
pretreatment procedures [31, 32] could further contribute to the study heterogeneity. However, the selected
studies did not report on these relevant factors and subgroup analyses using these factors were not undertaken.
The Begg’s test and funnel plots were conducted to assess potential publication bias initially.
3. Results
3.1. Results of the search
Of the 4578 published articles retrieved through the original literature search, a specific evaluation of 14
full-text articles resulted in the exclusion of six studies for the following reasons: 1) two studies [33, 34] were
dismissed owing to a small sample size; 2) two clinical trials [35, 36] were eliminated due to a short duration of
treatment; and 3) two additional studies [37, 38] were excluded due to incomplete data (Figure 1). After
evaluation, eight clinical studies, including a total of 626 subjects, met the inclusion criteria and were selected
for review and qualitative analysis.
ACCEPTED MANUSCRIPT

10. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
9
3.2 Included studies
The characteristics of the included studies are summarized in Table 1. Out of the included studies, three studies
evaluated curcumin among patients with type 2 diabetes mellitus (T2DM) [18, 23, 24]; one study included
patients with metabolic syndrome (MetS) [21]; two studies evaluated patients with osteoarthritis [19, 20]; one
study enrolled patients with end-stage renal disease [22], one study focused on patients with chronic pruritic
skin lesions [25]. All studies were double-blind, except from the trials by Selvi et al. [23] and Usharani et al.
[24]. Five studies were conducted in Iran [18, 20-22, 25] and the remaining studies were conducted in India [19,
23, 24]. The study duration ranged from 4 weeks to 4 months. Two studies used turmeric powder [22, 23] and
the remaining studies used curcuminoids [18-21, 24, 25]. Parameters of oxidative stress among subjects in the
included studies were different at the baseline measurements and the measurements following intervention
(Table 2).
3.3 Assay methods
The included studies used different methods to measure plasma MDA concentrations and SOD activity. Most of
the studies [18, 20, 21, 24] used a spectrophotometer to measure plasma MDA concentrations and SOD activity.
Pakfetrat et al. [22] analyzed plasma MDA concentration as thiobarbituric acid reactive substances (TBARS)
using a colorimeter. Panahi et al. [25] analyzed plasma SOD activity using commercial kits. Two studies [22, 23]
failed to report the specific measurement of the GPX activity in RBC, from which Selvi et al. [23] also failed to
ACCEPTED MANUSCRIPT

11. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
10
report the specific measurement of plasma MDA concentrations. These two studies [22, 23] only mentioned the
related methodological references. However, one study that used the TBARS assay to determine serum MDA
concentrations did not describe the specific measuring method [19].
3.4 Risk of bias in included studies
Details of the quality of bias are shown in Table 3. Three studies used appropriate randomization methods, such
as computerized randomization schedule [19] or a table of random numbers [20, 23]. Allocation concealment
was carried out in four studies [19, 22, 23, 25]. Six of the studies used a double-blind design (practitioners and
patients) [18-22, 25]. There was a lack of information regarding “free of other bias” information in most of the
included studies [19-25].
3.5 Effects of curcumin
Pooled data from seven trials (n = 263, in experimental groups and n = 262, in control groups) showed that the
use of curcumin led to a significant reduction in serum MDA concentrations (SMD = -0.769, 95% CI: -1.059 to
-0.478, P < 0.0001) [18-24]. As there was a significant heterogeneity observed among the seven trials. Hence, a
random-effects model was used in this study (P = 0.022, I2
= 59.5%, Fig. 2).
With regard to plasma SOD activity, the pooled analysis of data from four studies [18, 20, 21, 25] showed
significant increases of curcumin (SMD = 1.084 , 95% CI: 0.487 to 1.680, P < 0.0001) in the experimental
group (n =165) as compared to the control group (n = 171). Although a random-effect model was used,
ACCEPTED MANUSCRIPT

12. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
11
significant heterogeneity was observed among these four trials (P < 0.0001, I2
= 84.2%; Fig. 3). Meta-analysis
of pooled data from two studies [22, 23] comprising 108 subjects indicated that curcumin therapy did not lead
to increased GPX activity in the RBC (P = 0.490, I2
= 42.4%; Fig. 4).
3.6Subgroup analyses
To assess the sources of potential bias, subgroup analyses were conducted with regard to the form of
intervention, administered dose, duration of supplementation, and the underlying disease.
3.6.1 Subgroup analyses for serum MDA concentrations
Analysis of pooled data from two studies comprised of 108 subjects did not show a significant effect of turmeric
powder therapy on circulating MDA concentrations [-0.248 (-0.627 to 0.131), P = 0.200, I2
= 0%] [22, 23].
However, the use of concentrated turmeric extract in two studies [19, 24] and use of curcuminoids with piperine
in three studies [18, 20, 21] showed a significant reduction in circulating MDA concentrations (P < 0.0001, I2
=
0% and P < 0.0001, I2
= 7.1% respectively). Moreover, the studies administering curcuminoids in combination
with piperine (SMD: -1.085, 95% CI: -1.357, -0.813, P < 0.0001) vs those administering unformulated
curcumin (SMD: -0.850, 95% CI: -1.158, -0.542, P < 0.0001) revealed a higher MDA-lowering effect in cases
when administration was combined with piperine (Fig. 5).
With respect to the comparison of doses evaluated, five studies [18-21] revealed significant differences (P <
0.0001, I2
= 0.00%) between the experimental group (n = 208) and the placebo group (n = 209) when
ACCEPTED MANUSCRIPT

13. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
12
experimental groups were administered doses of curcuminoids ≥ 600 mg/d. However, pooled data from two
trials [22, 23] did not show a significant difference (P = 0.200, I2
= 0%) between groups taking < 600 mg/d
[experimental group, (n = 55)] and the placebo group (n = 53; Fig. 6).
In regards to the duration of supplementation (duration ≥ 8 weeks or < 8 weeks) and underlying diseases
(T2DM or osteoarthritis); the duration of curcumin supplementation and the underlying diseases did not have a
significant effect on the lowering of circulating MDA concentrations (Table 4).
3.6.2 Subgroup analyses for serum SOD activity
Due to the use of the same form of intervention in the treatment of various underlying diseases in the included
studies, subgroup analyses were conducted regarding the administered dose (1000 mg/d curcumin with 10 mg
of piperine) and the duration of supplementation (durations ≥ 8 weeks or < 8 weeks). However, these factors do
not have a significant effect on the change in serum SOD activity (Table 4).
Because there were a small number of studies, meta-regression analysis was not performed to evaluate the
relationship between serum SOD activity and potential confounders including the duration and dose of
supplementation with curcumin, and the baseline levels. Additionally, as fewer than ten studies were included in
the meta-analysis, the Begg’s test and funnel plots could not be conducted.
3.7Adverse effects of turmeric and curcumin
ACCEPTED MANUSCRIPT

14. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
13
Srivastava et al. [19] reported that two patients experienced adverse effects caused by Curcuma longa L extract.
The adverse effects included dyspepsia and nausea/vomiting in the one patient, respectively. Moreover, four
patients experienced adverse effects in the placebo group. Two patients reported dyspepsia, one patient reported
nausea/vomiting, and another patient reported constipation. Usharani et al. [24] stated that two subjects
complained of mild diarrhea in the experimental group. Selvi et al. [23] and Pakfetrat et al. [22] reported no
adverse events in either group. Two clinical studies [18, 24] demonstrated that curcumin had induced no serious
adverse reactions. Moreover, three studies [20, 21, 25] did not report any study-related adverse reaction.
4. Discussion
The results of the present meta-analysis of randomized controlled trials of 4 weeks or longer indicate that
curcumin can significantly reduce plasma MDA concentrations and increase serum SOD activity, with no
significant change in the GPX activity in RBC. The subgroup analysis suggests that curcuminoids may play a
role in the lowering of MDA at doses ≥ 600mg/d. The natural preparations (turmeric powder) failed to have an
MDA-lowering effect; however, curcuminoids with piperine showed better activity as compared to turmeric
extract (SMD = -1.085, 95% CI: -1.357 to -0.813 vs SMD = -0.850, 95% CI: -1.158 to -0.542). There was a
non-significant association between the MDA-lowering effect of curcumin and the duration of treatment and
underlying diseases independently. Similarly, no significant association was found between the SOD-increasing
effect of curcumin and the duration of treatment or the administered dose.
While reactive species are believed to be harmful, dietary antioxidants are thought to be safe for humans.
However, this is not supported by the current studies. Both reactive species and dietary antioxidants can have
ACCEPTED MANUSCRIPT

15. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
14
positive and negative effects on the body [39, 40]. ROS are indispensable mediators in many crucial cellular
processes, but high concentrations of ROS can hinder the redox homeostasis and induce ROS [39]. For the
dietary antioxidants, it is uncertain whether antioxidant, pro-oxidant, or other biological effects potentially
exerted by flavonoids (a kind of dietary polyphenols) contribute to the health benefits of dietary antioxidants
[40]. Since phytochemicals with low oral absorption may be rapidly metabolized into molecules with altered
chemical structures and different biological properties, the use of total antioxidant activity assays should be
discouraged for the assessment of dietary antioxidants [41]. The direct measurement of specific oxidation
end-product markers, such as F2-isoprostanes, protein carbonyls, but also the ratio of glutathione to
glutathione-disulfide, and the activity and expression of additional relevant enzymes (e.g., glutathione reductase
and others), are possible biomarkers of redox status. However, the randomized controlled trials related to these
parameters are limited. Hence, serum MDA concentrations, serum SOD activity, and GPX activity in RBC were
selected as markers.
There are some plausible explanations for the effects of curcumin. Curcumin may not have direct antioxidant
activity. Curcumin is considered to be an antioxidant owing to its phenolic hydroxyl and the β-diketone groups
in its structure, and the protons provided from these active groups can block free radical reaction [42-45].
Moreover, the degraded products of curcumin have been shown to exhibit higher free radical scavenging ability
than the parent curcumin [46]. However, these studies used experimental and theoretical approaches, which are
different to the physiological conditions found in the body. A direct antioxidant effect of curcumin is highly
unlikely since hydroxyl groups are required for antioxidant activity, and these hydroxyl groups are conjugated
by phase II enzymes. Nearly all circulating curcuminoids are in the form of conjugated metabolites, which do
not have direct antioxidant activity.
ACCEPTED MANUSCRIPT

16. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
15
Curcumin may have indirect antioxidant activity. In normal physiological conditions, nuclear factor
erythroid 2-related factor 2 (Nrf2) is inactive owing to its combination with the cytosolic protein Keap1 [47].
ROS causes activation of Nrf2, and Nrf2 moves into the nucleus where it binds to antioxidant response elements
(ARE) [47]. Curcumin was shown in reporter gene assays to activate Nrf2, a vital component of the
ARE-mediated induction of antioxidant enzymes, such as glutathione-S-transferase (GST), hemeoxygenase-1
(HO-1), NAD(P)H:quinone oxidoreductase 1(NQO1) [47]. Moreover, curcumin can activate the Nrf2 signal
pathway, which increases the activity of SOD and GPX [48]. These actions decrease lipid peroxidation and
alleviate the hepatic damage [49, 50]. Hence, we tentatively propose that the reduced plasma MDA
concentrations and increased SOD activity may be an indirect effect of curcumin. However, the increased SOD
activity may also be a compensatory mechanism for increased generation of ROS.
Piperine may also have an indirect antioxidant effect as an inducer of Nrf2[51], and negligible effects on
curcumin bioavailability [52]. Through the mentioned-above signaling pathway piperine may induce
antioxidant enzyme expression. Hence, both curcumin and piperine may play indirect roles in providing
antioxidant effects. However, it remains unresolved that if piperine alone has an indirect antioxidant effect, or if
it plays this effect only in combination with curcumin, or if this effect was based on improved-bioavailability or
combined action on transcription factors. As the existing data are insufficient to prove the mentioned-above
uncertainties, we are unable to draw definitive conclusions at this time. Furthermore, more large-scale trials are
required to assess the roles of curcumin, piperine, or curcuminoids-piperine in lowering plasma MDA
concentrations and increasing serum SOD activity.
However, the MDA-lowering effect of curcuminoids seems to be dose-dependent. Turmeric powder contains
ACCEPTED MANUSCRIPT

17. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
16
only 3-5% curcumin(oids) [2]. The doses used in some of the included studies were artificially high and
unlikely to be found in natural diets. In our review, we found that turmeric powder failed to have an
MDA-lowering effect. Therefore, the dose of curcumin contained in turmeric powder is too low to produce an
MDA-lowering effect. Some studies have reported that the dose of curcumin and the presence of metal ions in
the medium expressed a dichotomy between its pro-oxidant and antioxidant effect [53-55]. Similarly, it is worth
noting that 300 mg/day supplementation of curcuminoids for three months did not affect plasma MDA
concentrations and GPX activity in diabetic individuals [38]. In our review, we found that 600 mg was the
minimum effective dose used in the included studies. However, due to the limited number of trials available,
further pharmacokinetic research on the adequate dose of curcuminoids would be helpful.
Low oral absorption, rapid metabolism and elimination from the body, along with poor bioavailability are the
major limiting factors for clinical use of curcumin [56]. Almost all ingested curcumin is excreted in the feces as
a mixture of un-metabolized and metabolized curcumin, while only a small portion is absorbed and converted to
its water-soluble metabolites [57]. Multiple studies have reported that, even with high doses of curcumin, the
concentrations of curcumin and its metabolites are eliminated in the serum and tissues after a short period of
time [30, 58]. Several strategies have been designed to overcome these deficits, including liposomal,
phosphatidylcholine, and micellar formulations of curcumin [59].
The majority of the included studies were performed with patients suffering from increased generation of
reactive species, such as metabolic syndrome [21], diabetes mellitus [18, 23, 24], and osteoarthritis [19, 20]
patients. The baseline concentration/activity of these relative markers may be higher in these patients than other
populations. Finally, the outcomes may differ in other populations (such as healthy people, in which the serum
ROS concentrations are lower than those in patients with ROS-related diseases, such as cardiovascular disease,
ACCEPTED MANUSCRIPT

18. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
17
cancer and diabetes [60-62]). Larger scale trials are required for patients with different rates of generation of
reactive species to explore the effects of curcumin in lowering serum MDA concentrations and increasing
serum SOD activity.
In our review, most selected studies used TBARS to analyze MDA concentrations, and only some studies
used chromatographic methods, while others failed to describe the specific measuring method. In our opinion,
the commonly used serum MDA determination methods possess poor reproducibility and high non-specificity
[63, 64]. Furthermore, the batch spectrofluorometric and spectrophotometric thiobarbituric acid-based methods
possess non-specificity because of the abundant chemically reactive carbonyl groups-containing compounds
from various classes of substances [65]. However, the use of high-performance liquid chromatography can
considerably improve the limitation of these assays [66-68], which potentially limits their applicability for
future studies.
Several limitations of the present review need consideration. The limitations of our review include the small
number of studies, the restriction of studies in the English language, and the inclusion of only published clinical
studies. Additionally, the presence of other ingredients in turmeric powder or turmeric extract besides curcumin
may have confounded the findings in this study. Therefore, we have not drawn definitive conclusions and have
merely provided available evidence to support large-scale trials that can further confirm the effect of curcumin
on serum MDA concentrations and SOD activity. The included studies recruited a small number of patients and
thus confirming evidence from further clinical trials is required. Lastly, some data were obtained indirectly, and
these data may have affected the accuracy of the results of our study.
5. Conclusion
ACCEPTED MANUSCRIPT

19. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
18
Our analysis indicates that curcumin decreases circulating MDA concentrations and increase serum SOD
activity; however, the MDA-lowering effect was independent of the duration of treatment and underlying
diseases. The MDA-lowering effect of curcuminoids seems to be dose-dependent; however, the most effective
dose of curcuminoids remains inconclusive. The efficacy of curcuminoids with piperine in reducing
circulating MDA concentrations was higher when compared with the unformulated preparations, but no
significant differences were observed when compared with the natural preparations. In addition, there was no
significant difference observed in the GPX activity in RBC.
Both reactive species and dietary antioxidants have positive and negative effects on the body, and there is no
evidence that changes in redox status would prevent disease. Even if the MDA-lowering effect and
SOD-increasing effect of curcumin were confirmed, there would be little reason to support the use of dietary
antioxidants. Nonetheless, the analysis does provide evidence in support of large-scale trials of curcumin to
investigate the uncertain and underlying mechanisms through which curcumin suppresses serum MDA
concentrations and increases SOD activity.
As adapted from a previous study [69], metabolomics and metabolite profiling can provide detailed and
timely analyses of how curcumin is metabolized by the human body. Future studies employing the
above-mentioned technology would help elucidate the underlying mechanisms of action. Large-scale trials are
required to test this effect with various biomarkers of redox status.
ACCEPTED MANUSCRIPT

20. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
19
Acknowledgments
This work was supported by the Natural Science Foundation of China [No. 81171560], the “Par-Eu Scholars
Program" of Chongqing City and the National Science and Technology Major Project of China [No.
2012ZX10002007001]. The funding bodies had no role in study design, data collection and analysis, decision to
publish, or the preparation of the manuscript. The authors declare that they have no competing interests. The
author contributions are as follows: SQ, HR, and HDH contributed to the conception and design of the study.
SQ, LFH, and HDH conducted the literature search and data extraction. LFH and SQ performed the statistical
analyses. SQ, LFH, JJG, SSS, JH, YT, and HDH drafted the manuscript. HR and HDH supervised the study. All
authors gave final approval of this manuscript.
ACCEPTED MANUSCRIPT

21. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
20
REFERENCES
[1] Goel A, Aggarwal BB. Curcumin, the golden spice from Indian saffron, is a chemosensitizer and
radiosensitizer for tumors and chemoprotector and radioprotector for normal organs. Nutr Cancer
2010;62:919-30.
[2] Goel A, Kunnumakkara AB, Aggarwal BB. Curcumin as "Curecumin": from kitchen to clinic. Biochem
Pharmacol 2008;75:787-809.
[3] Kunnumakkara AB, Bordoloi D, Padmavathi G, Monisha J, Roy NK, Prasad S, et al. Curcumin, the
golden nutraceutical: multitargeting for multiple chronic diseases. Br J Pharmacol 2017;174:1325-48.
[4] Ng QX, Koh SSH, Chan HW, Ho CYX. Clinical use of curcumin in depression: a meta-analysis. J Am
Med Dir Assoc 2017;18:503-8.
[5] Pulido-Moran M, Moreno-Fernandez J, Ramirez-Tortosa C, Ramirez-Tortosa M. Curcumin and health.
Molecules 2016;21:264.
[6] Sies H. Role of reactive oxygen species in biological processes. Klin Wochenschr 1991;69:965-8.
[7] Lobo V, Patil A, Phatak A, Chandra N. Free radicals, antioxidants and functional foods: Impact on
human health. Pharmacogn Rev 2010;4:118-26.
[8] White PA, Oliveira RC, Oliveira AP, Serafini MR, Araujo AA, Gelain DP, et al. Antioxidant activity
and mechanisms of action of natural compounds isolated from lichens: a systematic review. Molecules
2014;19:14496-527.
[9] Mao S, Zhang A, Huang S. Serum levels of malondialdehyde, vitamin C and E in idiopathic nephrotic
syndrome: a meta-analysis. Ren Fail 2014;36:994-9.
ACCEPTED MANUSCRIPT

22. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
21
[10] Marrocco I, Altieri F, Peluso I. Measurement and clinical significance of biomarkers of oxidative stress
in humans. Oxid Med Cell Longev 2017;2017:6501046.
[11] Wang XN, Zhang CJ, Diao HL, Zhang Y. Protective Effects of curcumin against sodium
arsenite-induced ovarian oxidative injury in a mouse model. Chin Med J (Engl) 2017;130:1026-32.
[12] Mise Yonar S, Yonar ME, Ural MS. Antioxidant effect of curcumin against exposure to malathion in
Cyprinus carpio. Cell Mol Biol (Noisy-le-grand) 2017;63:68-72.
[13] Xie YL, Chu JG, Jian XM, Dong JZ, Wang LP, Li GX, et al. Curcumin attenuates
lipopolysaccharide/d-galactosamine-induced acute liver injury by activating Nrf2 nuclear translocation
and inhibiting NF-kB activation. Biomed Pharmacother 2017;91:70-7.
[14] Akar I, Ince I, Arici A, Benli I, Aslan C, Senol S, et al. The protective effect of curcumin on a spinal
cord ischemia-reperfusion injury model. Ann Vasc Surg 2017;42:285-92.
[15] Liu YF, Yang CW, Liu H, Sui SG, Li XD. Efficacy and therapeutic potential of curcumin against
sepsis-induced chronic lung injury in male albino rats. J Nutr Health Aging 2017;21:307-13.
[16] Samarghandian S, Azimi-Nezhad M, Farkhondeh T, Samini F. Anti-oxidative effects of curcumin on
immobilization-induced oxidative stress in rat brain, liver and kidney. Biomed Pharmacother
2017;87:223-9.
[17] Yonar ME. Chlorpyrifos-induced biochemical changes in Cyprinus carpio: Ameliorative effect of
curcumin. Ecotoxicol Environ Saf 2018;151:49-54.
[18] Panahi Y, Khalili N, Sahebi E, Namazi S, Karimian MS, Majeed M, et al. Antioxidant effects of
curcuminoids in patients with type 2 diabetes mellitus: a randomized controlled trial.
Inflammopharmacology 2017;25:25-31.
ACCEPTED MANUSCRIPT

23. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
22
[19] Srivastava S, Saksena AK, Khattri S, Kumar S, Dagur RS. Curcuma longa extract reduces inflammatory
and oxidative stress biomarkers in osteoarthritis of knee: a four-month, double-blind, randomized,
placebo-controlled trial. Inflammopharmacology 2016;24:377-88.
[20] Panahi Y, Alishiri GH, Parvin S, Sahebkar A. Mitigation of systemic oxidative stress by curcuminoids
in osteoarthritis: results of a randomized controlled trial. J Diet Suppl 2016;13:209-20.
[21] Panahi Y, Hosseini MS, Khalili N, Naimi E, Majeed M, Sahebkar A. Antioxidant and anti-inflammatory
effects of curcuminoid-piperine combination in subjects with metabolic syndrome: A randomized
controlled trial and an updated meta-analysis. Clin Nutr 2015;34:1101-8.
[22] Pakfetrat M, Akmali M, Malekmakan L, Dabaghimanesh M, Khorsand M. Role of turmeric in oxidative
modulation in end-stage renal disease patients. Hemodial Int 2015;19:124-31.
[23] Maithili Karpaga Selvi N, Sridhar MG, Swaminathan RP, Sripradha R. Efficacy of turmeric as adjuvant
therapy in type 2 diabetic patients. Indian J Clin Biochem 2015;30:180-6.
[24] Usharani P, Mateen AA, Naidu MU, Raju YS, Chandra N. Effect of NCB-02, atorvastatin and placebo
on endothelial function, oxidative stress and inflammatory markers in patients with type 2 diabetes
mellitus: a randomized, parallel-group, placebo-controlled, 8-week study. Drugs R D 2008;9:243-50.
[25] Panahi Y, Sahebkar A, Amiri M, Davoudi SM, Beiraghdar F, Hoseininejad SL, et al. Improvement of
sulphur mustard-induced chronic pruritus, quality of life and antioxidant status by curcumin: results of a
randomised, double-blind, placebo-controlled trial. Br J Nutr 2012;108:1272-9.
[26] Qin S, Huang L, Gong J, Shen S, Huang J, Ren H, et al. Efficacy and safety of turmeric and curcumin in
lowering blood lipid levels in patients with cardiovascular risk factors: a meta-analysis of randomized
controlled trials. Nutr J 2017;16:68.
ACCEPTED MANUSCRIPT

24. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
23
[27] Halbert JA, Silagy CA, Finucane P, Withers RT, Hamdorf PA, Andrews GR. The effectiveness of
exercise training in lowering blood pressure: a meta-analysis of randomised controlled trials of 4 weeks
or longer. J Hum Hypertens 1997;11:641-9.
[28] Moher D, Liberati A, Tetzlaff J, Altman DG, Group P. Preferred reporting items for systematic reviews
and meta-analyses: the PRISMA statement. PLoS Med 2009;6:e1000097.
[29] Higgins JP, Altman DG, Gotzsche PC, Juni P, Moher D, Oxman AD, et al. The Cochrane
Collaboration's tool for assessing risk of bias in randomised trials. BMJ 2011;343:d5928.
[30] Sharma RA, McLelland HR, Hill KA, Ireson CR, Euden SA, Manson MM, et al. Pharmacodynamic and
pharmacokinetic study of oral Curcuma extract in patients with colorectal cancer. Clin Cancer Res
2001;7:1894-900.
[31] Lepage G, Munoz G, Champagne J, Roy CC. Preparative steps necessary for the accurate measurement
of malondialdehyde by high-performance liquid chromatography. Anal Biochem 1991;197:277-83.
[32] Wasowicz W, Neve J, Peretz A. Optimized steps in fluorometric determination of thiobarbituric
acid-reactive substances in serum: importance of extraction pH and influence of sample preservation and
storage. Clin Chem 1993;39:2522-6.
[33] Durgaprasad S, Pai CG, Vasanthkumar, Alvres JF, Namitha S. A pilot study of the antioxidant effect of
curcumin in tropical pancreatitis. Indian J Med Res 2005;122:315-8.
[34] Yang H, Xu W, Zhou Z, Liu J, Li X, Chen L, et al. Curcumin attenuates urinary excretion of albumin in
type II diabetic patients with enhancing nuclear factor erythroid-derived 2-like 2 (Nrf2) system and
repressing inflammatory signaling efficacies. Exp Clin Endocrinol Diabetes 2015;123:360-7.
[35] Takahashi M, Suzuki K, Kim HK, Otsuka Y, Imaizumi A, Miyashita M, et al. Effects of curcumin
ACCEPTED MANUSCRIPT

25. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
24
supplementation on exercise-induced oxidative stress in humans. Int J Sports Med 2014;35:469-75.
[36] Wongcharoen W, Jai-Aue S, Phrommintikul A, Nawarawong W, Woragidpoonpol S, Tepsuwan T, et al.
Effects of curcuminoids on frequency of acute myocardial infarction after coronary artery bypass
grafting. Am J Cardiol 2012;110:40-4.
[37] Panahi Y, Ghanei M, Hajhashemi A, Sahebkar A. Effects of curcuminoids-piperine combination on
systemic oxidative stress, clinical symptoms and quality of life in subjects with chronic pulmonary
complications due to sulfur mustard: a randomized controlled trial. J Diet Suppl 2016;13:93-105.
[38] Na LX, Yan BL, Jiang S, Cui HL, Li Y, Sun CH. Curcuminoids target decreasing serum adipocyte-fatty
acid binding protein levels in their glucose-lowering effect in patients with type 2 diabetes. Biomed
Environ Sci 2014;27:902-6.
[39] Zuo L, Zhou T, Pannell BK, Ziegler AC, Best TM. Biological and physiological role of reactive oxygen
species--the good, the bad and the ugly. Acta Physiol (Oxf) 2015;214:329-48.
[40] Halliwell B. Dietary polyphenols: good, bad, or indifferent for your health? Cardiovasc Res
2007;73:341-7.
[41] Pompella A, Sies H, Wacker R, Brouns F, Grune T, Biesalski HK, et al. The use of total antioxidant
capacity as surrogate marker for food quality and its effect on health is to be discouraged. Nutrition
2014;30:791-3.
[42] Barclay LR, Vinqvist MR, Mukai K, Goto H, Hashimoto Y, Tokunaga A, et al. On the antioxidant
mechanism of curcumin: classical methods are needed to determine antioxidant mechanism and activity.
Org Lett 2000;2:2841-3.
[43] Priyadarsini KI, Maity DK, Naik GH, Kumar MS, Unnikrishnan MK, Satav JG, et al. Role of phenolic
ACCEPTED MANUSCRIPT

26. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
25
O-H and methylene hydrogen on the free radical reactions and antioxidant activity of curcumin. Free
Radic Biol Med 2003;35:475-84.
[44] Sun YM, Zhang HY, Chen DZ, Liu CB. Theoretical elucidation on the antioxidant mechanism of
curcumin: a DFT study. Org Lett 2002;4:2909-11.
[45] Miriyala S, Panchatcharam M, Rengarajulu P. Cardioprotective effects of curcumin. Adv Exp Med Biol
2007;595:359-77.
[46] Shen L, Liu CC, An CY, Ji HF. How does curcumin work with poor bioavailability? Clues from
experimental and theoretical studies. Sci Rep 2016;6:20872.
[47] Strimpakos AS, Sharma RA. Curcumin: preventive and therapeutic properties in laboratory studies and
clinical trials. Antioxid Redox Signal 2008;10:511-45.
[48] Carmona-Ramirez I, Santamaria A, Tobon-Velasco JC, Orozco-Ibarra M, Gonzalez-Herrera IG,
Pedraza-Chaverri J, et al. RETRACTED: Curcumin restores Nrf2 levels and prevents quinolinic
acid-induced neurotoxicity. J Nutr Biochem 2013;24:14-24.
[49] Reddy AC, Lokesh BR. Effect of curcumin and eugenol on iron-induced hepatic toxicity in rats.
Toxicology 1996;107:39-45.
[50] Rukkumani R, Aruna K, Varma PS, Menon VP. Curcumin influences hepatic expression patterns of
matrix metalloproteinases in liver toxicity. Ital J Biochem 2004;53:61-6.
[51] Choi BM, Kim SM, Park TK, Li G, Hong SJ, Park R, et al. Piperine protects cisplatin-induced apoptosis
via heme oxygenase-1 induction in auditory cells. J Nutr Biochem 2007;18:615-22.
[52] Shoba G, Joy D, Joseph T, Majeed M, Rajendran R, Srinivas PS. Influence of piperine on the
pharmacokinetics of curcumin in animals and human volunteers. Planta Med 1998;64:353-6.
ACCEPTED MANUSCRIPT

27. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
26
[53] Joe B, Vijaykumar M, Lokesh BR. Biological properties of curcumin-cellular and molecular
mechanisms of action. Crit Rev Food Sci Nutr 2004;44:97-111.
[54] Ahsan H, Hadi SM. Strand scission in DNA induced by curcumin in the presence of Cu(II). Cancer Lett
1998;124:23-30.
[55] Ahsan H, Parveen N, Khan NU, Hadi SM. Pro-oxidant, anti-oxidant and cleavage activities on DNA of
curcumin and its derivatives demethoxycurcumin and bisdemethoxycurcumin. Chem Biol Interact
1999;121:161-75.
[56] Panahi Y, Hosseini MS, Khalili N, Naimi E, Simental-Mendia LE, Majeed M, et al. Effects of curcumin
on serum cytokine concentrations in subjects with metabolic syndrome: A post-hoc analysis of a
randomized controlled trial. Biomed Pharmacother 2016;82:578-82.
[57] Anand P, Kunnumakkara AB, Newman RA, Aggarwal BB. Bioavailability of curcumin: problems and
promises. Mol Pharm 2007;4:807-18.
[58] Lao CD, Ruffin MTt, Normolle D, Heath DD, Murray SI, Bailey JM, et al. Dose escalation of a
curcuminoid formulation. BMC Complement Altern Med 2006;6:10.
[59] Schiborr C, Kocher A, Behnam D, Jandasek J, Toelstede S, Frank J. The oral bioavailability of curcumin
from micronized powder and liquid micelles is significantly increased in healthy humans and differs
between sexes. Mol Nutr Food Res 2014;58:516-27.
[60] Mantovani G, Maccio A, Madeddu C, Mura L, Gramignano G, Lusso MR, et al. The impact of different
antioxidant agents alone or in combination on reactive oxygen species, antioxidant enzymes and
cytokines in a series of advanced cancer patients at different sites: correlation with disease progression.
Free Radic Res 2003;37:213-23.
ACCEPTED MANUSCRIPT

28. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
27
[61] Salardi S, Zucchini S, Elleri D, Grossi G, Bargossi AM, Gualandi S, et al. High glucose levels induce an
increase in membrane antioxidants, in terms of vitamin E and coenzyme Q10, in children and
adolescents with type 1 diabetes. Diabetes Care 2004;27:630-1.
[62] Vassalle C, Boni C, Di Cecco P, Landi P. Elevated hydroperoxide levels as a prognostic predictor of
mortality in a cohort of patients with cardiovascular disease. Int J Cardiol 2006;110:415-6.
[63] Hackett C, Linley-Adams M, Lloyd B, Walker V. Plasma malondialdehyde: a poor measure of in vivo
lipid peroxidation. Clin Chem 1988;34:208.
[64] Schoenmakers AW TB. Reliability of the thiobarbituric acid test in the presence of inorganic iron.
Nature 1966;210:1153.
[65] Tsikas D. Assessment of lipid peroxidation by measuring malondialdehyde (MDA) and relatives in
biological samples: Analytical and biological challenges. Anal Biochem 2017;524:13-30.
[66] Lykkesfeldt J. Determination of malondialdehyde as dithiobarbituric acid adduct in biological samples
by HPLC with fluorescence detection: comparison with ultraviolet-visible spectrophotometry. Clin
Chem 2001;47:1725-7.
[67] Bird RP, Hung SS, Hadley M, Draper HH. Determination of malonaldehyde in biological materials by
high-pressure liquid chromatography. Anal Biochem 1983;128:240-4.
[68] Wade CR, Jackson PG, Van Rij AM. Quantitation of malondialdehyde (MDA) in plasma, by ion-pairing
reverse phase high performance liquid chromatography. Biochem Med 1985;33:291-6.
[69] Zhang B, Zhang H, Du C, Ng QX, Hu C, He Y, et al. Metabolic responses of the growing Daphnia
similis to chronic AgNPs exposure as revealed by GC-Q-TOF/MS and LC-Q-TOF/MS. Water Res
2017;114:135-43.
ACCEPTED MANUSCRIPT

29. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
28
Figure Legends
Fig. 1 Flow diagram of the literature selection process.
Fig. 2 Forest plot of the meta-analysis for comparison of serum malondialdehyde concentrations between
experimental and control groups.
Fig. 3 Forest plot of the meta-analysis for comparison of serum superoxide dismutase activity between
experimental and control groups.
Fig. 4 Forest plot of the meta-analysis for comparison of glutathione peroxidase activity in RBC between
experimental and control groups.
Fig. 5 Forest plot displaying subgroup analyses for the impact of dosage form on the
malondialdehyde-lowering activity.
Fig. 6 Forest plot displaying subgroup analyses for the impact of treatment dose on the
malondialdehyde-lowering activity.
ACCEPTED MANUSCRIPT

30. ACCEPTED
M
ANUSCRIPT
29
Table 1 Characteristics of the studies included in the meta-analysis.
RCT: randomized controlled trial; NA: not available.
Values are expressed as mean ± SD
Study Region Type Duration Target
Population
Intervention Turmeric
preparation
N Age Male
Panahi,
2017
Iran RCT
double-blind
8 weeks Type 2 diabetic
patients
1000 mg/d curcumin (with 10 mg of piperine) Curcuminoids 50 43 ± 8 25
Placebo 50 41 ± 7 26
Srivastava,
2016
India RCT
double-blind
4 months Osteoarthritis 950 -1000mg/d curcumin +Diclofenac Curcuminoids 66 50.23 ± 8.08 25
Placebo + Diclofenac 67 50.27 ± 8.63 32
Panahi,
2016
Iran RCT
double-blind
6 weeks Osteoarthritis 1500 mg/d curcumin (with 15 mg of piperine) Curcuminoids 19 57.32 ± 8.78 5
Placebo 21 57.57 ± 9.05 4
Panahi,
2015
Iran RCT
double-blind
8 weeks Metabolic
syndrome
1000 mg/d curcumin (with 10 mg of piperine) Curcuminoids 50 44.80 ± 8.67 27
Placebo 50 43.46 ± 9.70 23
Selvi,
2015
India RCT 4 weeks Type 2 diabetic
patients
46 mg/d curcumin + Metformin Turmeric
powder
30 47.00 ± 7.17 30
Metformin 30 46.80 ± 6.10 30
Pakfetrat,
2015
Iran RCT
double-blind
8 weeks End-stage renal
disease
66.3 mg/d curcumin Turmeric
powder
25 46.80 ± 14.5 15
Placebo 23 52.3 ± 14.7 11
Panahi,
2012
Iran RCT
double-blind
4 weeks Chronic pruritic
skin lesions
1000 mg/d curcumin (with 10 mg of piperine) Curcuminoids 46 37–59 years 96
Placebo 55
Usharani,
2008
India RCT 8 weeks Type 2 diabetic
patients
600 mg/day curcumin Curcuminoids 23 55.52 ± 10.76 12
Placebo 21 49.75 ± 8.18 11
ACCEPTED MANUSCRIPT

31. ACCEPTED
M
ANUSCRIPT
30
Table 2 Oxidative stress parameters in the studies before and after the intervention
Study
Outcome Unit
Experimental Control
Before After Before After
Panahi, 2017 MDA nmol/ ml 3.9046 ± 1.06 3.0490 ± 0.91 5.03 ± 0.16 3.69 ± 0.12
SOD U/mL 3.4610 ± 0.99 3.8632 ± 0.76 3.3872 ± 1.01 2.9588 ± 0.78
RBC GPX NA NA NA NA NA
Srivastava, 2016 MDA nmol/ ml 5.03 ± 0.16 3.69 ± 0.12 5.15 ± 0.14 4.91 ± 0.11
SOD NA NA NA NA NA
RBC GPX NA NA NA NA NA
Panahi, 2016 MDA nmol/ ml 23.04 ± 2.30 NA 23.03 ± 2.32 NA
SOD U/mL 4.03 ± 1.36 NA 4.17 ± 1.39 NA
RBC GPX NA NA NA NA NA
Panahi, 2015 MDA nmol/ ml 19.38 ± 3.08 15.62 ± 2.59 19.56 ± 2.73 20.16 ± 3.11
SOD U/mL 54.30 ± 14.02 60.95 ± 15.68 60.35 ± 15.96 55.00 ± 11.09
RBC GPX NA NA NA NA NA
Selvi, 2015 MDA nmol/ ml 0.67 ± 0.16 0.51 ± 0.11 0.63 ± 0.3 0.53 ± 0.2
SOD NA NA NA NA NA
RBC GPX U/g Hb 13 ± 6.0 14 ± 8.4 17.3 ± 12 15 ± 5
Pakfetrat, 2015 MDA nmol/ ml 7.5 ± 2.5 6.0 ± 2.4 8.6 ± 1.4 7.5 ± 1.1
SOD NA NA NA NA NA
RBC GPX units/g Hb 28.6 ± 22.1 56.2 ± 28.2 25.6 ± 26.1 58.2 ± 46.3
Panahi, 2012 MDA NA NA NA NA NA
SOD mKat/l 57.68 ± 12.67 65.85 ± 13.84 58.01 ± 12.17 57.01 ± 8.84
RBC GPX NA NA NA NA NA
Usharani, 2008 MDA nmol/ ml 4.09 ± 0.70 2.48 ± 0.30 3.88 ± 1.51 4.04 ± 1.19
SOD NA NA NA NA NA
RBC GPX NA NA NA NA NA
ACCEPTED MANUSCRIPT

32. ACCEPTED
M
ANUSCRIPT
31
Data are shown as mean ± SD;
MDA: malondialdehyde; SOD: superoxide dismutase; RBC GPX = red blood cell glutathione peroxidase activity; NA: not available.
ACCEPTED MANUSCRIPT

33. ACCEPTED
M
ANUSCRIPT
32
Table 3 Quality of studies assessed by the Cochrane guidelines
H: high risk of bias; L: low risk of bias; U: unclear or unrevealed risk of bias.
Study
Random sequence Allocation
Blinding
Incomplete Selective Free of other bias
generation concealment outcome data reporting
Panahi, 2017 U U L L L H
Srivastava, 2016 L L L L U U
Panahi, 2016 L U L L U U
Panahi, 2015 U U L L L U
Selvi, 2015 L L H L U U
Pakfetrat, 2015 U L L L L U
Panahi, 2012 U L L L L U
Usharani, 2008 U U H L L U
ACCEPTED MANUSCRIPT

34. ACCEPTED
M
ANUSCRIPT
33
Table 4 Subgroup analysis for the impact of treatment duration and underlying diseases on the oxidative stress biomarkers
Outcome
of interest
No. of No. of Effects SMD 95% CI Heterogeneity P-value
study patients model
P value I2
(%)
arms
MDA Treatment duration Durations ≥ 8 weeks 5 425 Random −0.87 −1.19 to −0.55 0.048 58.40 < 0.0001***
Durations < 8 weeks 2 100 Fixed −0.44 −0.84 to −0.05 0.324 0.000 0.029*
Type of disease T2DM 3 204 Random −0.71 −1.20 to −0.23 0.064 63.70 0.004*
Osteoarthritis 2 173 Fixed −0.84 −1.15 to −0.53 0.606 0.000 < 0.0001***
SOD Treatment duration Durations ≥ 8 weeks 2 200 Random 1.30 0.21 to 2.39 0.000 91.900 0.020*
Durations < 8 weeks 2 136 Random 0.83 0.21 to 1.45 0.112 60.500 0.009*
Administered dose 1000 mg/d curcumin with
10 mg of piperine
3 296 Random 1.05 0.30 to 1.80 0.000 89.30 0.006*
MDA: malondialdehyde; SOD: superoxide dismutase; T2DM: type 2 diabetes mellitus
*p < 0.05
**p < 0.001
***p < 0.0001
ACCEPTED MANUSCRIPT

35. A
C
C
E
P
T
E
D
M
A
N
U
S
C
R
I
P
T
34
Highlights
1.Eight randomized, controlled trials with 626 patients were reviewed.
2.Curcumin is the active component of turmeric (an Indian spice).
3.Malondialdehyde, SOD and glutathione peroxidase are oxidative stress biomarkers.
4.Curcumin can reduce serum malondialdehyde levels and increase SOD levels.
5.The MDA-lowering effect of curcumin was associated with dosage and dosage form.
ACCEPTED MANUSCRIPT

36. Figure 1

37. Figure 2

38. Figure 3

39. Figure 4

40. Figure 5

41. Figure 6