Zinc levels in seminal plasma and their correlation with male infertility: A systematic review and meta-analysis

Abstract

Zinc is an essential trace mineral for the normal functioning of the male reproductive system. Current studies have investigated the relationship between seminal plasma zinc and male infertility but have shown inconsistent results. Hence, we systematically searched PubMed, EMBASE, Science Direct/Elsevier, CNKI and the Cochrane Library for studies that examined the relationship between seminal plasma zinc and male infertility, as well as the effects of zinc supplementation on sperm parameters. Twenty studies were identified, including 2,600 cases and 867 controls. Our meta-analysis results indicated that the seminal plasma zinc concentrations from infertile males were significantly lower than those from normal controls (SMD (standard mean differences) [95% CI] −0.64 [−1.01, −0.28]). Zinc supplementation was found to significantly increase the semen volume, sperm motility and the percentage of normal sperm morphology (SMD [95% CI]: −0.99 [−1.60, −0.38], −1.82 [−2.63, −1.01], and −0.75 [−1.37, −0.14], respectively). The present study showed that the zinc level in the seminal plasma of infertile males was significantly lower than that of normal males. Zinc supplementation could significantly increase the sperm quality of infertile males. However, further studies are needed to better elucidate the correlation between seminal plasma zinc and male infertility.

Full text

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Scientific RepoRts | 6:22386 | DOI: 10.1038/srep22386
www.nature.com/scientificreports
Zinc levels in seminal plasma
and their correlation with male
infertility: A systematic review and
meta-analysis
Jiang Zhao, Xingyou Dong, Xiaoyan Hu, Zhou Long, Liang Wang, Qian Liu, Bishao Sun,
Qingqing Wang, Qingjian Wu & Longkun Li
Zinc is an essential trace mineral for the normal functioning of the male reproductive system. Current
studies have investigated the relationship between seminal plasma zinc and male infertility but have
shown inconsistent results. Hence, we systematically searched PubMed, EMBASE, Science Direct/
Elsevier, CNKI and the Cochrane Library for studies that examined the relationship between seminal
plasma zinc and male infertility, as well as the eects of zinc supplementation on sperm parameters.
Twenty studies were identied, including 2,600 cases and 867 controls. Our meta-analysis results
indicated that the seminal plasma zinc concentrations from infertile males were signicantly lower
than those from normal controls (SMD (standard mean dierences) [95% CI] −0.64 [−1.01, −0.28]).
Zinc supplementation was found to signicantly increase the semen volume, sperm motility and the
percentage of normal sperm morphology (SMD [95% CI]: −0.99 [−1.60, −0.38], −1.82 [−2.63, −1.01],
and −0.75 [−1.37, −0.14], respectively). The present study showed that the zinc level in the seminal
plasma of infertile males was signicantly lower than that of normal males. Zinc supplementation could
signicantly increase the sperm quality of infertile males. However, further studies are needed to better
elucidate the correlation between seminal plasma zinc and male infertility.
Infertility is dened as the lack of ability to conceive within one year of unprotected intercourse with the same
partner1. It is estimated that nearly 8–12% of couples are infertile2, and approximately 30–40% of infertility cases
are caused by male factors3. Several risk factors are involved in the pathogenesis of infertility, some of which
include alterations in spermatogenesis due to testicular cancer, aplasia of the germinal cells, varicocele, defects
in the transport of sperm, or environmental factors as well as congenital anomalies, infectious diseases, bilateral
spermaducts, pregnancy-related infections, alterations in the characteristics of semen such as a decrease in sperm
motility and sperm count, the presence of antisperm antibodies (ASAs), and nutritional deciency of trace ele-
ments such as selenium and zinc (Zn)4–9.
Trace elements play an important role in the male reproductive process because of their high activity at the
molecular level, although they are known to exist in the body at very low levels. Zn is second only to iron as the
most abundant element in human tissues. Although Zn is found in most types of foods such as red meat, white
meat, sh, and milk, the World Health Organization (WHO) estimates that one-third of the world’s population is
decient in zinc. Zinc and citrate are excreted from the prostate gland as a low-molecular-weight complex; thus,
it is estimated that the zinc levels in seminal plasma typically represent prostatic secretory function. Aer ejacula-
tion, half of the quantity of this complex is redistributed and linked to medium- and high-molecular-weight com-
pounds generated from the seminal vesicles9. e decrease in the seminal plasma zinc concentration may result
in inadequate intake, reduced absorption, increased losses, or increased demand. Additionally, the commonest
worldwide cause is inadequate intake as a result of a diet low in Zn or rich in phytate. Additionally, increased
urinary losses can occur under conditions associated with muscle catabolism, such as sepsis, or iatrogenically
from the prolonged use of drugs10,11. Furthermore, some studies have reported that a sharp decrease in zinc in the
prostatic uid must result in a decreased zinc concentration in seminal plasma11–13.
Department of Urology, Second Aliated Hospital, Third Military Medical University, Chongqing, 400037, China.
Correspondence and requests for materials should be addressed to L.L. (email: lilongk@hotmail.com)
received: 14 October 2015
accepted: 10 February 2016
Published: 02 March 2016
OPEN

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During reproduction, zinc has numerous important functions, and it is essential for conception, implantation,
and a favorable pregnancy outcome10,14. Zinc is present in high concentrations in the seminal uid, and it could
play a multifaceted role in sperm functional properties. It inuences the uidity of lipids and, thus, the stability of
biological membranes15. It aects the stability of sperm chromatin16. It is involved in the formation of free oxygen
radicals17, and it could play a regulatory role in the process of capacitation and the acrosome reaction18. However,
little is known concerning the role of zinc in seminal plasma or serum regarding the global functional compe-
tence of human spermatozoa, such as the sperm’s ability to penetrate cervical mucus (CM) or its fertilizing capac-
ity. e relationship of zinc to the routinely determined variables of semen quality has been controversial19–21.
Current studies have investigated the correlations between seminal plasma zinc concentrations and male
infertility but have shown contradictory results, with some studies showing that the seminal plasma zinc con-
centrations of infertile men were signicantly lower than those of normal controls; however, other studies did
not report this outcome22–41. Moreover, some studies have reported that zinc supplementation in the treatment
of infertility could signicantly increase the sperm quality of infertile males, while other studies have shown
opposing results25,26,28,30,31,37. erefore, we systematically reviewed the available literature and performed a
meta-analysis to evaluate the correlations between seminal plasma zinc concentrations and male infertility and
the eects of zinc supplementation on sperm parameters to possibly provide valuable insights into the diagnosis
and treatment of male infertility.
Results
Characteristics of the included studies. Figure1 shows the detailed review process. In total, 1,320 undu-
plicated studies were identied, and twenty studies were ultimately selected according to the eligibility criteria.
Aer group discussion, all of the reviewers were in agreement to include all twenty papers. Table1 summarizes
the general data from the eight studies. e retrieved studies involved 2,600 infertile males and 867 normal con-
trols. e age ranges of the patient and control groups were 29.2–49.3 years and 30.9–36.6 years, respectively. e
mean ages of the patient and control groups were unavailable for thirteen studies23,24,26,27,29,32,34,36–41. All of these
studies reported exclusion/inclusion criteria22–32,34–41. irteen of 20 studies included the abstinence time before
semen collection23,25,30,32–41. Of the seventeen studies22–25,28,30–32,34–37,39–41 that studied the correlation of seminal
plasma zinc concentrations with male infertility (Table2), 1,893 infertile males and 792 normal controls were
included. Six studies26,27,29,31,32,38 studied the eects of zinc supplementation on sperm parameters and included
563 infertile males.
Meta-analysis
Seminal plasma zinc concentration between normal and infertile males. e test of heterogeneity
suggested a random-eects model, and the meta-analysis revealed that the seminal plasma zinc concentrations
from infertile patients were signicantly lower than those from normal controls (SMD [95% CI]: − 0.64 [− 1.01,
− 0.28]) (Fig.2). Because one study detected the zinc concentration by XFR (radionuclide-induced energy dis-
persive X-ray uorescence), and two studies detected the zinc concentration by chemical chromatometry testing,
we also conducted a sub-analysis that excluded XFR and chemical chromatometry testing; however, the nal
conclusion was not changed (the seminal zinc concentrations from the infertile patients were signicantly lower
than those from the normal controls) (Fig.3).
Eect of zinc supplementation on sperm parameters. For the curative eects of zinc supplementa-
tion in the treatment of male infertility, the meta-analysis revealed that zinc supplementation can signicantly
Figure 1. Flow diagram of the selection of eligible studies.

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increase the percentage of normal sperm morphology, sperm motility and semen volume (SMD [95% CI]: − 0.75
[− 1.37, − 0.14], − 1.82 [− 2.63, − 1.01], and − 0.99 [− 1.60, − 0.38], respectively) (Figs4–6). However, there were
no signicant eects of zinc supplementation on the sperm viability, sperm concentration, sperm count or per-
centage of abnormal sperm morphology (Figs4 and 7–9).
Publication bias of the included studies. Begg’s funnel plot showed no substantial asymmetry (Fig.10).
Egger’s regression test of publication bias of the seminal plasma zinc in infertile and normal males indicated little
evidence of publication bias (t = − 0.19 P = 0.85 > 0.05) (Table3).
Sensitivity analysis of the meta-analysis. We omitted one study sequentially, and the calculated com-
bined SMD for the remaining studies yielded consistent results. In the overall meta-analysis, no single study sig-
nicantly changed the combined results, indicating that the results were statistically stable and reliable (Fig.11).
Discussion
In our study, seventeen articles studied the correlation between seminal plasma zinc concentrations and male
infertility. Nine studies reported that the zinc concentrations in seminal plasma from infertile men were signif-
icantly lower than those from normal men24,30,32,34,35,37–40; one study reported that the zinc concentration in the
seminal plasma from infertile men was signicantly higher than that in normal men28, and the other seven studies
showed no signicant dierence between infertile and normal males22–24,31,33,36,41. In this meta-analysis, the zinc
Study Country Mean age (cases/controls) Cases Zinc supplementation Abstinence Assay Semen parameters
Türk S 2014 Estonia 31/31 32 NI NI
Fuse H 1999 Japan NI 14 5 AAS
Colagar AH 2009 Iran NI 15 NI AAS
Camejo MI 2011 Venezuela 33.6 ± 9.6/34.3 ± 6.4 67 3–5 XFR
Hadwan MH 2012 Iran NI 37 zinc sulfate 220 mg NI AAS SV, STC, SM, SNM
Hadwan MH 2014 Iran NI 60 zinc sulfate 220 mg NI AAS SV, STC, SM, SNM
Akinloye O 2010 Nigeria 35 ± 1.2/36.6 ± 1.0 30 NI AAS
Haidar M 2013 Iran NI 18 zinc sulfate 220 mg NI AAS SV, SC, SM, SNM, SPV
Chia SE 2000 Singapore 34.8 ± 5.3/34.2 ± 4.3 107 3 AAS
Wong WY 2002 South Africa 34.1 ± 4.1/35.3 ± 4.4 107 zinc sulfate 66 mg NI NI SV, SC, SM, SNM, SPA
Li Y 2013 China NI 58 NI 3–7 CCT SM, SPV
Li FB 2008 China 49.3 ± 2.4/32.6 ± 2.9 63 3–7 AAS
Liao CS 2011 China NI 28 5 CCT
Shi KH 2014 China 29.2 ± 2.9/30.9 ± 3.1 154 5 AAS
Wang R 2006 China NI 119 2–4 AAS
Xu X 1997 China NI 17 3–5 AAS
Zhang DT 2003 China NI 876 zinc gluconate 10 ml 7AAS SV, SPV,STC,SPA
Zheng LP 2012 China NI 265 3–7 AAS
Li P 2012 China NI 500 3–7 AAS
He Y 2011 China NI 33 5–7 AAS
Table 1. Characteristics of the included studies investigating seminal plasma zinc concentrations and male
infertility. Abbreviations: SV, semen volume; SC, sperm concentration (density); SPV, sperm viability; SNM,
sperm normal morphology; SPA, sperm abnormal morphology; SM, sperm motility; STC, sperm count; AAS,
atomic absorption spectrophotometry; XRF: radionuclide-induced energy dispersive X-ray uorescence test;
CCT, chemical chromatometry test. NI, not indicated in the study.
Zn dose
Sperm
concentration
Semen
volume
Sperm
viability
Sperm normal
morphology
Sperm
abnormal
morphology
Sperm
count
Sperm
motility
Hadwan MH 2012 zinc sulfate 220 mg + + − +
Hadwan MH 2014 zinc sulfate 220 mg + + + +
Haidar M 2013 zinc sulfate 220 mg − − + + +
Wong WY 2002 zinc sulfate 66 mg + − − − +
Li Y 2013 NI + +
Zhang DT 2003 zinc gluconate 10 ml + + + +
Table 2. Zn dosages and sperm parameters of the included studies. NI, not indicated in the study; + , a
signicant dierence between before and aer zinc supplementation; − , no signicant dierence between
before and aer zinc supplementation.

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concentrations in seminal plasma from infertile males were signicantly lower than those in normal males. Six
of twenty articles studied the curative eects of zinc supplementation in the treatment of male infertility and its
eect on sperm parameters. Our results revealed that zinc supplementation could signicantly increase the sperm
volume, sperm motility and percentage of normal sperm morphology of infertile men. Aer zinc supplementa-
tion, the sperm quality of infertile men was signicantly increased.
e concentration of zinc in human seminal plasma is higher than that in other tissues42. In fact, during the
early stages of sperm development, spermatogenic cells reside within the seminiferous tubules, which have a Zn
content similar to or lower than that of other organs, such as the liver or kidneys. Subsequently, sperm encoun-
ter, in succession, the epididymis, vas deferens and seminal vesicles, which are characterized by a progressively
increased tissue Zn content43,44. Finally, spermatozoa are ejaculated into seminal plasma, which is essentially
formed by prostate secretions in which Zn is nearly 100 times more concentrated than in blood serum. Foresta C
also suggested that, along the entire genital tract, there is a prevalent expression of Zn transporters that supply Zn.
Additionally, from the germ cells to mature sperm, there is an overall uptake of Zn, and, before ejaculation, the
prostate secretions concur to stabilize sperm10. All of these mechanisms are a prerequisite for mature sperm to be
able to undergo capacitation, motility hyperactivation and the acrosome reaction when the Zn levels fall during
their transit in the female genital tract.
In the human reproductive system, Zn plays an important role in spermatogenesis, from its forma-
tion and contribution to the ultrastructural stabilization of chromatin compaction to the modulation of
mitochondria-dependent processes, such as cell respiration and programmed cell death45,46. Zinc is a metallo-
protein cofactor for DNA-binding proteins with Zn ngers. It is part of copper (Cu)/zinc superoxide dismutase,
and several proteins are involved in the repair of damaged DNA and transcription and translation processes of
DNA47,48.
Several studies have investigated the curative eects of zinc supplementation in the treatment of male infertil-
ity and its eects on sperm parameters, but they have shown inconsistent results. ere are several mechanisms
by which zinc might interfere with sperm function. First, zinc is a cofactor for several hundred metalloenzymes,
particularly the enzymes responsible for protein synthesis49–52. It inuences phospholipases53, thus modulating
the stability of biological membranes. It has been suggested that the removal of zinc from the sperm cell surface
destabilizes the plasma membrane, playing an important role in preparation for the completion of capacitation
and the acrosome reaction. Some studies have reported that zinc supplementation can also improve the synthesis
of metallothioneins (low-molecular-weight Zn-binding proteins), which have properties of enhancing the quality
of seminal uids to protect sperm against damage54; metallothioneins have the property of protecting biological
tissues from the damage of oxidative stress via the capture of harmful oxidant species, such as superoxide and
hydroxyl radicals55. Second, zinc in seminal plasma is involved in maintaining the stability of sperm chroma-
tin56. Studies have shown that chromatin stability is high in normal men with high zinc content in their seminal
plasma, but it is low in infertile men with less stable sperm chromatin57–59. ird, zinc exerts an in vitro eect on
oxidative changes in human semen and is considered a scavenger of excessive O2 production by defective sper-
matozoa and/or leukocytes aer ejaculation60. Fourth, Zn plays an important role in the development of testes
and secondary sexual characteristics and in a few sperm physiologic functions. Zinc acts as a growth factor, an
immune-regulator, and a cryoprotectant with anti-inammatory eects, and decreased zinc levels cause hypog-
onadism, decreases in the testis volume, inadequate development of secondary sexual characteristics, and atrophy
of seminiferous tubules, with negative eects on sperm development61–63.
In addition, some studies have suggested that zinc plays an important role in prostate health; several studies in
the last decade have attempted to correlate zinc levels with semen quality parameters. Although some studies have
Figure 2. Forest plot showing the meta-analysis outcomes of the plasma zinc between infertile and normal
men. Abbreviations: IV: inverse variance; Random: random-eects model.

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Figure 3. Sub-group forest plot showing the meta-analysis outcomes of seminal plasma zinc between
infertile and normal men. Abbreviations: IV: inverse variance; Random: random-eects model. Exclude XFR
test: studies only included AAS and the chemical chromatometry test. Exclude chemical chromatometry test:
studies only include AAS and XRF test.
Figure 4. Forest plot showing the meta-analysis outcomes of the eect of zinc supplementation on
abnormal and normal sperm morphology. Abbreviations: IV: inverse variance; Random: random-eects
model. Normal (%): percent of normal morphology; abnormal (%): percent of abnormal morphology.

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reported that pathologic conditions of the prostate gland do not necessarily implicate interference with sperm
function, other studies have also reported that, in prostatitis patients, zinc concentrations in the seminal plasma
and prostatic uid were also decreased64–68. It is known that prostatic uid contributes greatly to the zinc content
of the ejaculate. A sharp decrease in zinc in the prostatic uid must result in a decreased zinc concentration in
seminal plasma, and some studies have also reported that chronic prostatitis has negative eects on sperm param-
eters69–75. us, the zinc concentrations in prostate tissue also play an important role in sperm quality. In this
meta-analysis, only Rui W et al. and Fuse H et al. discussed this factor. Some studies have suggested a relationship
between zinc levels and standard variables, such as sperm motility76,77 and/or sperm count78, but they have shown
contradictory results. Our study revealed that zinc supplementation could signicantly increase the sperm vol-
ume, sperm motility and percentage of normal sperm morphology. Some studies have reported that zinc is con-
sidered one of the major factors that aect spermatozoa motility; it controls its eects by modulating the activity
of the Ca2+ ATPase enzyme and reducing antisperm antibodies, particularly IgG. Regarding the mechanism of
the inuence of Zn on the semen volume, as discussed above, prostatic uid contributes greatly to ejaculate the
zinc content. In the prostate, zinc is involved in regulating the growth and apoptosis of prostate epithelial cells.
e increase in prostatic uid may contribute to the increase in semen volume. Zn was also shown to be necessary
for maintaining the stability of sperm chromatin and membrane stabilization and inhibiting apoptosis for normal
sperm morphology. However, the mechanism by which zinc supplementation increases the sperm quality needs
further study. is meta-analysis suggested that zinc supplementation might increase male reproduction func-
tion, and these ndings could open new avenues of future fertility research and treatment and could aect public
health. However, this eld requires further study.
ere were some limitations in our study that should be considered when interpreting the results of this
meta-analysis. First, the sample size of each study was relatively small, and 2,600 infertile men and 867 normal
controls were investigated in all twelve studies; thus, the control group size was particularly small. Second, sev-
eral studies related to the subject were excluded due to a lack of control data, means or standard deviations or
the inability to obtain the full text. ird, although this meta-analysis showed that the seminal plasma zinc level
decreases in infertile patients, it is not clear whether the change in the seminal plasma zinc concentration is
the result of male infertility or whether the change in zinc concentration led to male infertility. Seminal plasma
zinc levels also have limited predictive value because zinc is a primarily intracellular ion whose levels uctuate
Figure 5. Forest plot showing the meta-analysis outcomes of the eect of zinc supplementation on sperm
motility. Abbreviations: IV: inverse variance; Random: random-eects model.
Figure 6. Forest plot showing the meta-analysis outcomes of the eect of zinc supplementation on the
semen volume. Abbreviations: IV: inverse variance; Random: random-eects model.
Figure 7. Forest plot showing the meta-analysis outcomes of the eect of zinc supplementation on sperm
viability. Abbreviations: IV: inverse variance; Random: random-eects model.

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according to circadian rhythm. As such, it is dicult to draw denitive conclusions concerning the clinical value
of seminal zinc concentrations in male infertility.
In summary, the present study illustrated that zinc in the seminal plasma of infertile males was signicantly
lower than that in normal males. Zinc supplementation could signicantly increase the semen volume, sperm
motility and percentage of normal sperm morphology of infertile males, suggesting that zinc supplementation
might increase male reproductive function. ese ndings could open new avenues of fertility research and treat-
ment and could aect public health. However, further studies with larger sample sizes are needed to better eluci-
date the correlation between seminal plasma zinc levels and male infertility.
Methods
Literature search. is meta-analysis was restricted to published studies that investigated the correlation
between seminal plasma zinc concentrations and male infertility and the eects of zinc supplementation on
sperm parameters. Two independent reviewers searched the PubMed, EMBASE, Science Direct/Elsevier, and
CNKI databases, as well as the Cochrane Library, from inception to July 2015; the language or study type was
not restricted. e search terms combined text words and MeSH terms. For example, the search terms for sem-
inal plasma zinc concentration were: ‘semen zinc concentration’, ‘semen zinc content’, ‘seminal plasma zinc con-
centration’, ‘seminal plasma zinc content’, ‘seminal plasma zinc level’, and ‘zinc level’. e search terms for male
infertility were ‘sterility’, ‘infertility’, and ‘dysgenesis’, and the search terms for zinc supplementation were ‘zinc
supplementation’, ‘added zinc’, ‘zinc supplements’, ‘oral zinc sulfate’, and ‘oral zinc gluconate’. e search terms for
semen parameters were ‘sperm’, ‘spermatozoa’, ‘semen analysis’, ‘seminal parameters’, ‘sperm count’, ‘spermatozoon
count’, ‘sperm motility’, ‘sperm parameters’ and ‘spermatozoon density’. All of the related articles and abstracts
were retrieved. In addition, references cited within relevant reviews were retrieved by hand; only full articles were
searched.
Eligibility criteria. Inclusion criteria. All patients presenting for infertility evaluations had a minimum of
one year of unprotected intercourse. e female partners of the selected men did not present hormonal dysfunc-
tions, tubal obstruction or reproductive system infections. e control cases were normal men and consisted of
healthy men with no history of fertility problems whose partners conceived spontaneously within 1 year of reg-
ular, unprotected intercourse. Semen samples were obtained before therapeutic interventions and were analyzed
according to the World Health Organization (WHO) criteria. Semen parameters included the semen volume,
sperm concentration (density), sperm motility, sperm count, sperm viability, and normal and abnormal sperm
morphology percentages. Available data were extracted from the articles, and the means and standard deviations
of the zinc concentrations and sperm parameters were calculated in all of the groups.
Exclusion criteria. Studies were excluded if they were case reports or review articles. Studies involving patients
with infertility accompanied by other disorders of the urogenital system and patients who were undergoing zinc
supplementation therapy were also excluded.
Study selection and validity assessment. Two independent reviewers screened the titles and abstracts
of all of the citations from the literature search. All of the relevant studies that appeared to meet the eligibility
criteria were retrieved. If an ambiguous decision was made based on the title and abstract, it was necessary to
analyze the full text. e nal decision of eligible studies was made by reviewing the articles. Disagreements were
resolved by consensus or a third reviewer.
Figure 9. Forest plot showing the meta-analysis outcomes of the eect of zinc supplementation on the
sperm count. Abbreviations: IV: inverse variance; Random: random-eects model.
Figure 8. Forest plot showing the meta-analysis outcomes of the eect of zinc supplementation on sperm
concentration. Abbreviations: IV: inverse variance; Random: random-eects model.

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Data extraction and statistical analysis. Data, including demographic data (authors, year of publica-
tion, country, number and mean age of the participants) and outcome data of the seminal plasma zinc concen-
trations and semen parameters in all of the included studies, were extracted from the studies by three reviewers.
Disagreements were resolved by consensus. Quantitative meta-analysis was performed by two reviewers using
Review Manager (RevMan) soware (version 5.2; the Nordic Cochrane Centre, the Cochrane Collaboration,
2012, Copenhagen, Denmark) and Stata soware (version 12.0; College Station, Texas, USA). Available data were
analyzed in a meta-analysis.
We pooled the standard mean dierences (SMDs) of the semen zinc concentrations and sperm parameters
from the included studies, which were identied with 95% condence intervals (95% CIs). Heterogeneity was
assessed by the P-value and I-square statistic (I2) in the pooled analyses, representing the percentage of total var-
iation across studies. If the P-value was less than 0.1, or the I2-value was greater than 50%, the summary estimate
was analyzed in a random-eects model. Otherwise, a xed-eects model was applied. To estimate the stability of
the meta-analysis, we conducted a sensitivity analysis. Publication bias was detected using the visual symmetry
of funnel plots, with asymmetry suggesting possible publication bias. Publication bias was also assessed by Begg’s
test and Egger’s test in the meta-analysis. If the P-value was less than 0.05, publication bias existed.
Figure 10. Begg’s publication bias plot of the seminal plasma zinc concentration between infertile and
normal men. e funnel plot did not show any substantial asymmetry, suggesting no evidence of publication
bias.
Std_E Coef. Std. Err. t P > |t| (95% Conf. Interval)
slope − 0.53 0.52 − 1.04 0.32 − 1.63 0.57
bias − 0.51 2.65 − 0.19 0.85 − 6.16 5.14
Table 3. Egger’s test of publication bias.
Figure 11. Sensitivity analysis plot of the seminal plasma zinc concentration between infertile and normal
men.

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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 81500580 and
581230017).
Author Contributions
J.Z., X.Y.D. and L.K.L. designed the research; J.Z., X.Y.D., L.W., X.Y.H., Q.L., Q.Q.W., B.S.S. and Z.L. conducted the
studies; J.Z., X.Y.H., Q.J.W. and X.Y.D. analyzed the data and prepared the manuscript; J.Z. and L.K.L. guided the
experiments and edited the paper. All of the authors read and approved the manuscript.
Additional Information
Competing nancial interests: e authors declare no competing nancial interests.
How to cite this article: Zhao, J. et al. Zinc levels in seminal plasma and their correlation with male infertility: A
systematic review and meta-analysis. Sci. Rep. 6, 22386; doi: 10.1038/srep22386 (2016).
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