Association between dietary total antioxidant capacity and semen quality among men attending an infertility clinic: a cross-sectional study

Abstract STUDY QUESTION Is dietary non-enzymatic antioxidant capacity related to semen quality? SUMMARY ANSWER The only statistically significant association of semen quality parameters with dietary total antioxidant capacity (DTAC) detected was an inverse association between DTAC and ejaculate volume. WHAT IS KNOWN ALREADY Growing interest exists regarding the role of diet in influencing semen quality. While DTAC is linked to favorable health outcomes, its association with semen quality, especially among men attending infertility clinics, remains understudied. STUDY DESIGN, SIZE, DURATION This cross-sectional study was carried out between June and December of 2020. In total, 1715 participants were included in the final analysis. PARTICIPANTS/MATERIALS, SETTING, METHODS Men who attended an infertility clinic in China were enrolled. Experienced clinical technicians performed the semen analysis. The DTAC indices included the ferric-reducing ability of plasma, oxygen radical absorbance capacity, total reactive antioxidant potential, and Trolox equivalent antioxidant capacity. The quantile regression model was used for multivariate analysis. MAIN RESULTS AND THE ROLE OF CHANCE After adjustment for a variety of confounding variables, a significant inverse association was identified between DTAC and ejaculate volume (βcontinuous FRAP = −0.015, 95% CI = −0.023, −0.006, βT3 vs T1 = −0.193, 95% CI = −0.379, −0.006, Ptrend = 0.007; βcontinuous TRAP = −0.019, 95% CI = −0.041, 0.002, βT3 vs T1 = −0.291, 95% CI = −0.469, −0.112, Ptrend = 0.002). The majority of DTAC indices have no statistically significant association with semen quality parameters. LIMITATIONS, REASONS FOR CAUTION We cannot infer causality because of the nature of the cross-sectional study design. The robustness of the conclusion may be compromised by the exactness of non-enzymatic antioxidant capacity estimation. WIDER IMPLICATIONS OF THE FINDINGS Our findings demonstrated no association between DTAC indices and semen quality parameters among men attending an infertility clinic, except for ejaculate volume. Even though our findings are mostly non-significant, they contribute novel knowledge to the field of study while also laying the groundwork for future well-designed studies. STUDY FUNDING/COMPETING INTEREST(S) This work was supported by the JieBangGuaShuai Project of Liaoning Province [grant number 2021JH1/10400050], the Clinical Research Cultivation Project of Shengjing Hospital [grant number M1590], and the Outstanding Scientific Fund of Shengjing Hospital [grant number M1150]. The sponsors had no role in study design, or in the collection, analysis, and interpretation of data, or in the writing of the report, or in the decision to submit the article for publication. There are no conflicts of interest to declare. TRIAL REGISTRATION NUMBER N/A.


Introduction
Infertility has been a persistent public health concern in recent decades, with the disease burden projected to have grown in 195 countries and territories between 1990 and 2017 (Sun et al., 2019).According to a recent large-scale demographic analysis based on 204 countries and territories, the global total fertility rate has significantly declined from 4.97 in 1950 to 2.31 in 2019 (GBD 2019Demographics Collaborators, 2020).Notably, global data suggest that male factors are estimated to account for 20-70% of infertility cases, with an estimated 30 million infertile males worldwide (Agarwal et al., 2015).Furthermore, a review of spatiotemporal trends suggested a deterioration of human semen quality in some specific areas (Auger et al., 2022).Debate regarding the decline in semen quality and the reasons for it has persisted for decades, with geographical differences, lifestyle, and environmental factors all being considered (Virtanen et al., 2017).In males, maintaining a diet rich in antioxidant vitamins, such as vegetables and fruits, has been linked to improved semen quality (Salas-Huetos et al., 2017).A systematic review and metaanalysis of randomized clinical trials also found that some dietary antioxidant supplements could have a beneficial effect on sperm quality parameters (Salas-Huetos et al., 2018).
Assessing the dietary total antioxidant capacity (DTAC) is a comprehensive method for determining the collective antioxidant impact of a diet (Serafini and Del Rio, 2004).Since its introduction, DTAC has been steadily employed in epidemiological studies (Nascimento-Souza et al., 2018).High DTAC has recently been linked to reducing the risk of a variety of health outcomes, including cognitive impairment (Sheng et al., 2022a), cardiovascular disease (Zujko et al., 2022), cancer (Parohan et al., 2019), and all-cause, cardiovascular, or respiratory disease mortality (Sheng et al., 2022b).A review indicated that lifestyle-related oxidative processes might be the primary cause of cellular and tissue damage, which has contributed to the development of noncommunicable diseases (Seyedsadjadi and Grant, 2020).However, to our best knowledge, no research has been conducted on the association between DTAC and semen quality, despite the fact that both diet and oxidative stress have been identified as important factors influencing semen quality (Evans et al., 2021;Ferramosca and Zara, 2022).
However, previous studies have mostly focused on the association between antioxidant-rich dietary patterns, dietary supplements, or antioxidant nutrients and semen quality (Salas-Huetos et al., 2017, 2018;Arab et al., 2018;De Cosmi et al., 2021), rather than DTAC that measures quality of the entire diet (Salari-Moghaddam et al., 2022).Meanwhile, the applicability of the aforementioned evidence may vary across different countries owing to local variations in dietary habits.Hence, the current study aimed to evaluate the association between DTAC and semen quality in Chinese men.

Participants
The research methodology has been previously reported (Liu et al., 2021(Liu et al., , 2022)).Briefly, this cross-sectional study was carried out between June and December 2020.Men who attended the infertility clinic at Shengjing Hospital of China Medical University were enrolled.Male participants were included based on the following criteria: provided informed consent to participate in the study; being 18 years of age or older; and being a first-time visitor who had not undergone any medical treatment, including medication, surgery, or combination therapy, for more than 3 months.Men who: refused to participate in the study; did not fully complete the questionnaire; or had previously been diagnosed with infertility at another hospital were excluded.
As shown in Fig. 1, a total of 1984 male participants were recruited.After excluding participants with missing basic information (n ¼ 31), missing dietary information (n ¼ 3), missing semen parameters (n ¼ 180), a history of varicocele (n ¼ 29), and unexplained total energy intake (<800 or >6000 kcal/day) (n ¼ 26) (Yuan et al., 2021), the final analysis included 1715 participants.PASS (Power Analysis and Sample Size) software, version 11.0 (NCSS, LLC.Kaysville, UT, USA) determined that this number matched the required sample size of 1645 men.

Ethical approval
All participants signed informed consent forms at the time of study enrollment.This study received approval by the ethics

WHAT DOES THIS MEAN FOR PATIENTS?
Male infertility is a global public health issue, and one of its main causes is poor semen quality.The dispute over why semen quality is diminishing has raged for decades, with geographical differences, lifestyle, and environmental factors all being considered.Oxidative stress (which causes tissue damage) is one of the chief mechanisms underlying male infertility and is defined as a disturbance in the balance between production of potentially damaging reactive oxygen species (called free radicals) and antioxidant defenses, i.e. the body's mechanisms of reducing this stress.Antioxidants are compounds found in foods, particularly certain fruits/vegetables, that scavenge and neutralize the damaging free radicals.A low intake of antioxidant-rich foods, may have a negative impact on semen quality, contributing to reduced male fertility.Because oxidative stress is increasingly recognized to cause sperm damage, there is growing public interest in a possible role of antioxidant-rich diets or antioxidant supplements in improving semen quality and, hence, fertility.The term 'dietary total antioxidant capacity' refers to the antioxidant properties derived from the entire diet.We carried out a study of 1715 Chinese men attending an infertility clinic to investigate if there was a link between dietary total antioxidant capacity and semen quality.Our results showed a link between dietary total antioxidant capacity and ejaculate volume but no other features of sperm were linked (e.g.number of sperm, their movement, or structure).However, it is difficult to determine if ejaculate volume alone affects semen quality, and therefore, overall, our study suggests that dietary total antioxidant capacity is not associated with semen quality.Furthermore, because we collected dietary and semen data almost simultaneously, we cannot show a 'cause and effect'.More well-designed studies into the link between dietary total antioxidant capacity and semen quality are required.committee of Shengjing Hospital of China Medical University (2017PS190K).

Data collection
Data were collected using relevant questionnaires, including a demographic and lifestyle questionnaire, a physical activity questionnaire, and a food frequency questionnaire (FFQ).We collected information such as region (Liaoning/other provinces), age (years), annual family income (RMB; thousand yuan), education level (junior secondary or below, senior high school/technical secondary school, and university/junior college or above), occupational status (employed/unemployed), alcohol intake status (yes/no), smoking status (yes/no), and use of nutritional supplements (yes/no) by using a demographic and lifestyle questionnaire.To determine physical activity levels, we employed the identical questionnaire as used in the China Kadoorie Biobank study (Du et al., 2013).The FFQ with 110 food items was used to assess dietary intake (Cui et al., 2023a).The auditor quickly checked for omissions and errors after participants completed the on-site self-administered questionnaire.Every participant underwent a physical examination and semen analysis.A trained assistant measured the participant's height and weight during the physical examination.

Semen analysis
Semen analyses were all performed by experienced clinical technicians.Prior to semen collection, the males were instructed to abstain for 3-7 days.Semen samples were collected by individuals in a designated private room adjacent to the laboratory, through masturbation into a sterile plastic container, abstaining from condom use.A WLJY9000 computer-assisted sperm analyzer (Beijing Weili New Century Science & Tech.Dev.Co. Ltd, Beijing, China) was used to evaluate the collected semen samples.The key detection metrics were semen volume, total sperm count, sperm concentration, progressive motility, total sperm motility, and sperm morphology.Total motility was defined as the sum of progressive and non-progressive motility.A semen smear was made and stained using the Papanicolaou method, and the sperm morphology was assessed by examining the semen smear under an optical microscope.The World Health Organization laboratory manual for the examination and processing of human semen (5th Edition, 2010) was employed to determine the quality of the semen (WHO, 2010).External quality control was implemented throughout the study (Li et al., 2022;Zhao et al., 2023).

Dietary intake assessment
A previously validated FFQ tailored exclusively for the population of Northeast China was employed to acquire dietary information (Cui et al., 2023a).Participants were asked to report on their average frequency of consumption of each FFQ food item in the previous year.The portion size (g/time) derived from the weighed diet records was used to translate these frequencies to grams of food consumed (g) (Cui et al., 2023a).The daily nutrient intake was then calculated by multiplying the mass of each food (g/day) by the nutrient content (per 100 g of food) in the Chinese Food Composition Tables (Yang et al., 2018b).

Dietary total antioxidant capacity
The ferric-reducing ability of plasma (FRAP) (Carlsen et al., 2010) Dietary antioxidant capacity and semen quality | 3 radical-trapping antioxidant parameter (TRAP) (Pellegrini et al., 2003(Pellegrini et al., , 2006)), and the Trolox equivalent antioxidant capacity (TEAC) (Pellegrini et al., 2003(Pellegrini et al., , 2006) ) were used for determining the DTAC value.Each FFQ 110 food item was cross-referenced with its corresponding DTAC database.If an FFQ item did not correspond to one in the database, a proxy estimate based on the average of similar items was employed.Each participant's DTAC was computed by multiplying the daily foods consumed by the relevant DTAC value per food portion and summing these values.The DTAC estimates did not account for nutritional supplements, including vitamin and mineral supplements, cod liver oil, or DHA supplements, as well as ginseng supplement.

Other variables
Physical activity was quantified by multiplying each type of activity by the corresponding metabolic equivalent of task (MET) intensity values (Ainsworth et al., 2011).The BMI was computed by dividing weight in kilograms by height in meters squared (kg/m 2 ).
The eating habits section of the FFQ was utilized to gather information on recent dietary changes and cooking methods.
Participants were asked if they had made any recent dietary changes, with four options available: 'None', 'From 3 years ago', 'From 1 to 2 years ago', and 'From this year'.These responses were translated to 'yes' or 'no' during the analysis to indicate the dietary change.Participants were asked how frequently they eat meat, vegetables, and seafood that had been steamed, stewed, broiled, deep-fried, stir-fried, or eaten raw.These frequencies were then converted to weekly times, and the values for each cooking type were tallied.

Statistical analysis
Demographic data were categorized into DTAC tertiles.If the continuous variables had a normal distribution, the means with SDs were reported and compared using one-way ANOVA.If the continuous variables were not normally distributed, they were expressed as medians with interquartile ranges (IQRs) and compared using the Kruskal-Wallis test.The Chi-square test was used to compare categorical variables presented as frequencies (%).The residual method was applied to adjust for energy in the DTAC indices and other dietary factors.The DTAC indices were analyzed as either continuous variables or tertiles.The linear trend across increasing tertiles was analyzed using the median value of each tertile as a continuous variable.The quantile (median) regression model was used for the multivariable analysis (Nassan et al., 2020).Furthermore, we used restricted cubic spline (RCS) with three knots positioned at the 5th, 50th, and 95th percentiles of the distribution to investigate potential nonlinear associations between the DTAC indices and semen quality parameters.
Subgroup analyses were performed depending on age (<32 and �32 years), smoking status (yes/no), nutritional supplements use (yes/no), and recent dietary changes (yes/no).The multiplicative and additive interactions of DTAC indices with the previously mentioned factors were examined.In addition, we conducted several sensitivity analyses.First, an analysis was carried out to rule out the possibility of normozoospermic men skewing the current data.Second, as is common in semen quality studies, a multivariate linear regression model with a logtransformed dependent variable was used.Third, we performed a sensitivity analysis based on the tertiles of semen quality parameters to help interpret the results.Finally, to evaluate the impact of missing values on the robustness of the present findings, we employed multiple imputation to address missing values (Austin et al., 2021).SAS software, version 9.4 (SAS Institute, Cary, NC, USA), was utilized for all analyses.In all statistical tests, a P-value of <0.05 was considered statistically significant.

Participant characteristics and dietary intake
The characteristics of all participants are shown in Table 1.The median age of the participants was 32.0 years, and 63.9% had a junior college/university degree or above.An estimated 71.6% of the participants originated from Liaoning Province (Fig. 2).Supplementary Table S1 shows the distribution of DTAC and semen quality parameters.The median FRAP (mmol/day) with IQR was 6.7 (5.1, 10.2).The median total sperm count (�10 6 /ml) was 161.0 (86.9, 261.8).The median percentage of normal sperm morphology was 4.0 (3.0, 7.0).In Supplementary Table S2, participant characteristics are shown by DTAC tertiles.Except for region, abstinence time, sperm concentration, and normal sperm morphology, all variables were significantly associated with at least one DTAC index (P < 0.05).As shown in Table 3, multivariable-adjusted quantile regression revealed that DTAC was significantly associated with ejacu-
With the exception of increased TEAC demonstrating a linear link with lower ejaculate volume (P overall ¼ 0.038, P nonlinear ¼ 0.823) (Fig. 3), all other RCS results were deemed non-significant after multivariable adjustments (P overall > 0.05).The RCS curves for FRAP and semen quality parameters are depicted in Supplementary Fig. S2.
In linear regression models (Table 4), DTAC indices were found to be related to ejaculate volume (b continuous TRAP ¼ −0.006,  S5.Moreover, sensitivity analysis based on semen parameter tertiles demonstrated a link between TRAP and ejaculative volume (Ejaculative volume T3 vs T1 : odds ratio (OR) T3 vs T1 ¼ 0.637, 95% CI: 0.470, 0.861, P trend ¼ 0.003) (Supplementary Table S6).Finally, as shown in Supplementary Table S7, the multiple imputation analysis also found that DTAC was associated with ejaculate volume

Discussion
To the best of our knowledge, this is the first study to examine the relationship between DTAC and semen quality.Given the lack of a perfect assay, the current study applied a variety of commonly used DTAC indices rather than a single index, which might compensate for each other's shortcomings and result in more accurate DTAC estimates.We found that the crude model related DTAC indices to ejaculate volume and progressive motility.An inverse association between DTAC and ejaculate volume was consistently observed after multivariable adjustment.Furthermore, following multiple sensitivity analyses, the relationship between DTAC and ejaculate volume remained robust.However, ejaculate volume alone is insufficient to predict semen quality, and given that the majority of our findings are nonsignificant, our study is more likely to conclude that the influence of DTAC on semen quality appears to be insignificant.The residual method was used to adjust the intake variables for energy intake.
Dietary antioxidant capacity and semen quality | 5 Biologically, oxidative stress is currently recognized as one of the primary mechanisms underlying male infertility (Cassina et al., 2015;Bisht et al., 2017;Barati et al., 2020).Antioxidants, which serve as 'scavengers' of reactive oxygen species (ROS), appear to be promising therapeutic approaches for reversing the negative impact of high ROS levels on semen parameters (Bisht et al., 2017;Barati et al., 2020).In recent years, the role of antioxidant-rich diets or antioxidant supplements in male fertility has gained considerable attention (Salas-Huetos et al., 2017;Nassan et al., 2018;Ferramosca and Zara, 2022;Zafar et al., 2023).For example, according to a review, a low intake of antioxidantrich foods, such as fruits/vegetables or products with antioxidant potential, may have a negative impact on semen quality, contributing to poorer male fertility (Skoracka et al., 2020).Moreover, antioxidant supplements, such as coenzyme Q10, appear to improve sperm quality by lowering testicular oxidative stress and sperm DNA fragmentation (Lucignani et al., 2022).In contrast, despite a plausible theoretical foundation, some studies have produced statistically non-significant results.For instance, in a recent case-control study, our colleagues found no significant link between DTAC and asthenozoospermia (Huang et al., 2023), and the present cross-sectional study on DTAC and semen quality parameters also yielded statistically non-significant findings.However, it should be noted that the two studies mentioned above, as well as many others on dietary antioxidants, are observational, thus limiting their ability to infer causality.In fact, the use of dietary antioxidants for male fertility has been impeded by an absence of high-quality evidence.For example, a 2018 review found little high-quality evidence to determine whether antioxidant consumption helps or harms infertile couples (Smits et al., 2018).Furthermore, a Cochrane systematic review on antioxidants for male subfertility, updated in 2022, revealed that the pooled results for total sperm motility, progressive sperm motility, and concentration at 3, 6, and 9 months were unreliable because of extremely high heterogeneity in each analysis (de Ligny et al., 2022).Therefore, additional high-quality randomized controlled trials are needed to determine factors such as the type of antioxidant utilized, the dose, the time to be administered, the duration of treatment, and the cost of antioxidants in male infertility (Dias et al., 2020;Agarwal et al., 2021).
There are several potential explanations for our findings.First, the quality of semen may be affected by a variety of factors, including monogenic factors (Houston et al., 2021), ambient air pollution (Xu et al., 2023), cigarette smoking (Sharma et al., 2016), as well as dietary factors (Salas-Huetos et al., 2017).If the oxidative stress only plays a secondary role in the pathogenesis, augmenting antioxidant defense alone may be insufficient for disease treatment or prevention (Forman and Zhang, 2021).Second, dietary antioxidants may be a double-edged sword.On the one hand, there is substantial evidence that antioxidant supplements, fruits, vegetables, and other antioxidant-rich products can help improve semen quality (Nassan et al., 2018;Skoracka et al., 2020;Zafar et al., 2023).On the other hand, maintaining a balance between ROS levels and antioxidant defense appears to be more critical (Chianese and Pierantoni, 2021) because a certain level of ROS is necessary for sperm function, and excessive antioxidants may inhibit the normal signaling pathways activated by ROS molecules, causing harm to the male reproductive system (Dias et al., 2020).
Another consideration is that the antioxidant defenseinducing compounds may not achieve effective concentrations in vivo (Forman and Zhang, 2021).Antioxidants may exhibit dosedependent effects.One review showed that many antioxidants could act as pro-oxidants at high doses, increasing oxidative    0.000 (−0.000, 0.000) −0.000 (−0.000, 0.000) 0.000 (−0.000, 0.000) 0.000 (−0.000, 0.000) −0.000 (−0.000, 0.000) stress and causing toxicity (Yang et al., 2018a).Dietary natural polyphenols were shown to have either positive or negative effects on sperm mitochondrial function, depending on the concentration (Ferramosca and Zara, 2022).An investigation of 16 individual compounds also revealed that certain polyphenols, at high concentrations, could act as pro-oxidants or alkylating agents in spermatozoa, resulting in severe negative effects on the biological competence of these cells, with only resveratrol, genistein and 2,2',4,4'-tetrahydroxydiphenyl maintaining an antioxidant function and having no adverse effects on sperm when the dose was <100 lM (Aitken et al., 2016).Furthermore, in a study of the modulation of plant polyphenols on human sperm  Dietary antioxidant capacity and semen quality | 9 mitochondrial respiratory efficiency, researchers found that quercetin, naringin, genistein, luteolin, and resveratrol could significantly improve mitochondrial respiratory efficiency at 0.1 nM but significantly decreased it at 10 nM (Ferramosca et al., 2021).
Collectively, it could be said that the effect of natural antioxidants on spermatozoa may be concentration dependent.
On the other hand, variability in semen quality could also be one of the factors influencing our findings.To clarify this issue, we compared the variability of semen parameters in the current study to that of similar Chinese studies (Wang et al., 2015;Yang et al., 2017;Miao et al., 2021), and we found that most semen parameters had roughly identical variability, with even lower variability in our data on normal sperm morphology and slightly higher variability in sperm count.Furthermore, a previous study that involved 10 laboratories assessing intra-and interindividual variability in human semen quality concluded that external quality control schemes were necessary for maintaining the quality of semen analysis (Auger et al., 2000).In our study, experienced technicians performed external quality control as part of the Chinese Medical Association Society of Reproductive Medicine's nationwide quality control program on semen analysis (Li et al., 2022;Zhao et al., 2023).Our thorough external quality control ensures the quality of semen analysis and, to some extent, minimizes the variability of semen quality parameter measures.
Finally, dietary constancy may be another essential factor to consider in the analysis of association between DTAC and semen quality because sperm collected during enrollment was produced over a 74-day period, whereas we employed a validated FFQ to follow dietary intake over the past 12 months (Cui et al., 2023a).However, according to a systematic review, FFQs with a 12month dietary recall interval exhibit superior FFQ reproducibility when compared to <12 months (Cui et al., 2021), indicating that our FFQ approach for assessing dietary intake may be stable.Moreover, to eliminate the influence of dietary changes on the conclusion, we included dietary change during the preceding year as an adjustment variable, as in previous studies (Huang et al., 2023;Zhao et al., 2023).In addition, we conducted a subgroup analysis and an interaction analysis based on the dietary change status.Finally, we found that dietary changes might act as an effect modifier in the association between DTAC and specific semen parameters.Except for the above results, most of the subgroup and interaction analysis results were non-significant, corresponding to the primary findings.Therefore, it is speculated that dietary changes were unlikely to affect the conclusion of this study.
The current study possesses certain strengths.First, as mentioned above, this is the first study to investigate DTAC in relation to semen parameters.Second, quantile regression analyses between seven DTAC indices and six semen parameters, as well as numerous subgroup analyses, interaction analyses, and sensitivity analyses, were performed.These analyses may contribute to a better understanding of the association, especially between certain DTAC indices and specific semen quality parameters, while also boosting the robustness and generalizability of the conclusion.
Nevertheless, some limitations should be noted.First, the cross-sectional nature of our study limits our ability to establish causality.Second, although a previous systematic review on the validity of the FFQ indicated that it was suitable to estimate overall dietary intake in a nutritional epidemiological study (Cui et al., 2023b), the FFQ employed is known to be subject to recall bias.Third, data on a previous coronavirus disease 2019  diagnosis, a known risk factor for sperm parameters, was not collected in this study (Li et al., 2020).However, according to the official data for 2020 (available at http://www.nhc.gov.cn/jkj/s3578/202103/f1a448b7df7d4760976fea6d55834966.shtml), the incidence of COVID-19 in China was low throughout the study year, with an estimated 6.2 per 100 000 people.Furthermore, while participants came from various provinces, most of them lived in Liaoning Province when they enrolled, and according to government data, Liaoning province was free of COVID-19 from June to December 2020 (available at https://wsjk.ln.gov.cn/wsjk/zfxxgk/fdzdgknr/tfggsj/bdc99f4d-2.shtml).In addition, everyone who visited Shengjing Hospital during the study period had to be screened for COVID-19 and then transferred to a fever clinic if they tested positive.As a result, despite not specifically excluding them, the current study included nearly no patients with COVID-19.Fourth, there are some limitations to the DTAC estimate.First of all, nutritional supplements and oils were not included in the DTAC calculation owing to the limited number of FFQ food items and the difficulties in quantifying them.Moreover, these DTAC indices may not accurately reflect the antioxidant status in vivo since assessing the in vitro antioxidant capacity of isolated compounds, beverages, or extracts is insufficient for estimating their antioxidant effects in-vivo, according to a recent review (Mota et al., 2023).In addition, we had to rely on international databases because we lacked DTAC values for local food items.This measurement bias could not be completely avoided despite the use of several DTAC indices and controlling for diet-related confounding factors to compensate for indices estimation shortcomings.Furthermore, food processing may affect DTAC estimation accuracy since thermal and nonthermal operations alter the phenolic antioxidants in fruits, vegetables, and grains (Nayak et al., 2015).Finally, although the confounding factors of cooking methods were taken into account in the current analysis, there were some unmeasurable food processing procedures, such as blanching and drying, that could influence the DTAC estimates.Fifth, while the variability of semen parameters was low in our study, and external quality control was performed to help ensure this, slight fluctuations may still limit our ability to discover the association between DTAC and semen quality.Variability of semen parameters is a common issue in similar studies that should be noted and addressed in future study.

Conclusion
Our findings demonstrated no association between DTAC indices and semen quality parameters among men attending an infertility clinic, with the exception of ejaculate volume.Even though our findings do not imply a substantial association, they contribute novel knowledge to the field of study while also laying the groundwork for future well designed studies.

Figure 1 .
Figure 1.Flow diagram for selecting participants in a study of dietary total antioxidant capacity and semen quality among Chinese men attending an infertility clinic.

Figure 2 .
Figure 2. Regional distribution within China of participants in the study.

Figure 4 .
Figure 4.The statistically significant results of the dietary change subgroup analysis.EV, ejaculate volume; FRAP, ferric-reducing ability of plasma; NSM, normal sperm morphology; TEAC, Trolox equivalent antioxidant capacity; TRAP, total radical-trapping antioxidant parameter.

Figure 3 .
Figure 3.The dose-response association between Trolox equivalent antioxidant capacity and ejaculate volume in infertile Chinese men.Diff, difference; TE, Trolox equivalents; TEAC, Trolox equivalent antioxidant capacity.Difference in ejaculate volume where the reference value for TEAC is median.

Table 1 .
Baseline demographic, reproductive, and nutritional characteristics of the infertile participants.
MET, metabolic equivalent task; No, number.Data are presented as a median (P 25 , P 75 ) or as a count (%).

Table 2 .
Energy-adjusted quantile regression coefficients with 95% CI for semen parameters in relation to the DTAC indices.

Table 3 .
Multivariate-adjusted quantile regression coefficients with 95% CI for semen parameters in relation to the DTAC indices.

Table 4 .
Multivariate-adjusted linear regression coefficients with standard errors for semen parameters in relation to the DTAC indices.