NAC has antioxidant properties as a prodrug of intracellular CySH (intracellular increase in CySH concentration) with a subsequent increase in GSH. Therefore, periods of continuous supplementation 24,27,30, of at least 3 days, would provide a higher concentration of NAC, and therefore a direct antioxidant effect, assessed by TAC 24,30 or pro-antioxidant ratio . This may explain the differences in the results of the studies included in this systematic review 21,24,30. However, the study conducted by Christensen et al. reported no difference between the two groups (IG, CG) after 1 day of NAC supplementation. However, there was no clear evidence of any beneficial effects of NAC supplementation on haematological markers, inflammatory response, and muscle behaviour. In general, participants supplemented with NAC showed significant improvements in exercise performance, antioxidant capacity, and glutathione homeostasis. Second, all possible subgroup and bias-controlling analyses were performed to obtain reliable results on the effects of NAC on the reproductive biomarkers. First, to our knowledge, current study is the first comprehensive systematic review and meta-analysis addressing this issue, so far. Therefore, future studies must focus on the proper design to diminish the possible sources of bias. Participants of included studies in seventeen of eighteen studies had PCOS. On the other hand, the hormone status and other metabolites levels, such as serum 25(OH)D in women younger than 30 years, are lower than that in women 30 years and older(Reference Nakamura, Nashimoto and Matsuyama45). Glutathione also acts as an antioxidant, neutralizing inflammatory molecules called free radicals or ROS that can damage cellular structures. It participates in conjugation reactions in the liver, where toxins are bound to glutathione to make them water-soluble and more easily excreted from the body. Glutathione is a tripeptide composed of three amino acids, glutamate, glycine, and cysteine. In addition, publication bias was revealed by visual inspection of the funnel plot (online Supplementary Fig. 6(b)). Moreover, funnel plot visual inspection revealed no sign of publication bias (online Supplementary Fig. 4(b)). WMD, weighted mean difference; SMD, standardised mean difference; LH, luteinising hormone; FSH, follicle-stimulating hormone; E2, oestradiol; NR, not reported. Removing an individual research at a time by sensitivity analysis did not affect the results. There were no significant changes in results following subgroup analysis (Table 2). Moreover, NAC has a positive effect on sperm concentration and ejaculate volume, whereas no obvious effect in serum hormones. In addition, selection bias and subjective bias may affect the final results of this study. Spermatogenesis is highly controlled by hormonal environment, in addition to affecting the speed and quality of spermatogenesis; any change of hormone distribution may profoundly affect the chromosome ploidy and the integrity of sperm chromatin (Safarinejad & Safarinejad, 2009). The study reported that although sperm density and sperm activity did not improve, sperm function tended to improve and ROS level decreased significantly after NAC administration (Akiyama, 1999). In the included literature, all patients were treated with 600 mg/d NAC for at least 3 months, which was considered as a safe drug dose with a higher median lethal dose (Ciftci et al., 2009; Jannatifar et al., 2019; Safarinejad & Safarinejad, 2009). Considering their mechanisms and functions, LC and LAC have been proposed as a possible treatment in selected forms of oligoasthenoteratozoospermia (Lenzi et al., 2003; Vicari et al., 2002). NAC appears to be a more appropriate supplement than GSH or CySH administration to modulate OS induced by drugs (paracetamol) or diseases that occur with low GSH levels, such as chronic obstructive pulmonary disease (COPD), by increasing intra-tissue GSH 47,48. GSH is the most abundant non-protein thiol in the body and one of the major antioxidants against ROS, and GSH is a cofactor of GPx . The reduction in lipid peroxidation occurs as with other antioxidant supplements, such as vitamin C and/or E, and prevents OS 43,44. This is in line with Yalçin et al. who reported that oral supplementation with 100 mg/day of NAC blocked lipid peroxidation in chronic blepharitis. Exercise stimulates an increase in peroxidation that damages cell membranes, alters lipoproteins, and breaks down structures containing lipid conjugates . The concentration of NAC is considered to be a limiting factor in its direct antioxidant activity . In this meta-analysis, compared with the control group, the viscosity, liquefaction time, and sperm volume of NAC group were significantly increased. They researched PubMed, EMBASE, and Cochrane Library databases and references of the included studies; Medical Subject Headings (MeSH) terms were "L-carnitine, L-acetyl-carnitine, N-acetyl-cysteine, NAC, LC, LAC, infertile, idiopathic, idiopathic infertile, men, male, idiopathic asthenozoospermia." Pooled analysis of eighteen randomised clinical trials with a total population of 3161 in the current study revealed that NAC supplementation had significant increasing and decreasing effects on FSH and TT levels, respectively. Therefore, the present meta-analysis aimed to evaluate the effects of NAC supplementation on infertility parameters such as ovulation biomarkers and serum levels of sex hormones in women with PCOS. The present study assessed the effects of NAC supplementation on semen parameters and antioxidant status in asthenoteratozoospermic men partners of infertile couples. Based on the above, the present study was conducted to investigate the effects of daily oral NAC supplementation on the quality of semen parameters, chromatin integrity and reproductive hormones in asthenoteratozoospermic men. We utilised a random-effects model to get the overall effect size if the amount of between-study heterogeneity was significant. If the trial had ‘low risk’ for all domains, a high-quality study was considered a low risk of bias. The Cochrane Collaboration’s risk of bias tool was employed to assess the risk of bias for each study(Reference Higgins, Altman and Gøtzsche19). Then, relevant studies were assessed to ensure the suitability of a study for full-text assessment. Numerous studies have indicated that NAC is effective for the induction of ovulation and pregnancy rates in PCOS patients.
