Since the 1990s, reports about decreasing human sperm counts and increasing abnormalities in human testes have focused research on the possible impacts of environmental factors such as lifestyle, environmental and occupational exposure to trace elements, environmental pollution, and pesticides on semen quality and male reproductive health
]. Merzenich et al.
] suggested that some meta-analyses of sperm-count data showed a global downward trend, but this conclusion should be interpreted with caution because of the heterogeneities among the studies in terms of geographical and/or ethnic variations, study designs and/or different methodological standards. More population-based studies are also needed to investigate the trends in male reproductive disorders and to explore the factors influencing male reproductive health.
Mn is a necessary element for humans, but although it has many important functions for normal reproductive health, overexposure may be toxic to reproductive health
]. One study reported that high Mn levels were associated with erectile dysfunction, but this study only involved workers with severe sickness induced by occupational exposure to Mn
]. The damaging effects of Mn on male reproductive function have mainly been examined in occupationally-exposed men, and showed prolonged time to semen liquefaction and decreased sperm motility among workers in contact with Mn, and decreasing percentage of motile sperm with increasing duration of employment as a miner
]. In workers who had been in contact with Mn for more than 10 years, the activities of protective enzymes such as catalase in erythrocytes were lower than in controls, suggesting that high Mn levels might result in decreases in the activities of antioxidant enzymes, resulting in subsequent damage
The role of Mn2+
was examined during the cryopreservation of cattle bull semen, and the addition of Mn to the egg-yolk-citrate extender + glycerol (EYC-G) dilutor was found to improve the quality/fertility of the semen, resulting in improved success rates of in vitro
fertilization and artificial insemination
]. An in vitro
experiment using semen with >75% motility and 80×106
sperm/mL from healthy males found that Mn2+
supplementation improved total thiols and reduced glutathione levels under normal and oxidative stress conditions, and that Mn2+
supplementation maintained the thiol level by reducing oxidative stress
]. In vitro
experiments using semen samples obtained from 12 normozoospermic healthy volunteer donors aged 19–23 years found that 50 μmol/L Mn(III)TMPyP, a kind of superoxide scavengers, named Mn(III) tetrakis (1-methyl-4-pyridyl) porphyrin, could attenuate the effects of superoxide (O2+
) on sperm motility parameters
]. In contrast, another experiment using semen from healthy male volunteers showed that 500 ppm Mn2+
significantly inhibited sperm motility with no accompanying change in seminal malondialdehyde levels, indicating that 500 ppm Mn2+
could affect sperm motility in vitro
A potential role of Mn in infertile males has been reported. Lafond et al.
] reported lower Mn levels in seminal plasma from men with lower sperm densities. Among 52 Nigerian male partners of infertile couples, normospermic infertile patients had higher serum Mn levels compared to those with oligospermia and azoospermia (P<0.001)
], suggesting a potential role for Mn in the evaluation of infertile males. Another study, however, showed higher seminal Mn levels in infertile men compared to controls
]. In a study of 200 infertile men, high Mn levels were associated with increased risk of low sperm motility (OR=5.4; 95%CI: 1.6–17.6) and low sperm concentration (OR=2.4; 95%CI: 1.2–4.9)
]. There are thus conflicting results regarding the effects of Mn on semen quality in infertile men, and more studies are needed to confirm its impacts and to explore the dose threshold values for infertility.
Few studies have investigated the harmful impacts of Mn on reproductive function in men without occupational Mn exposure
]. Some studies examined the in vitro
effects of Mn on semen quality in healthy men
] and found some useful results, but no consistent conclusions. The current study found that serum Mn concentrations >19.40 μg/L (P80
) had negative impacts on sperm morphology, suggesting that high serum Mn levels can adversely affect semen quality in healthy men with no occupational Mn exposure, however, the value of 19.40 μg/L was only the 80th percentile cut-off point for all subjects, and could not be considered as the threshold value for the harmful effects of Mn on sperm morphology. No apparent risks were associated with lower (<20%) or higher (>80%) Mn levels in terms of semen volume, and a lower proportion of abnormal semen volume was found in the >80% Mn group, but semen quality assessment should not focus only on semen volume. In this study, the negative impacts of high Mn levels on sperm viability, progressive motility, and morphology were more obvious than the beneficial effects. Our results indicated that higher Mn concentration might be harmful to semen routine parameters among no-occupational Mn-contact men. So, long-term or lage amount of Mn contact should be avoid in life, especially for people at reproductive age.
However, this study was a cross-sectional survey and was therefore unable to demonstrate any cause-effect relationships. In addition, only serum Mn concentrations were measured, which do not fully reflect the status of Mn in the seminal plasma. Further studies are therefore needed to explore the effects of Mn on human semen quality and reproductive function.