Nickel and human sperm quality: a systematic review

Reproductive effects of chronic exposure to nickel (Ni), including sperm quality, have been a matter of debate given that published studies yielded contrasting results. We have, therefore, planned to systematically search and analyze medical literature with the aim to ascertain the association of exposure to nickel with the sperm quality in humans.

We systematically searched Pubmed, Scopus and Embase for studies reporting the association of Ni with the sperm quality in humans with no time or language limits and used PRISMA to report the findings. The risk of bias was assessed using JBI critical appraisal checklist and SIGN tool. Because the reported effects were no coherent, meta-analysis was not possible.

All included studies were observational and planned to test the effect of a group of trace elements, but not Ni alone. We identified and included 19 studies from 23 publications, published from 12 countries, which assessed sperm quality, sperm DNA damage and sperm metabolome. Ni was quantified in blood, semen plasma, spermatozoa and urine. Sixteen included cross-sectional studies were of acceptable quality, whereas three more case-control reports were of poor quality. Multivariate models were reported in only eight studies. Overall, studies were inconsistent in the direction of effect, when elevated Ni was not associated with the outcome (N = 8 studies), or some association was present (N = 11 studies). In the latter, 9 studies yielded elevated risk and 2 studies exhibited protective effect. Only one report was in an occupationally exposed population with some association with tail defects, but present in both welders and controls.

Existing evidence from the studies in humans is inconsistent and does not confirm a clear adverse effect of higher Ni concentrations in blood, urine or semen on the sperm quality. Robust methodology must be a key issue in the future studies. Studies with more powerful evidence, such as cohort or experimental reports are needed.