Sperm protected with melatonin

Note: This article was previously published and has been updated according to scientific and divulgative criteria. Its purpose is informative and does not replace medical advice.

IN BRIEF

• Melatonin is a molecule produced by the pineal gland with proven antioxidant properties and roles in circadian regulation.
• Laboratory studies show that melatonin can reduce oxidative stress and apoptosis in sperm and follicular cells.
• Clinical research in reproductive medicine shows mixed results: some improvements in oocyte/embryo parameters, but effects on pregnancy and live births are not yet definitively confirmed.
• Biological plausibility is strong (antioxidant action, receptors in reproductive cells), but clinical translation requires larger and more standardized studies.

Abstract: What does science say?

Melatonin is an endogenous molecule with antioxidant action and receptor activity in reproductive tissues. Experimental evidence on human sperm and follicular cells shows a reduction in markers of oxidative damage, less caspase activation, and preservation of mitochondrial function after melatonin treatment. Clinical studies in in vitro fertilization cycles have evaluated oral supplements or laboratory use (insemination, cryopreservation), reporting improvements in parameters such as oocyte quality or reduction of sperm DNA fragmentation in some cases; however, the effects on clinical pregnancies and births are not yet consolidated. Outcomes depend on dose, duration, route of administration, and context (in vitro vs. in vivo). Available evidence supports biological plausibility but requires caution in interpretation: much research is experimental, on animal models, or in small clinical trials, and does not always demonstrate definitive causality. Therefore, melatonin may appear as a possible adjuvant in laboratory procedures and as a subject of clinical study, not as a standard treatment without specialist evaluation.

Biological mechanisms and experimental evidence

Melatonin is a lipophilic and hydrophilic molecule capable of reaching various cellular compartments, including the nucleus and mitochondria. This allows for both a direct scavenging action of free radicals and an indirect effect through the modulation of endogenous antioxidant enzymes. In sperm, excess reactive oxygen species (ROS) alters the plasma membrane, compromises motility, damages DNA, and can activate apoptotic pathways; melatonin has been shown in the laboratory to reduce caspase activation and DNA fragmentation, contributing to greater cell survival. On ovarian follicular cells and oocytes, antioxidant action limits DNA oxidation products such as 8-OHdG and protects mitochondrial function, important elements for oocyte maturation and early embryonic development.

Sperm: apoptosis, ROS, and the role of melatonin

Laboratory studies on human sperm indicate that melatonin can attenuate the toxic effects induced by hydrogen peroxide (H2O2) by reducing caspase activation and DNA fragmentation; some experimental research suggests an involvement of the MT1 membrane receptor and the ERK signaling pathway in the protective response [1]. Other experimental work suggests that, in addition to direct radical scavenging, melatonin modulates intracellular antioxidant capacity by improving mitochondrial resilience [2]. These results are solid at the in vitro level, but their clinical translation requires attention: experimental concentrations and conditions may differ from those achievable in vivo.

Oocytes and follicle: biomarkers of oxidative damage

Follicular fluid is a microenvironment sensitive to oxidative balance. In human samples, the marker 8-OHdG, an indicator of oxidative DNA damage, was found to be higher in follicles associated with degenerated oocytes; in clinical and experimental studies, melatonin has been correlated with a reduction in this marker and improvements in fertilization in IVF/ICSI contexts [3]. Experiments on granulosa cells show that melatonin reduces DNA damage, lipid peroxidation, and apoptotic activity, preserving mitochondrial function [4]. Such evidence consolidates the biological plausibility of an intrafollicular protective effect.

Clinical evidence in assisted reproductive technologies

The translation of laboratory observations into clinical practice has been evaluated in several clinical studies, including randomized trials and meta-analyses. Controlled pilot trials and dosage studies have explored oral melatonin administration during ovarian stimulation and its use in the laboratory (e.g., integration of manipulation media or cryoprotectants). The results are heterogeneous: some trials have shown improvements in the number of mature oocytes or embryo quality, but the effect on clinical pregnancy and live births remains uncertain and not unequivocal [5]. Subsequent reviews and meta-analyses have suggested possible benefits on some reproductive endpoints, while highlighting the limitation of available evidence and the need for larger and more standardized studies [6]. Current clinical recommendations therefore require caution and case-by-case evaluation.

Randomized controlled trials

A pilot, randomized, double-blind trial evaluated different melatonin dosages (2–8 mg, twice daily) during IVF/ICSI cycles to verify effects on reproductive outcome parameters. The study did not demonstrate a clear increase in clinical pregnancies at the tested dosage, while documenting some differences in intermediate parameters and short-term safety [5]. These results indicate that clinical experimentation is active but still insufficient for universal recommendations.

Meta-analyses and reviews

Systematic reviews and meta-analyses of the literature have gathered experimental and clinical evidence suggesting an improvement in some laboratory outcomes (oocyte maturation, embryo quality), while the effect on live births is less consistent and often not significant among available studies [6][7]. The authors of the reviews point out methodological heterogeneity, different doses and durations, and the need for larger, multicenter studies.

Dose, method of use, and application limits

The studied administration methods vary: systemic oral supplementation (from a few mg to repeated dosages), laboratory pretreatment of sperm or oocyte samples, and addition to cryopreservation media. Experimental and clinical results depend heavily on the dose, duration of exposure, phase of the reproductive cycle, and experimental model. Experimental studies show protective effects at concentrations that often do not exactly correspond to plasma concentrations achievable with standardized in vivo doses; moreover, the effect can be biphasic (different responses at low vs. high concentrations). For these reasons, clinical evaluation requires well-designed trials comparing different doses, times, and forms in homogeneous populations [7].

What it means in practice

For those involved in fertility or interested in the topic, the practical implications are as follows: melatonin has a solid biological basis as an antioxidant and a molecule that interacts with receptors present in reproductive tissues; in the laboratory (sperm manipulation, cryopreservation, oocyte culture), its use as a protective agent is supported by preclinical studies and some experimental protocols. In the clinical setting, oral melatonin administration during ovarian stimulation cycles has been studied, but there are currently no consolidated recommendations for routine use aimed at improving live births: clinical adoption should only occur in experimental centers or on specialist indication, preferably within the framework of trials or clinical registries. For patients, it is important not to interpret experimental results as a standard prescription: every therapeutic choice should be discussed with the reference team, who will evaluate specific benefits and limitations.

KEY POINTS TO REMEMBER

• Melatonin acts both as a direct radical scavenger and as a modulator of cellular antioxidant defenses.
• In vitro and in animals, protection of sperm and follicular cells from oxidative stress and apoptosis is observed [1][4][8].
• Clinical studies show partial results on laboratory parameters; impacts on pregnancies and births remain uncertain [5][6].
• Outcomes depend on dose, route of administration, and context (in vitro vs. in vivo); there are no "universal" dosages validated for all scenarios [7].
• Melatonin does not replace established quality practices in assisted reproduction laboratories or specialist medical advice.

Limitations of the evidence

Available evidence includes laboratory studies, animal models, observational studies, and some randomized clinical trials. It is essential to distinguish: in vitro and animal observations show plausible biological mechanisms but do not prove that the same results occur in humans under real clinical conditions; observational studies can indicate associations but not causality; only randomized and adequately sized clinical trials can provide causal evidence regarding clinical outcomes such as pregnancies and live births. Furthermore, many studies differ in dosage, formulation, duration, and participant selection criteria, increasing heterogeneity and limiting the possibility of robust synthesis. For responsible clinical translation, larger studies, multicenter designs, and long-term outcome evaluations are needed.

Editorial conclusion

Research on melatonin and reproductive health has made significant progress: laboratory and animal model evidence supports a protective role against oxidative stress in gametes and follicular cells; some controlled clinical studies suggest benefits on intermediate parameters in assisted reproduction. However, definitive proof of a relevant and reproducible clinical benefit (clinical pregnancy, live births) is not yet consolidated. Proceeding with caution, promoting well-designed clinical studies, and evaluating on a case-by-case basis remains the most prudent approach. Melatonin remains a promising research subject and a possible laboratory adjuvant, but not an automatic treatment without specialist supervision.

Editorial note

The article has been updated by integrating experimental and clinical studies available in peer-reviewed literature and including verifiable DOI references for transparency. The content is for informational purposes and does not replace specialist medical advice.

SCIENTIFIC RESEARCH

1. Javier Espino et al., "Melatonin protects human spermatozoa from apoptosis via melatonin receptor- and extracellular signal-regulated kinase-mediated pathways", Fertility and Sterility, 2011. DOI: https://doi.org/10.1016/j.fertnstert.2011.03.063 [1]
2. Javier Espino et al., "Melatonin as a potential tool against oxidative damage and apoptosis in ejaculated human spermatozoa", Fertility and Sterility, 2010. DOI: https://doi.org/10.1016/j.fertnstert.2009.12.082 [2]
3. Hiroshi Tamura et al., "Oxidative stress impairs oocyte quality and melatonin protects oocytes from free radical damage and improves fertilization rate", Journal of Pineal Research, 2008. DOI: https://doi.org/10.1111/j.1600-079X.2007.00524.x [3]
4. Manabu Tanabe et al., "Melatonin protects the integrity of granulosa cells by reducing oxidative stress in nuclei, mitochondria, and plasma membranes in mice", Journal of Reproduction and Development, 2015. DOI: https://doi.org/10.1262/jrd.2014-105 [4]
5. S. Fernando et al., "Melatonin in Assisted Reproductive Technology: A Pilot Double-Blind Randomized Placebo-Controlled Clinical Trial", Frontiers in Endocrinology, 2018. DOI: https://doi.org/10.3389/fendo.2018.00545 [5]
6. Systematic review and meta-analysis, "Melatonin Application in Assisted Reproductive Technology: A Systematic Review and Meta-Analysis of Randomized Trials", Frontiers in Endocrinology, 2020. DOI: https://doi.org/10.3389/fendo.2020.00160 [6]
7. Yonghui Jiang et al., "Applications of Melatonin in Female Reproduction in the Context of Oxidative Stress", Oxidative Medicine and Cellular Longevity, 2021. DOI: https://doi.org/10.1155/2021/6668365 [7]
8. V.S. et al., "Melatonin Protects Bovine Spermatozoa by Reinforcing Their Antioxidant Defenses", Animals (MDPI), 2023. DOI: https://doi.org/10.3390/ani13203219 [8]