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Infertile men possess substantially more sperm DNA damage than do fertile men, damage that may impact negatively on reproductive outcomes. In this era of assisted reproductive technologies there is mounting concern regarding the safety of utilizing DNA-damaged spermatozoa in this setting. Therefore, it is important to identify strategies that may reduce sperm DNA damage. The purpose of this review is to discuss the rationale for antioxidant therapy in men with sperm DNA damage and to evaluate the data on the efficacy of dietary and in vitro antioxidant preparations on sperm DNA damage.
We reviewed the literature on antioxidants and sperm DNA damage.
To date, the data suggest that dietary antioxidants may be beneficial in reducing sperm DNA damage, particularly, in men with high levels of DNA fragmentation. However, the mechanism of action of dietary antioxidants has not been established and most of the clinical studies are small. A beneficial effect of in vitro antioxidant supplements in protecting sperm DNA from exogenous oxidants has been demonstrated, however, the effect of these antioxidants in protecting sperm from endogenous ROS, gentle sperm processing and cryopreservation has not been established.
There is evidence to show that infertile men possess substantially more sperm DNA damage than do fertile men and that this DNA damage may adversely affect reproductive outcomes [1–4]. This is particularly relevant in an era where advanced forms of assisted reproductive technologies (ARTs) are commonly utilized (ARTs often bypass the barriers to natural selection), as there is some uncertainty and mounting concern regarding the safety of utilizing DNA-damaged spermatozoa in this setting . Therefore, it is important to identify strategies that may reduce sperm DNA damage. The proposed strategies include eliminating testicular gonadotoxins or hyperthermia, treatment of semen or genital tract infections, correction of varicoceles and the use of antioxidants [6–8]. The purpose of this review is to explore the rationale for antioxidant therapy in men with sperm DNA damage and to present data on the efficacy of dietary and in vitro antioxidant preparations on sperm DNA damage.
The etiology of sperm DNA damage is multi-factorial and may be due to primary testicular or secondary (e.g. environmental) factors. Sperm DNA damage is believed to be the result of aberrant protamine expression, excessive ROS (reactive oxygen species) generation and abortive apoptosis during spermatogenesis [9–12].
The association between sperm DNA damage and semen ROS is the basis for the use of antioxidants in the treatment of sperm DNA damage. High levels of ROS have been detected in the semen of 25% of infertile men [13, 14]. The levels of sperm-derived ROS (measured in sperm preparations having minimal leukocyte contamination) have been associated with sperm DNA damage, although no ROS threshold level above which sperm DNA damage is detected has been established [15–17]. Moreover, the levels of sperm DNA oxidation are higher in infertile compared to fertile men [18, 19]. Semen ROS are generated by spermatozoa (especially, defective or immature) and semen leukocytes [16, 20–23]. While the controlled release of low levels of ROS is necessary for normal sperm function, high levels of ROS can cause sperm dysfunction . The susceptibility of human spermatozoa to oxidative stress stems primarily from the abundance of unsaturated fatty acids in the sperm plasma membrane. These unsaturated fatty acids provide fluidity that is necessary for membrane fusion events (e.g. the acrosome reaction and sperm-egg interaction) and for sperm motility. However, the unsaturated nature of these molecules predisposes them to free radical attack and ongoing lipid peroxidation throughout the sperm plasma membrane. Once this process has been initiated, accumulation of lipid peroxides occurs on the sperm surface and oxidative damage to DNA can ensue [6, 24, 25]. Studies have demonstrated that exogenous and endogenous ROS can induce sperm DNA damage in vitro, confirming that ROS may play a role in the etiology of sperm DNA damage in infertile men [25–27].
Seminal fluid is an important source of antioxidants (ROS scavengers) and is key in protecting spermatozoa from oxidative injury [14, 28, 29]. This is particularly important because spermatozoa have little cytoplasmic fluid (antioxidant enzymes are generally intracellular), virtually no capacity for protein synthesis and little antioxidant capacity . The endogenous free radical scavenging enzymes in the male reproductive tract include superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPX) [14, 30–33]. These same antioxidant enzymes (SOD, catalase and GPX) are found in semen . Moreover, several non-enzymatic antioxidants (e.g. vitamins C and E, hypotaurine, taurine, L-carnitine, lycopene) are also found in semen and this non-enzymatic component accounts for much of the total seminal antioxidant activity [14, 35].
Several clinical studies have evaluated the relationship between semen antioxidant levels and sperm DNA damage and have reported conflicting results. Some studies have shown that a deficiency in semen antioxidants is associated with sperm DNA damage, whereas, other studies have not observed such a relationship [6, 36–39]. Similarly, some studies have found that seminal antioxidant activity is reduced in infertile men with high levels of seminal ROS (relative to those with normal levels of ROS) whereas others have not shown this [14, 40–42]. To date, there are no studies to indicate a relationship between systemic antioxidant or vitamin deficiency and male infertility. Silver et al, 2005 evaluated a cohort of fertile men and did not identify any relationships between dietary antioxidant intake (vitamins C, E or ß-carotene) and sperm DNA damage . Nonetheless, it is possible that a subset of infertile men may be at risk for antioxidant deficiency, particularly, vitamin C deficiency . Moreover, infertile men with specific lifestyles may also be at risk for antioxidant or vitamin deficiency (e.g. smoking, increased alcohol intake, dieting)[45, 46].
In order to be active, a dietary antioxidant should be effectively absorbed and concentrated in reproductive tract organs. The antioxidant should also replete a deficiency (in the testis, epididymis or semen) and play a role in reproductive function. The antioxidant must either improve spermatogenesis and/or epididymal function, ultimately, resulting in improved sperm function and chromatin compaction and integrity. Alternatively, the antioxidant should enhance semen antioxidant capacity in order to reduce oxidative DNA damage.
There are a small number of reports on the effects of dietary antioxidant supplementation on sperm DNA integrity. In general, these are small studies that do not evaluate the mechanism of action of antioxidants: the only endpoint that is measured is the integrity of the sperm DNA or pregnancy rate. Moreover, most studies evaluate the effects of a short treatment course (with no long-term follow-up), are not randomized and fail to include a placebo-control group. Most of these clinical studies have evaluated men with high levels of sperm DNA damage. In these men, treatment with antioxidant supplements is generally associated with reduced levels of sperm DNA integrity and/or improved fertility potential (Table 1) [19, 47–52].
In 1991, Fraga et al. demonstrated that dietary vitamin C increases semen vitamin C levels and improves sperm DNA integrity (lowers DNA oxidation levels) in men with a vitamin C deficiency (on a vitamin C depleted diet). More recent studies of infertile men with high levels of sperm DNA damage (2 randomized controlled and 3 uncontrolled trials) have shown that antioxidant therapy is effective in improving sperm DNA integrity or pregnancy rates (Table 1). In men with unselected infertility, the effect of dietary antioxidants on sperm DNA integrity is equivocal with one of two controlled trials showing a benefit of antioxidants on sperm DNA integrity (Table 1).
Several studies have examined the role of in vitro antioxidant supplementation in protecting the sperm DNA from oxidative damage. This is clinically relevant as sperm washing is routinely performed prior to ARTs (e.g. intrauterine insemination and in vitro fertilization) and the process may result in injury to the sperm DNA, particularly, as spermatozoa are now vulnerable to oxidative stress because seminal plasma (rich in antioxidants) has been removed in the process . However, it is important to note that subpopulations of spermatozoa will exhibit variable susceptibility to oxidative stress: the DNA of normal spermatozoa is reportedly less susceptible to gentle processing techniques than is the DNA of abnormal or immature spermatozoa [21, 54].
The studies on in vitro antioxidant supplementation have looked at the role of antioxidants in protecting sperm from exogenous and endogenous ROS, and, from the effects of semen processing and cryopreservation. Antioxidants (e.g. vitamins C and E, catalase, glutathione) have been shown quite clearly to protect sperm DNA from the effects of exogenous ROS (see Table 2) [55–58]. This is of clinical relevance as many of the semen samples contain leukocytes and these cells have the potential to generate exogenous ROS. In contrast, antioxidants appear to be of limited value in protecting the DNA of normal spermatozoa from endogenous ROS production (e.g. NADPH-induced or centrifugation-induced) (see Table 3) [25, 59–61]. In samples with poor morphology and poor sperm chromatin compaction, antioxidants may protect the sperm DNA from endogenous ROS production, as these samples are more vulnerable to oxidative stress [21, 54]. In general, antioxidants appear to be of limited value in protecting sperm DNA from gentle semen processing (e.g. incubation or density-gradient centrifugation) (see Table 4) [62–65]. In some cases, antioxidants supplementation in vitro (e.g. combination of vitamins C and E) may cause sperm DNA damage [63, 64]. The one study evaluating the effects of sperm cryopreservation suggests that antioxidants (vitamin E) do not protect sperm DNA in this setting .
In summary, the data suggest that ROS appear to play an important role in the generation of sperm DNA damage. Although in vitro studies have demonstrated a beneficial effect of antioxidant supplements in protecting sperm DNA from exogenous oxidants, the effect of these antioxidants in protecting sperm from endogenous ROS, gentle sperm processing and cryopreservation has not been established. The data suggest that dietary antioxidants may be beneficial in reducing sperm DNA damage, particularly, in men with high levels of DNA fragmentation. However, the mechanism of action of dietary antioxidants has not been established and most of the clinical studies are small.
Capsule Dietary and in vitro antioxidant preparations may help reduce sperm DNA damage, however, larger, well-designed studies are needed to firmly establish the utility of antioxidants in this setting.