What is DNA fragmentation in sperm?

Deoxyribonucleic acid (DNA) is the genetic material present in all cells and is the main component of chromosomes. Among other things, it is responsible for instructing cells to make different proteins and other substances required for growth and development.

Like all human cells, sperm cells contain DNA. The role of the sperm cell is to transmit the sperm DNA into an egg (oocyte), thus creating the fertilized zygote — with half of the zygote DNA coming from the oocyte and half from the sperm. Sperm DNA integrity is critical to the normal development of the embryo and for the achievement of a healthy pregnancy.ⁱⁱ

DNA fragmentation describes a type of DNA damage where nicks or breaks appear in the DNA molecules. This type of damage can occur within sperm DNA and within DNA in other cells in our bodies.

Developing sperm have the ability to repair errors in their DNA, but once sperm reach maturity and are stored in a structure near the testicle called the epididymis, they are no longer able to make repairs. Thus, damage that occurs to sperm DNA after storage or post-ejaculation is carried with the sperm if it fertilizes the egg. In some cases, the oocyte and embryo can repair the damaged sperm DNA after fertilization.

The percentage of sperm with DNA fragmentation in a semen sample is called the DNA fragmentation index (DFI); this number is related to male fertility. A high proportion of sperm DNA fragmentation (SDF) corresponds to a decrease in fertility.ⁱⁱⁱ

What causes sperm DNA fragmentation?

Multiple different mechanisms can induce sperm DNA fragmentation. On a molecular level, here are a few of the most common hypothesized causes:

• ‍DNA breaks during sperm development: For sperm DNA to be stored inside the tiny sperm head, it needs to be compactly coiled. During sperm development, the DNA is coiled and recoiled multiple times, and it is thought that small nicks may be made in the DNA to allow the DNA to bend and be tightly packaged. It is possible that these nicks can accumulate as DNA fragmentation if they are not repaired by the time the sperm matures.^iv
• ^‍Reactive oxygen species (ROS): The presence of high amounts of damaging molecules known as ROS within the male reproductive tract may also contribute to SDF. ROS are unstable oxygen-containing molecules that easily react with other molecules. When they react with neighboring molecules — including DNA, RNA (ribonucleic acid), and proteins — they can cause damage. Interestingly, immature sperm produce ROS, and it is thought that mature sperm may be exposed to these ROS when they are housed next to immature sperm in the seminiferous tubules or part of the epididymis.^v
• ^‍Apoptosis: Apoptosis is the process by which cells undergo programmed cell death. It is a normal process triggered by cellular damage, such as too-low or too-high levels of oxygen or nutrient deficiency. A full 50 to 60 percent of germ cells starting the process to become sperm will be marked for apoptosis before maturity because they are defective. Normally, these cells marked for apoptosis will break down and not become sperm. However, it is thought that some of these early defective DNA-containing germ cells may not be broken down appropriately and may ultimately develop into fully mature sperm. As the genetic content of the sperm may not be associated with any morphologic (structural) or motility issues, the sperm may appear normal but contain defective — or fragmented — DNA.^vi

Other factors may influence SDF, too:

• ‍Age, high body mass index, and smoking: A 2018 study of Saudi Arabian males undergoing intracytoplasmic sperm injection cycles (ICSI) found that 14 percent had high SDF, defined by the researchers as a DNA fragmentation index (DFI) of greater than 30 percent.^vii They showed that increasing age and a higher body mass index (BMI) were associated with high SDF. However, age and obesity (as well as smoking) have not been associated with high SDF in all studies; thus, more research is needed in this area.^viii,ix,x
• ^‍Gonadotoxins: Substances that damage the testicles and ovaries, such as chemotherapy or radiation used for cancer treatment, can also lead to SDF through the molecular mechanisms described above.^xi
• ^‍Varicoceles: Similarly, varicoceles (abnormal enlargement of the veins within the scrotum) can lead to an increase in ROS, which has been shown to increase sperm DNA fragmentation.^xii

How does high sperm DNA fragmentation impact fertility?

A high DNA fragmentation index (DFI) may have different impacts depending on the attempted method of conception:

Natural conception

Multiple studies reveal an association between high SDF and lower male fertility. A large prospective study by Louis et al (2013) followed 501 couples that discontinued contraception and were trying to conceive.^xiii They measured SDF in human semen samples and showed that a higher DFI was associated with a longer time to pregnancy.

Spanò et al (2000) followed 231 couples trying to conceive and found that male fertility may begin to decline once the SDF index exceeds 20 percent and decreases significantly once the SDF index exceeds 40 percent.^xiv A 2018 meta-analysis also reported that males experiencing infertility suffered from increased rates of SDF and similarly set the DFI threshold for infertility at 20 percent.^xv

In vitro fertilization (IVF)

Some studies also reveal an association between SDF and assisted reproductive technology outcomes. A cohort study by Oleszczuk et al (2016) showed that semen samples with an SDF index of greater than 20 percent had a significantly decreased live birth rate with in vitro fertilization (IVF) treatment, compared to the control group where the SDF index was less than 10 percent.^xvi Interestingly, there was no difference between the groups if ICSI was used, leading the researchers to conclude that infertility due to high SDF may be overcome by using ICSI instead of conventional IVF.

In a 2024 study, researchers analyzed clinical data from 6,330 patients undergoing IVF or ICSI in China.^xvii They did not find that higher DFI impacted rates of pregnancy, stillbirth, or congenital anomaly rates. However, they observed that a DFI of 15 to 30 percent and greater than 30 percent were associated with a higher rate of miscarriage and lower birth weight compared to a DFI of less than 15 percent.^xviii

Furthermore, a large meta-analysis from 2019 demonstrated that higher rates of SDF in the male partner was associated with recurrent pregnancy loss.^xix It is worth noting, however, that not all studies have observed that DFI influences birth and pregnancy outcomes in IVF; the evidence is not consistent.^xx,xxi More studies are needed to better understand these relationships.

Intrauterine insemination (IUI)

Intrauterine insemination (IUI) has also been found to be less effective in males with a high rate of SDF. Bungun et al (2007) found that in patients with an SDF index less than or equal to 30 percent, the per cycle rate of deliveries following IUI was 19 percent, but in patients with an SDF index greater than 30 percent, the delivery rate dropped to 1.5 percent.^xxii,xxiii

How does sperm DNA fragmentation testing work?

SDF testing methods rely on standardized protocols with stringent quality control. Here are some examples of SDF tests:

• Sperm chromatin structure assay (SCSA)
• Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay (TUNEL)
• Sperm chromatin dispersion (SCD)
• Single cell gel electrophoresis assay (Comet)

These are all genetic tests that measure the level of various types of sperm DNA damage,^xxiv and involve laboratory testing of sperm from a semen sample. A two- to three-day period of abstinence is generally recommended prior to the semen analysis test.^xxv

The inability to conceive is a primary reason for undergoing sperm DNA fragmentation testing. Here are some other indications, as summarized in a 2017 study:^xxvi

• Varicocele (an abnormal enlargement of the veins within the scrotum) with normal or borderline semen analysis parameters
• Unexplained infertility
• Recurrent pregnancy loss (RPL)
• Recurrent IUI failure
• Recurrent IVF and/or ICSI failure
• Modifiable lifestyle risk factors, such as smoking and obesity, with borderline abnormal or normal semen analysis parameters

Clinical testing for SDF is on the rise. However, the American Society for Reproductive Medicine (ASRM) and the European Society of Human Reproduction (ESHRE) have not yet recommended SDF assessment as a routine test.^xxvii The American Urological Association (AUA) and the European Association of Urology (EAU) have acknowledged the importance of DNA fragmentation’s impact on sperm quality.^xxviii

How can sperm DNA fragmentation be improved?

Males with high SDF and other sperm defects may be able to improve sperm genetic integrity by adopting healthier lifestyle habits. These habits include consuming antioxidants, losing weight, and quitting smoking.

Antioxidant intake

Showell et al (2019) completed a systematic review and meta-analysis of studies that focused on subfertile males referred to fertility clinics and found an association between oral antioxidant intake and lower SDF.^xxix The group estimated that the baseline chance of live birth was 12 percent following placebo or no treatment, and 14 to 26 percent following the use of antioxidants. However, these findings were based on a small sample size of 124 live births in a total of 750 couples.

The antioxidants used in the studies included arginine, carnitines, carotenoids, coenzyme Q10, cysteine, micronutrients (folate, selenium, zinc), vitamin E, and vitamin C. However, there has not been an antioxidant regimen found to significantly improve male factor infertility or specifically SDF.^xxx,xxxi

Weight loss

Weight loss may also improve one’s SDF index. In one study from 2018, 105 Indian males were enrolled in a weight loss program and their SDF index was studied.^xxxii On average, the SDF index improved from 20.2 percent to 17.5 percent after the weight loss program.

Quitting smoking

Likewise, quitting smoking may improve SDF and subsequent fertility, as there is an established association between smoking and increased DNA fragmentation.^xxxiii

Reducing time between ejaculations

There is a small body of evidence that a shorter time between ejaculations may reduce SDF, which is particularly relevant to those providing semen for IUI, IVF, or ICSI. In one study by Gosalvez et al (2011), there was a 48 percent reduction in SDF in patients following a three-hour abstinence period compared to a 24-hour abstinence period.^xxxiv Additional studies are required on this subject, and currently there is no recommendation that several semen samples be obtained within 24 hours in the case of high SDF index.

What fertility treatments can support people with high DNA sperm fragmentation?

Patients with high SDF may have an increased risk of infertility. However, they have several options for assisted reproductive treatment to improve reproductive outcomes if they are unable to conceive at home. Success rates vary due to factors such as the severity of SDF, fertility treatment type, and other concurrent health conditions or infertility diagnoses.

IVF and IUI

High SDF has been associated with poor success with intrauterine insemination (IUI); one study demonstrated a live birth rate of just 1.5 percent with an SDF index above 30 percent.^xxxv

Likewise, IVF may be less successful when the sperm used have a high SDF index. A study by Simon et al (2013) of 203 couples undergoing IVF found that couples with less than 25 percent SDF had a live birth rate of 33.3 percent.^xxxvi In comparison, couples with SDF of up to 50 percent had a live birth rate of just 13.1 percent.

IVF with ICSI

Given that IVF may be less successful in the case of a high SDF index, intracytoplasmic sperm injection (ICSI) may give couples a higher chance of success. In ICSI, a single sperm cell is selected and injected directly into the egg. A meta-analysis of 20 articles by Zhang et al (2015) found that the clinical pregnancy rate was similar between patients with high SDF and low SDF when ICSI was used, indicating that ICSI can improve fertility in patients with high SDF.^xxxvii However, another study did not find any improvement in fertilization or clinical pregnancy rates using ICSI versus IVF in patients with SDF.^xxxviii

Testicular sperm extraction (TESE)

SDF may occur when sperm travels from the testes to the epididymis. In order to avoid fragmentation during this travel, surgical removal of sperm directly from testicular tissue via a testicular sperm extraction (TESE) procedure may improve sperm DFI. A study published in 2017 reported a significant increase in pregnancy rates following IVF cycles using TESE-retrieved sperm in comparison to ejaculated sperm in males with high levels of SDF.^xxxix

Conclusion

Sperm DNA fragmentation (SDF) is one possible contributor to male factor infertility and is a developing area of research. SDF testing may sometimes be recommended if a person is struggling with infertility and has other health conditions or factors including a varicocele, unexplained infertility, or recurrent pregnancy losses with normal or nearly normal semen analysis parameters. If high SDF is diagnosed, interventions such as weight loss, quitting smoking, and taking antioxidant supplements may help improve male fertility potential. However, in some cases, physicians may recommend advanced reproductive techniques such as IVF with ICSI, which has had successful outcomes in patients with high SDF. Individuals experiencing subfertility are encouraged to speak to their healthcare providers.