The different surrogate measures of adiposity in relation to semen quality and serum reproductive hormone levels among Estonian fertile men

Association between body mass index and semen quality: a systematic review and meta-analysis

The continuous decline of human semen quality during the past decades has drawn much concern globally. Previous studies have suggested a link between abnormal BMI and semen quality decline, but the results remain inconsistent. This systematic review and meta-analysis aimed to evaluate the association between body mass index (BMI) and semen quality. We searched PubMed, Embase, and Web of Science for eligible studies from inception to April 17, 2022. We considered men with BMI < 25.0 kg/m2 as the reference and calculated the pooled weighted mean difference of men with overweight (BMI 25.0-29.9 kg/m2), obesity (BMI ≥ 30.0 kg/m2), class I obesity (BMI 30.0-34.9 kg/m2), and class II/III obesity (BMI ≥ 35.0 kg/m2). A total of 5070 articles were identified, of which 50 studies were included (71,337 subjects). Compared with men with BMI < 25.0 kg/m2, men with obesity had an average reduction of 0.24 ml in semen volume, 19.56 × 106 in total sperm number, 2.21% in total motility, 5.95% in progressive motility, and 1.08% in normal forms, respectively, while men with overweight had an average reduction of 0.08 ml in semen volume and 2.91% in progressive motility, respectively. The reduction of semen quality was more pronounced among men with obesity than that among men with overweight. Moreover, significant reductions in semen quality were identified in men with different classes of obesity, which were more pronounced in men with class II/III obesity than that in men with class I obesity. Across men from the general population, infertile or subfertile men, and suspiciously subfertile men, we identified significant semen quality reductions in men with obesity/overweight. In conclusion, obesity and overweight were significantly associated with semen quality reductions, suggesting that maintaining normal weight may help prevent semen quality decline.

Toward clinical exomes in diagnostics and management of male infertility

Infertility, affecting ∼10% of men, is predominantly caused by primary spermatogenic failure (SPGF). We screened likely pathogenic and pathogenic (LP/P) variants in 638 candidate genes for male infertility in 521 individuals presenting idiopathic SPGF and 323 normozoospermic men in the ESTAND cohort. Molecular diagnosis was reached for 64 men with SPGF (12%), with findings in 39 genes (6%). The yield did not differ significantly between the subgroups with azoospermia (20/185, 11%), oligozoospermia (18/181, 10%), and primary cryptorchidism with SPGF (26/155, 17%). Notably, 19 of 64 LP/P variants (30%) identified in 28 subjects represented recurrent findings in this study and/or with other male infertility cohorts. NR5A1 was the most frequently affected gene, with seven LP/P variants in six SPGF-affected men and two normozoospermic men. The link to SPGF was validated for recently proposed candidate genes ACTRT1, ASZ1, GLUD2, GREB1L, LEO1, RBM5, ROS1, and TGIF2LY. Heterozygous truncating variants in BNC1, reported in female infertility, emerged as plausible causes of severe oligozoospermia. Data suggested that several infertile men may present congenital conditions with less pronounced or pleiotropic phenotypes affecting the development and function of the reproductive system. Genes regulating the hypothalamic-pituitary-gonadal axis were affected in >30% of subjects with LP/P variants. Six individuals had more than one LP/P variant, including five with two findings from the gene panel. A 4-fold increased prevalence of cancer was observed in men with genetic infertility compared to the general male population (8% vs. 2%; p = 4.4 × 10-3). Expanding genetic testing in andrology will contribute to the multidisciplinary management of SPGF.

A ripple effect? The impact of obesity on sperm quality and function

Infertility affects approximately 15% of couples trying to conceive. Male-related causes account for roughly 50% of cases, with obesity emerging as a possible significant factor. Obesity, defined as a body mass index of 30.0 or higher, has become a widespread epidemic associated with numerous health issues, including a decrease of fertility. This review discusses the relationship between obesity and male infertility, particularly focusing on sperm quality and function. An overview of the literature suggests that obesity may influence the male reproductive system via disruptions in hormonal profiles, oxidative stress, and inflammation, leading to changes in sperm parameters. Several studies have discussed if obesity causes a decrease in sperm concentration, motility, and normal morphology, so far without a consensus being reached. However, available evidence suggests an impairment of sperm function in obese men, due to an increase in DNA damage and oxidative stress, impaired mitochondrial function and acrosome reaction in response to progesterone. Finally, the relationship between obesity and assisted reproductive technologies outcomes remains debatable, with conflicting evidence regarding the influence on fertilisation, pregnancy, and live birth rates. Therefore, the actual impact of obesity on human spermatozoa still needs to be clarified, due to the multiple factors potentially in play.

Association between central obesity and semen quality: A cross-sectional study in 4513 Chinese sperm donation volunteers

BACKGROUND

General obesity classified by body mass index has been linked to a reduction in semen quality; however, evidence on the adverse effect of central obesity on semen quality remains limited.

OBJECTIVES

To investigate the association between central obesity and semen quality.

MATERIALS AND METHODS

We conducted a cross-sectional study of 4513 sperm donation volunteers in Guangdong Provincial Human Sperm Bank during 2018-2021. Three central obesity indicators, including waist circumference, waist-to-hip ratio, and waist-to-height ratio, were measured using a multi-frequency bioelectrical impedance analysis for each subject. Semen analysis was conducted according to the World Health Organization laboratory manual for the examination and processing of human semen 5th edition. Linear regression models and unconditional logistic regression models were used to quantify the association between central obesity and semen parameters.

RESULTS

With adjustment for age, race, education level, marital status, fertility status, occupation, year of semen collection, abstinence period, ambient temperature, and relative humidity, central obesity defined as waist circumference ≥90 cm, waist-to-hip ratio ≥0.9, or waist-to-height ratio ≥0.5 was significantly associated with a 0.27 (95% confidence interval: 0.15, 0.38) mL, 14.47 (3.60, 25.34) × 106 , 7.06 (0.46, 13.76) × 106 , and 6.80 (0.42, 13.18) × 106 reduction in semen volume, total sperm number, total motile sperm number, and total progressive motile sperm number, respectively, and a 53% (10%, 112%) increase in odds of below the World Health Organization 2010 reference value for semen volume. These associations did not significantly vary across age. Similar results were observed for central obesity defined using each of the three indicators, except that subjects with a waist circumference ≥90 cm had a slightly higher total motility (estimated change: 1.30%; 95% confidence interval: 0.27%, 2.34%) and progressive motility (estimated change: 1.27%; 95% confidence interval: 0.23%, 2.31%).

DISCUSSION AND CONCLUSION

We found that central obesity was significantly associated with a reduction in semen volume, total sperm number, total motile sperm number, and total progressive motile sperm number. Future studies are warranted to confirm our results in other regions and populations.

A North American study of anthropometric factors and semen quality

OBJECTIVE

To evaluate the association between anthropometric measures and semen parameters.

DESIGN

Cross-sectional study.

SUBJECTS

Male participants aged ≥21 years. We analyzed data from 659 males (1185 samples) participating in a semen testing substudy of the Pregnancy Online Study (PRESTO), a North American preconception cohort study. After enrollment, we invited males aged ≥21 years to perform at-home semen testing using the Trak system.

EXPOSURE(S)

Participants reported selected anthropometric variables (current weight, height, waist circumference, and weight at age 17 years) and covariate data via an online baseline questionnaire.

MAIN OUTCOME MEASURE(S)

We used generalized estimating equations models to estimate the percent difference in mean log-transformed semen parameter values and 95% confidence intervals (CI) for associations between selected anthropometric variables and semen volume (mL), sperm concentration (million/mL), and total sperm count (million), adjusting for sociodemographics, lifestyle factors, and medical history. We also evaluated World Health Organization-defined thresholds for low semen quality.

RESULT(S)

Percentage differences in mean log-transformed semen volume, sperm concentration, and total sperm count (95% CI) comparing current body mass index ≥35 vs. <25 kg/m2 were -6.3 (-15.8, 4.3), -6.4 (-24.6, 16.2), and -12.2 (-31.1, 11.8), respectively. Percentage differences (95% CIs) comparing waist circumferences of ≥42 vs. <31 inches were -4.2 (-15.0, 8.0), -6.4 (-27.6, 21.0), and -10.4 (-31.9, 17.9) for semen volume, sperm concentration, and total sperm count, respectively. Greater adult weight gain since age 17 years was associated with reduced semen volume (≥25 vs. <5 kg; percent difference, -9.7; 95% CI, -18.4, 0.1), but not sperm concentration or total sperm count. The highest categories of each anthropometric variable generally were associated with World Health Organization-defined low total sperm count (≤39 million).

CONCLUSION(S)

Selected anthropometric factors were associated modestly with poorer semen quality.

Microdeletions and microduplications linked to severe congenital disorders in infertile men

Data on the clinical validity of DNA copy number variants (CNVs) in spermatogenic failure (SPGF) is limited. This study analyzed the genome-wide CNV profile in 215 men with idiopathic SPGF and 62 normozoospermic fertile men, recruited at the Andrology Clinic, Tartu University Hospital, Estonia. A two-fold higher representation of > 1 Mb CNVs was observed in men with SPGF (13%, n = 28) compared to controls (6.5%, n = 4). Seven patients with SPGF were identified as carriers of microdeletions (1q21.1; 2.4 Mb) or microduplications (3p26.3, 1.1 Mb; 7p22.3-p22.2, 1.56 Mb; 10q11.22, 1.42 Mb, three cases; Xp22.33; 2.3 Mb) linked to severe congenital conditions. Large autosomal CNV carriers had oligozoospermia, reduced or low-normal bitesticular volume (22-28 ml). The 7p22.3-p22.2 microduplication carrier presented mild intellectual disability, neuropsychiatric problems, and short stature. The Xp22.33 duplication at the PAR1/non-PAR boundary, previously linked to uterine agenesis, was detected in a patient with non-obstructive azoospermia. A novel recurrent intragenic deletion in testis-specific LRRC69 was significantly overrepresented in patients with SPGF compared to the general population (3.3% vs. 0.85%; χ² test, OR = 3.9 [95% CI 1.8-8.4], P = 0.0001). Assessment of clinically valid CNVs in patients with SPGF will improve their management and counselling for general and reproductive health, including risk of miscarriage and congenital disorders in future offspring.

Male waist circumference in relation to semen quality and partner infertility treatment outcomes among couples undergoing infertility treatment with assisted reproductive technologies

BACKGROUND

Male obesity has been related to poor semen quality and may also have a negative effect on assisted reproductive technologies (ART) outcomes. Whether male waist circumference (WC), as a measure of central obesity, impacts a couple's fertility independently of BMI is unclear.

OBJECTIVES

To examine the associations of male WC with semen quality and couples' outcomes of infertility treatment with ART.

METHODS

Couples presenting to the Massachusetts General Hospital Fertility Center were invited to participate in the study. Between 2009 and 2019, 269 males provided 671 semen samples and 176 couples underwent 317 ART cycles. Height, weight, and WC were measured on site. We analyzed the association of male WC with semen quality and pregnancy outcomes using cluster-weighted regression models to account for repeated observations while adjusting for potential confounders. Models were also stratified by male BMI (<25 kg/m2 compared with ≥25 kg/m2).

RESULTS

The median male age, WC, and BMI were 36.1 years, 96.0 cm, and 26.8 kg/m2, respectively. A 5-cm increase in WC was associated with a 6.3% (95% CI, 2.1-10.5%) lower sperm concentration after adjustment for potential confounders, including BMI. Male WC was also inversely related to the probability of achieving a live birth. For each 5-cm increase in male WC, the odds of a live birth per initiated cycle decreased by 9.0% (95% CI, 1.1%-16.4%) after accounting for several anthropometric and demographic characteristics of both partners. These associations were stronger among males in the normal BMI category (<25 kg/m2) than among overweight or obese males.

CONCLUSIONS

A higher male WC may be an additional risk factor for poor outcomes of infertility treatment, even after accounting for male and female partner BMIs, particularly in couples where the male partner has a normal BMI.

Mycoplasma genitalium Provokes Seminal Inflammation among Infertile Males

The impact of sexually transmitted infections (STI) on male fertility is controversial. Aims: To investigate the prevalence of urethritis-associated STIs (chlamydia, gonorrhoeae, Mycoplasma genitalium, trichomoniasis) among infertile males; to analyze the effect of STIs on semen parameters and blood PSA. Case-control study. Study group (n = 2000): males with fertility problems or desire for fertility check. Control group (n = 248): male partners of pregnant women. Analyses: polymerase chain reaction for STI, seminal interleukin 6 (IL-6), semen and fractionated urine, blood analyses (PSA, reproductive hormones). The prevalence of M. genitalium and chlamydia in the study group was 1.1% and 1.2%, respectively. The prevalence of chlamydia in the control group was 1.6%, while there were no M. genitalium cases. No cases with gonorrhoeae or trichomoniasis or combined infections were observed in neither group. There was a higher seminal concentration of neutrophils and IL-6 among M. genitalium positives compared with STI negatives. There was a trend toward a lower total count of spermatozoa and progressive motility among STI positives. No impact of STIs on PSA was found. The prevalence of STIs among infertile males is low. M. genitalium is associated with seminal inflammation. The impact of STIs on semen parameters deserves further investigations.

Does obesity based on body mass index affect semen quality?-A meta-analysis and systematic review from the general population rather than the infertile population

Whether obesity affects the quality of semen has become the focus of research. However, there are some deficiencies in the past research, because the vast majority of known infertile patients were included in the study samples. Taking infertile men as the research object to analyse the impact of obesity on semen quality, which cannot accurately prove that the impact on semen quality is caused by obesity, because the impact on semen quality may also be caused by other factors. Therefore, we selected ordinary obese men rather than infertile patients to conduct a systematic review and meta-analysis of the effects of obesity on semen parameters. The results showed that obesity had no effect on sperm concentration (SMD: -0.15, 95% CI: -0.32 ~ 0.02, p = .088) and percentage of normal sperm morphology (SMD: -0.17, 95% CI: -0.66 ~ 0.32, p = .487), but decreased semen volume (SMD: -0.32, 95% CI: -0.52 ~ -0.12, p = .002), total sperm number (SMD: -0.77, 95% CI: -1.31 ~ -0.23, p = .005), percentage of forward progression (SMD: -0.95, 95% CI: -1.7 ~ -0.19, p = .014) and percentage of viability (SMD: -0.812, 95% CI: -1.532 ~ -0.093, p = .027). Therefore, obesity affects semen quality to a certain extent, and maintaining normal weight may be one of the effective ways to improve male fertility.

Male adiposity, sperm parameters and reproductive hormones: An updated systematic review and collaborative meta-analysis

The present updated systematic review and meta-analysis aims to summarize the evidence from published studies with low risk for any important bias (based on methodological quality assessment) investigating the potential associations of adiposity with sperm quality and reproductive hormones. We conducted a systematic search of the literature published in MEDLINE-PubMed and EMBASE through June 2019. Based on the criteria in our review, 169 eligible publications were used for data abstraction. Finally, 60 articles were included in the qualitative analysis and 28 in the quantitative analysis. Our systematic review results indicated that overweight and/or obesity were associated with low semen quality parameters (i.e., semen volume, sperm count and concentration, sperm vitality and normal morphology) and some specific reproductive hormones (e.g., inhibin B, total testosterone and sex hormone-binding globulin). Overweight and/or obesity were also positively associated with high estradiol concentrations. Meta-analysis indicated that overweight and/or obesity categories were associated with lower sperm quality (i.e., semen volume, sperm count and concentration, sperm vitality, total motility and normal morphology), and underweight category was likewise associated with low sperm normal morphology. In conclusion, our results suggest that maintaining a healthy body weight is important for increasing sperm quality parameters and potentially male fertility.

Modeling the contribution of the obesity epidemic to the temporal decline in sperm counts

OBJECTIVE

Total sperm count (TSC) has been declining worldwide over the last several decades due to unknown etiologies. Our aim was to model the contribution that the obesity epidemic may have on declining TSC.

MATERIALS AND METHODS

Obesity rates were determined since 1973 using the WHO's Global Health Observatory data. A literature review was performed to determine the association between TSC and obesity. Using the measured obesity rates and published TSC since 1973, a model was created to evaluate the association between temporal trends in obesity/temperature and sperm count.

RESULTS

Since 1973, obesity prevalence in the United States was increased from 41% to 67.9%. A review of the literature showed that body mass index (BMI) categories 2, 3, and 4 were associated with TSC (millions) of 164.27, 155.71, and 142.29, respectively. The contribution to change over time for obesity from 1974 to 2011 was modeled at 1.8%. When the model was changed to represent the most extreme possible contribution to obesity reported, the modeled change over time rose to 7.2%. When stratified according to fertility status, the contribution that BMI had to falling sperm counts for all comers was 1.7%, while those presenting for fertility evaluation was 2.1%.

CONCLUSIONS

While the decline in TSC may be partially due to rising obesity rates, these contributions are minimal which highlights the complexity of this problem.

Early pubertal timing is associated with lower sperm concentration in college students

To study the associations between pubertal timing and semen quality and reproductive hormones, 680 volunteers were recruited from universities in Chongqing, China. Pubertal timing was obtained using a questionnaire. The main measurements were five routine semen parameters and six reproductive hormones. After adjusting for potential confounders, we found that early pubertal timing was associated with lower sperm concentration. An one-year increase in age of peak height velocity was associated with a 4.7% (95% confidence interval [CI] = 1.0 to 8.6) increase in sperm concentration. An one-year increase in age of first spermatorrhea was associated with a 6.4% increase in sperm concentration and a 2.9% decrease in semen volume (95% CI = 1.7 to 11.3, -5.5 to -0.3; respectively). Regarding reproductive hormones, an one-year increase in age of height spurt and peak height velocity was associated with a 6.5% and a 6.7% decrease in estrogen (95% CI = -9.8 to -3.0, -10.4 to -2.8; respectively). While an one-year increase in age of height spurt was associated with higher follicle-stimulating hormone (% change = 2.6, 95% CI = 0.2 to 4.7). This was the first report that has suggested that early pubertal timing is associated with lower sperm concentration. However, further study is still needed to validate this association and fully elucidate the mechanism behind it.

The impact of BMI on sperm parameters and the metabolite changes of seminal plasma concomitantly

The development of male infertility increased rapidly worldwide, which coinciding with the epidemic of obesity. However, the impact of weight abnormalities on sperm quality is still contestable. To assess the correlation between BMI and sperm parameters, we searched relevant articles in PubMed, Embase, Web of science, and Wanfang database published until June 2015 without language restriction. Otherwise, we also recruited some participants who attended fertility clinic as well as some general populations in this report. We performed a systematic review and meta-analysis about BMI and sperm parameters containing total sperm count, concentration, semen volume and sperm motility (overall and progressive). Metabolomic analysis of seminal plasma was performed to explore the mechanism from a new perspective. This study found standardized weighted mean differences (SMD) in sperm parameters (total sperm count, sperm concentration, and semen volume) of abnormal weight groups decreased to different degree compared to normal weight. Dose-response analysis found SMD of sperm count, sperm concentration and semen volume respectively fell 2.4%, 1.3% and 2.0% compared with normal weight for every 5-unit increase in BMI. Metabolomic analysis of seminal plasma showed that spermidine and spermine were likely to play a vital role in the spermatogenesis progress. This systematic review with meta-analysis has confirmed there was a relationship between BMI and sperm quality, suggesting obesity may be a detrimental factor of male infertility.

The effect of metabolic syndrome on male reproductive health: A cross-sectional study in a group of fertile men and male partners of infertile couples

This study aimed to determine the effect of metabolic syndrome (MS) on the reproductive function in fertile (FM) and male partners of infertile couples (MPIC). We performed a cross-sectional study formatting two study groups: partners of pregnant women (n = 238; mean age 32.0) as FM and male partners of infertile couples (n = 2642; mean age 32.6) as MPIC. A standard semen analysis was performed and clinical, laboratory and lifestyle data were analysed. The adapted NCEP-ATPIII criteria were used to define MS. 12.2% of FM and 17.8% of MPIC had MS. In both groups, men with MS were older, they were centrally obese and had higher triglycerides, systolic and diastolic blood pressure and decreased HDL cholesterol values as compared to men without MS. However, glucose concentrations as well as fasting insulin levels were significantly higher only in the MPIC-MS⁺ group. MS was not associated with semen parameters. Testosterone levels were negatively correlated to MS in both groups. This negative association persisted within the BMI categories between MPIC-MS^- and MPIC-MS⁺ groups. LH was negatively correlated to MS but only in MPIC. FSH and oestradiol were not correlated to MS. Smoking and alcohol consumption were higher among men with MS. This study shows that except for testosterone, MS has no independent effect on major fertility parameters in different subgroups of men.

Relationships between total testicular volume, reproductive parameters and surrogate measures of adiposity in men presenting for couple's infertility

The objective of this study was to investigate the relationships between total testicular volume (TTV), reproductive parameters and adiposity measures: body mass index, waist circumference and waist-to-height ratio. Semen analysis was performed, and reproductive hormone levels were measured in 2,672 male patients (mean age 32.6) due to couple's infertility. Significant, positive correlations between semen parameters and the TTV were found. Gonadotrophins were negatively related to the TTV, and testosterone was not related to the TTV. Three anthropometric parameters were negatively correlated to the total sperm count, and sperm concentration seen in men with a TTV of ≤46 ml. In the case of a TTV >46 ml, only the semen volume was inversely correlated with WC and WHtR. These changes occurred from a WHtR ≥0.56, WC ≥102 cm and BMI ≥29 and were more pronounced between WHtR and the TTV. Adiposity was associated with a significant testosterone level decline but did not have a major impact on the gonadotrophin levels. This study shows the divergent results in sperm parameters in different TTV groups in the presence of central adiposity.

Overweight in young males reduce fertility in rabbit model

Semen quality has certainly declined over the past few decades, possibly owing to modern lifestyle factors. In this sense, the role of overweight and obesity in the development of subfertility in males has generated a considerable amount of interest in recent years. However, there is no consensus on whether overweight or obesity impaired sperm quality. Thus, based on the ongoing debate about risk factors for subfertility associated with overweight and obesity in men, this study was designed to investigate the effect of overweight on sperm quality parameters and fertility success in randomized controlled trial in a rabbit model. Fourteen male rabbits were randomly assigned to a control group in which nutritional requirements were satisfied or a group fed to satiety from 12 to 32 weeks of age. At 24 weeks of age, semen samples were analysed weekly by conventional semen analysis for 8 weeks. In addition, during the trial female rabbits were artificially inseminated by each male to assess the fertility success and the number of offspring. Young males fed to satiety were associated with a significant increase in body weight (13.6% overweight) and perirenal fat thickness (5%). Male overweight presented a significant decrease in sperm concentration. There were no differences in the remaining sperm parameters. However, male overweight showed a clear and significant decrease in fertility success (control group, 64±8.9% versus fed to satiety group, 35±9.2%), but not in the number of offspring. Taken together, our findings provide new evidence on the loss of fertility induced by overweight in males.

Causes of male infertility: a 9-year prospective monocentre study on 1737 patients with reduced total sperm counts

STUDY QUESTION

What are the primary causes of severe male factor infertility?

SUMMARY ANSWER

Although 40% of all patients showed primary causes of infertility, which could be subdivided into three groups based on the severity of their effect, ~75% of oligozoospermia cases remained idiopathic.

WHAT IS KNOWN ALREADY

There are few large-scale epidemiological studies analyzing the causes of male factor infertility.

STUDY DESIGN, SIZE, DURATION

A prospective clinical-epidemiological study was conducted at the Andrology Centre, Tartu University Hospital between 2005 and 2013, recruiting male partners of couples failing to conceive a child for over ≥12 months. Among 8518 patients, 1737 (20.4%) were diagnosed with severe male factor infertility. A reference group of fertile controls was comprised of 325 partners of pregnant women.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The mean age of infertility patients and fertile controls was 33.2 ± 7.3 and 31.7 ± 6.3 years, respectively. All participants were examined using a standardized andrology workup, accompanied by a structured medical interview. Hormonal analysis included serum FSH, LH and testosterone. Semen quality was determined in accordance to the World Health Organization recommendations. Cases with spermatozoa concentrations of ≤5 million/ml were screened for chromosomal aberrations and Y-chromosomal microdeletions.

MAIN RESULTS AND THE ROLE OF CHANCE

The primary cause of infertility was defined for 695 of 1737 patients (~40%). The analyzed causal factors could be divided into absolute (secondary hypogonadism, genetic causes, seminal tract obstruction), severe (oncological diseases, severe sexual dysfunction) and plausible causal factors (congenital anomalies in uro-genital tract, acquired or secondary testicular damage). The latter were also detected for 11 (3.4%) men with proven fertility (diagnoses: unilateral cryptorchidism, testis cancer, orchitis, mumps orchitis). The causal factors behind the most severe forms of impaired spermatogenesis were relatively well understood; causes were assigned: for aspermia in 46/46 cases (100%), for azoospermia in 321/388 cases (82.7%), and for cryptozoospermia in 54/130 cases (41.5%). In contrast, 75% of oligozoospermia cases remained unexplained. The main cause of aspermia was severe sexual dysfunction (71.7% of aspermia patients). Azoospermia patients accounted for 86.4% of all cases diagnosed with secondary hypogonadism and 97.1% of patients with seminal tract obstruction. Of patients with a known genetic factor, 87.4% had extreme infertility (azoo-, crypto- or aspermia). The prevalence of congenital anomalies in the uro-genital tract was not clearly correlated with the severity of impaired sperm production. Previously defined 'potential contributing factors' varicocele and leukocytospermia were excluded as the primary causes of male infertility. However, their incidence was >2-fold higher (31.0 vs 13.5% and 16.1 vs 7.4%; P < 0.001) in the idiopathic infertility group compared to controls. In addition, the proportions of overweight (or obese) patients and patients suffering from a chronic disease were significantly increased in almost all of the patient subgroups.

LIMITATIONS REASONS FOR CAUTION

The study included only subjects with reduced total spermatozoa counts. Thus, these findings cannot be automatically applied to all male factor infertility cases.

WIDER IMPLICATIONS OF THE FINDINGS

The novel insights and improved clarity achieved in the comprehensive analysis regarding the absolute, causative and plausible factors behind male infertility, as well as the 'potential contributing factors', will be valuable tools in updating the current clinical guidelines. The study highlights knowledge gaps and reiterates an urgent need to uncover the causes and mechanisms behind, and potential treatments of, oligozoospermic cases, representing the majority of idiopathic infertility patients (86.3%).

STUDY FUNDING/COMPETING INTERESTS

The project was financed by the EU through the ERDF, project HAPPY PREGNANCY, no. 3.2.0701.12-004 (M.P., M.L.) and the Estonian Research Council: grants PUT181 (M.P.) and IUT34-12 (M.L.). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. We have no competing interests to declare. TRAIL  REGISTRATION NUMBER: Not applicable.