Improving Male Sexual Health, Function & Fertility
Summary
Dr. Michael Eisenberg, a Stanford urologist and leading expert in male sexual and reproductive health, discusses the science behind sperm quality, testosterone levels, erectile dysfunction, and fertility. The conversation covers what causes decline in these metrics, what men can do to maintain or improve them, and how reproductive health serves as a broader barometer of overall male vitality. Many popular myths are addressed and dispelled with reference to current clinical and research evidence.
Key Takeaways
- Sperm quality and testosterone levels appear to be declining over recent decades, though the causes remain debated — leading candidates include chemical exposure (phthalates, BPA), obesity, and sedentary lifestyles.
- Male factors contribute to roughly 50% of infertility cases in couples, yet men are bypassed in fertility evaluation approximately one-third of the time.
- Exogenous testosterone therapy significantly reduces sperm production — at least 1 in 20 infertile men seen clinically are infertile because of testosterone therapy.
- Semen quality is a “sixth vital sign” — men with higher semen quality tend to live longer, have lower cancer rates, and visit doctors less frequently.
- Heat is the primary enemy of sperm production — hot tubs, laptop heat, prolonged cycling, and insulation from obesity all raise scrotal temperature and reduce spermatogenesis.
- Step count and physical activity are independently associated with higher testosterone, even after controlling for BMI.
- Getting 7–9 hours of sleep nightly is associated with optimal semen quality; both too little and too much sleep correlate with lower parameters.
- Testosterone therapy does not increase prostate cancer risk, likely due to the “saturation model” — once testosterone receptors in the prostate are saturated, additional testosterone has no further stimulatory effect.
- Paternal age matters — average paternal age in the US has risen from ~27.5 to ~31 years over the last four decades, with associated implications for offspring health risks including autism.
- A baseline semen analysis is worth doing for men in their 20s–30s, even without fertility concerns, as a general health marker.
Detailed Notes
Semen Quality: What It Is and Whether It’s Declining
- A standard semen analysis measures: ejaculate volume, sperm concentration, motility, and morphology. Advanced testing includes DNA fragmentation and epigenetic profiling.
- The World Health Organization periodically updates reference ranges for normal versus subfertile parameters.
- A landmark 1990s Danish study suggested a ~50% decline in sperm quality over 50 years, but this remains controversial.
- A more recent global meta-analysis of tens of thousands of men across Africa, Asia, and Western countries also supports a decline.
- The only longitudinal tracking study (Denmark, ~2000 onward) found that semen quality was stable over ~20 years — but only ~25% of Danish men had normal semen quality to begin with.
- Obesity explains only ~10% of the purported decline, suggesting additional environmental or chemical contributors.
Testosterone: Levels and Decline
- Normal testosterone range: approximately 300–900 ng/dL in serum; levels inside the testicle are roughly 10-fold higher.
- Testosterone peaks in the early 20s and declines approximately 1% per year thereafter.
- Well-designed cohort studies and national survey data (NHANES) show declining testosterone levels across generations when age is matched.
- Obesity lowers testosterone via two mechanisms:
- Peripheral fat tissue contains aromatase, converting testosterone to estrogen.
- Enlarged thighs insulate the testes, raising scrotal temperature and reducing production efficiency.
- Despite these trends, there is high individual variation — some men in their 80s and 90s maintain testosterone levels comparable to men in their 30s.
- You cannot predict testosterone levels by appearance alone; objective testing is necessary.
Fertility: Male Contribution and Evaluation
- Male factors contribute to approximately 50% of infertility cases.
- IVF with intracytoplasmic sperm injection (ICSI) requires only a single viable sperm, which has reduced the clinical urgency to identify and treat male-factor infertility.
- Without assisted reproduction, achieving pregnancy typically requires 20–40 million motile sperm.
Testosterone Therapy and Sperm Production
- Exogenous testosterone suppresses the hypothalamic-pituitary-gonadal (HPG) axis, reducing LH and FSH, which are necessary to stimulate:
- Leydig cells → testosterone production
- Sertoli cells / seminiferous tubules → sperm production
- Testosterone therapy has been studied as a male contraceptive due to its reliable suppression of sperm production.
- At least 1 in 20 infertile men presenting clinically are infertile due to testosterone therapy.
- HCG (human chorionic gonadotropin) at 500–1,000 IU every other day is used to maintain sperm production in men on testosterone therapy by mimicking LH.
- Sperm cryopreservation before starting testosterone therapy is recommended for reproductive-age men.
Lifestyle Factors Affecting Sperm and Testosterone
Avoid:
- Heat exposure: hot tubs, saunas (without scrotal cooling), laptops on lap, seat warmers
- Cycling >5 hours/week associated with lower sperm counts; standing up in the saddle helps dissipate heat
- Cold packs designed to be worn in shorts during sauna use may offset heat effects
- Smoking: consistently linked to lower semen quality across all parameters
- Cannabis: daily use associated with significantly lower sperm concentration, motility, and morphology in a study of ~1,200 men
- Heavy alcohol use: effects seen above ~20 drinks/week; moderate drinking not clearly harmful
- Note: individuals with the ALDH2 mutation (common in East Asians, ~40–50% prevalence in some populations) may be more susceptible to alcohol-related sperm harm due to impaired acetaldehyde metabolism — flushing is a clinical signal
- Opioids and illicit drugs: associated with lower semen quality; data limited but clinical consensus is avoidance
- Sedentary behavior / obesity: strongly linked to declining testicular function
Beneficial:
- Increased step count and physical activity: independently associated with higher testosterone across BMI categories
- Maintaining healthy body weight
- 7–9 hours of sleep per night: U-shaped relationship with semen quality; both insufficient and excessive sleep correlate with lower parameters
- Regular sexual activity / erections: maintaining nocturnal penile tumescence supports penile tissue health (“use it or lose it”)
Cycling and Male Sexual Health
- Prolonged saddle pressure compresses the pudendal neurovascular bundle between the ischial tuberosities, reducing blood flow and nerve supply to the penis.
- Symptoms: post-ride numbness, temporary erectile dysfunction lasting hours to a day or two.
- Approximately 20–30% of cyclists are susceptible.
- Recommendations:
- Proper saddle fit at a professional bike shop
- Saddles with a cutout or split-nose design
- Standing in the saddle periodically during long rides
- Limiting cycling to under 5 hours/week if fertility is a concern
Prostate Health and Erectile Function
- Nocturnal penile tumescence occurs multiple times per night, predominantly during REM sleep, which increases in proportion toward morning.
- Waking erections are one sign, but absence of waking erections does not confirm absence of nocturnal erections.
- Urinary symptoms to evaluate: waking more than once per night, weakening stream, urinary frequency. Once per night is considered normal.
- Testosterone therapy does not increase prostate cancer risk, supported by decades of longitudinal data. The saturation model explains this: prostate androgen receptors become saturated at relatively low testosterone levels.
Paternal Age and Offspring Health
- Average paternal age at birth in the US has increased from ~27.5 to ~31 years over the past 40 years.
- Older paternal age is associated with increased risk of certain conditions in offspring, including autism spectrum disorder.
- **Sperm cryopreserv