Researchers have known for a while that sperm count and quality are declining at alarming rates in westernized countries. But what, exactly, is the cause?
Over a half-century, researchers have tracked a steady, consistent drop in sperm counts among men in Westernized countries. This isn’t news to many, particularly since a much publicized 2017 review of research illustrated the decline in sperm among men in North America, Europe, Australia, and New Zealand. But why are sperm counts dropping? Experts still debate that. Or at least, they debate the most significant factors contributing to the decline.
Most experts agree that even if you have a low sperm count, you’re probably still able to conceive a baby. The bad, or more complicated, news is that decreasing sperm count is what the authors of the 2017 study, published in Human Reproduction Update, have called a “canary in the coal mine” in terms of men’s overall health. That’s because low sperm counts are associated with a greater risk for health problems such as cancer and earlier death. In addition, although humans probably won’t die out in our lifetime (at least, not because of low sperm counts), the decline does have real-world consequences for men today.
“It’s true you only need one sperm to conceive a baby,” says epidemiologist Shanna H. Swan, Ph.D., professor at the Icahn School of Medicine at Mount Sinai, one of the authors of the aforementioned Human Reproduction Update study and author of Count Down: How Our Modern World Is Threatening Sperm Counts, Altering Male and Female Reproductive Development, and Imperiling the Future of the Human Race. “Even with a low sperm count, it’s possible. But the number of months you have to try has gotten longer and longer.”
Couples are infertile, generally, if they’ve been trying to have a baby — having sex regularly without using contraception — for a year without success. Currently, nearly one in seven couples has trouble conceiving, the Mayo Clinic notes. Decades ago, it was assumed it was the woman’s fault if couples couldn’t conceive, but now doctors say fertility issues relate to women one-third of the time and men for another third. The rest are undetermined or might be some combination of fertility issues in both parents.
A man’s sperm count is low if he produces less than 15 million sperm per milliliter of semen. Fifteen million probably sounds like a lot when it only takes a single sperm to fertilize an egg. But a good illustration of how widespread this drop is, as well as how quickly sperm counts are falling, is to look at what sperm banks accept, Swan says. She points to a study that found the percentage of acceptable sperm among donor applicants in Boston went from 69% in 2003 to a 44% acceptance rate in 2013, according to research presented at a meeting of the American Society for Reproductive Medicine in 2015. (“Acceptable” samples have to contain enough sperm that swim properly and show no DNA damage, she says.) That’s a 25 percentage point drop in acceptance rates in 10 years.
Some doctors, however, say that sperm counts are dropping partly because people are having children later in life, and therefore older men are showing up in fertility clinics, where some researchers get their data. Swan strongly disagrees with this, pointing out that the study she co-authored included many young men — college students and those applying for military service — who had no idea what their sperm counts were.
Nevertheless, as reproductive medical technologies continue to advance, it’s allowing many people to procreate who previously wouldn’t have been able to, says New York City urologist Alex Shteynshlyuger, MD. “We’re getting better and better at defying biology and revising what we call natural selection,” Shteynshlyuger says. “Which is wonderful for many couples, but the downside of that contributes to the next generation having lower fertility.”
How big an impact expensive, high-tech fertility treatments are having on sperm counts is difficult to quantify. There are many other, and overlapping, factors that experts say are contributing to declining sperm counts. Parsing out causes is extremely difficult because they’re not separable, Swan says. They’re a mix of environmental factors that are difficult to avoid and lifestyle changes you can make to increase your sperm count.
“Many people are older, fatter, and more stressed, and all of those things to do affect sperm count,” Swan continues. “Lifestyle factors do matter — I’m not poo-pooing them at all. But these changes are unlikely due to genetics because it’s too fast. You don’t see changes like this in just 10 years. Then you’re left with chemicals.”
How Hormone-Disrupting Chemicals Could Affect Sperm
Industrialization has had an impact on our diet, how we package food, and has ushered in the widespread use of plastics, says urologist Bobby B. Najari, MD, MSc, assistant professor at New York University Grossman School of Medicine and director of Male Infertility at NYU Langone Health. The chemicals used to make plastics hard or soft are found in innumerable products we use every day, such as thermal receipts, water bottles, the lining of cans, plastic food packaging, detergents, flooring, and personal care products such as shampoo, soap, and nail polish.
“I do believe all those play a potential role in lower sperm counts and declining sperm quality,” Najari says. “Environmental exposure to certain chemicals affects how the body expresses the hormonal signals to produce sperm. Part of it is within your genetics, and then there’s also the regulation of that genetic information.”
Chemical exposure can affect men’s fertility via two avenues, Swan says: Prenatal exposure, when he is a fetus in utero, and exposure postnatally, or after he’s born. If a pregnant person smokes, for example, research has suggested that as an adult, their son may have a 40% lower sperm count, Swan says.
“The other scenario is if he smoked, then it’s lowered by 15%,” Swan says. “But if he stops, he can recover his sperm count.”
Of course, smoking cigarettes isn’t the only way chemicals can affect fertility. Before the 1950s, production of plastics, pesticides, and PCBs was close to zero, Swan says. In addition to bisphenol-A (BPA) and phthalates, other classes of chemicals are suspected endocrine (or hormone) disruptors, including flame retardants, surfactants, waterproofing, and Teflon, Swan says.
There’s robust research supporting a link between in utero exposure to known endocrine-disrupting chemicals — such as BPA and phthalates — and later fertility problems in people (and animals). Put very simply, hormone disruptors scramble the signals of hormones, suppressing healthy function or speeding up functions in unhealthy and unhelpful ways.
Sperm cell development in embryos is a delicate process that has to be done right, Swan says. During the short window in which it takes place, the mother needs to have enough testosterone present. When mothers are exposed to chemicals that can lower her testosterone, she might not have enough testosterone at the right time to complete the process of proper sperm development.
Adults’ hormone function can be scrambled by chemical exposure as well. “When released by products, these chemicals make their way into our bodies where they’re going to be processed and affect body functioning,” Swan says. “There needs to be a new class of chemicals not hormonally active and that do not persist in the environment.”
Testosterone levels have decreased plus or minus around 1% per year, and miscarriages are going up at the same rate, Swan says.
“It’s a pretty consistent picture,” she continues. “It’s not just sperm counts and not just men, and not just humans either. It’s a global problem.”
Although there has been a movement to ban certain endocrine disruptors, not all countries have the same bans in place or even ban the same phthalates, says Chris Airey, director of a testosterone replacement therapy clinic in the UK. What’s worse, he says, these chemicals remain in our environments for years and years, so even if they were banned universally, their effects could persist for many years to come.
Could Smoking Weed Harm Sperm Count?
There are myriad cannabis-based products claiming to enhance sexuality, but weed consumption appears unhelpful for couples wanting to get pregnant. Cannabis use has been linked to poorer quality sperm, meaning sperm are not good swimmers and less likely to reach and fertilize an egg, a 2018 study noted. The authors of an earlier study published in the American Journal of Epidemiology came to the same conclusion.
Najari includes questions about smoking marijuana as well as cigarettes on his patient intake forms and says that, anecdotally, there appears to be a correlation between pot and infertility.
“It’s interesting because I have a co-worker from Jamaica who has told me, ‘Look, if marijuana had a negative effect on fertility, I would’ve seen it where I’m from,’” he says. “And there is conflicting scientific evidence, but in my practice, I definitely see an impact.”
There might be a difference between men who abuse marijuana and those who use it more recreationally, he says. And whether marijuana use affects sperm count also probably depends on the sperm count a person already has, Najari says.
“What I tell couples about using it is that for men whose sperm production is robust, a small negative hit from being exposed to marijuana might not have a profound effect. But for some men whose sperm production is more fragile or tenuous, any type of potential insult can lower their count,” Najari says.
Illicit drugs and alcohol use might also depress sperm count. Decades of studies have suggested a link between alcohol misuse and low sperm counts and many other medications might depress counts as well, Najari says.
“This is not just anecdotal — testosterone supplementation is relatively common, and it’s well documented to have a very negative effect on sperm production,” Najari says.
Certain antidepressants, an incredibly common class of medications, appear to negatively affect sperm count as well, he adds.
The Low Sperm Count and Obesity Connection
Obesity is a very significant factor in declining counts, Najari says. “Part of it is poor diet,” he says, “but resulting obesity itself also has a negative impact.”
Adipose tissue, or fat, converts testosterone into estrogen, he explains. In men, estrogen is involved in how the reproductive tract is regulated by the pituitary gland. The pituitary gland doesn’t know how much testosterone is circulating, so it uses estrogen as a surrogate to get a sense of how much there is. If a man is obese, he’s converting more of his testosterone into estrogen. So the pituitary gland thinks there’s more testosterone than there actually is, which then dampens the gland’s signal to testicles to tell them to make testosterone and make sperm.
“In addition to lower testosterone and lower sperm count, we think excess adipose tissue causes a somewhat inflammatory state, and any kind of inflammatory state can suppress sperm production,” Najari says.
The Role Diet Plays in Low Sperm Count
Dairy products might adversely affect sperm’s ability to swim properly, and full-fat dairy products in particular appear to have a negative effect on sperm quality, according to a study published in 2013.
“When it comes to dairy, I say to stick to organic and hormone-free products if you can afford it,” Najari says. “The data is relatively sparse on this, but to me, it intuitively makes sense to minimize exposure to those types of chemicals, both from a fertility and overall health standpoint.”
Shteynshlyuger says there isn’t a lot of solid evidence regarding how particular nutrients might affect fertility. But we know certain diets tend to affect the body in certain ways, he says. “For example, high-fat animal diets are more likely to cause free radical issues and inflammatory changes in the body that can affect sperm quality,” he says.
Making healthy changes to your diet is worth doing for your health. But, he says, it’s unlikely to affect sperm count after a short period. In general though, a Mediterranean diet is associated with better sperm counts, Swan notes.
“If you’re eating healthy foods, you tend to be doing a good job eating unprocessed foods and avoiding the higher amounts of hormone disruptors in processed foods,” she says.
Some doctors are more optimistic that healthy dietary changes can in fact improve sperm counts relatively swiftly. “In general, a diet that’s good for the brain, heart, and belly is good for the sperm, too,” says John La Puma, M.D.
The Link Between Stress and Low Sperm
“Little things can propagate,” Shteynshlyuger says. “Not getting enough sleep can be a factor, but it’s more complicated than just not getting a normal amount of rest. Working too much and too hard can cause hormonal changes that are detrimental to overall health, including fertility. Stress hormones can be elevated, and if it’s chronic, it can affect other hormones in the body.”
What Else to Know
There are medical issues that could be affecting your sperm count too, which a urologist can identify and possibly treat. Urologists can also figure out if you might be at risk for other health problems associated with low sperm count. There’s increasing awareness of that link — the most recent American Urological Association and American Society for Reproductive Medicine guidelines now recommend that urologists urge men to see primary care physicians as well if their sperm counts are low, Najari says.
“There’s a large amount of epidemiologic data that correlates low sperm count with other health outcomes, and enough data that we should be mentioning this to our patients, they said,” according to Najari. “I wouldn’t say low sperm counts are the cause, but it could be more of a marker of poorer overall health. If you have low sperm count and don’t have a primary care doctor, you ought to establish care and be mindful you’re at higher risk of having other issues as well.”
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