Scientists Puzzle Over Declining Sperm Counts; a 'Crisis' or Not Enough Data?
Experts here for the European Society of Human Reproduction and Embryology annual conference last week debated the issue for an entire day.
Are today's young men less fertile than their fathers were? It's a controversy in the fertility field, with some experts raising the alarm over what some are calling a "sperm crisis" because they believe men's sperm counts have been decreasing for a decade or more.
One recent analysis found that in France, the sperm concentration of men decreased by nearly one-third between 1989 and 2005. Most but not all studies from several European nations with large databases and the ability to track health records have found that over the past 15 years or so, the counts of healthy men ages 18 to 25 have significantly decreased. This comes after a prominent study from the 1990s suggested that sperm count has decreased by half over the last half-century.
Many experts questioned the validity of those findings. There are huge variations in results by country and region. Certain areas, especially in the developing world, haven't been studied at all. In the U.S., some historical data suggest a decrease in sperm count among American men, but no published recent data exist.
Understanding if men are producing less sperm has implications beyond male fertility and couples who want children. The same environmental factors that might harm reproduction may also impact other parts of the body. Sperm count has even been linked with life expectancy, independent of cause of death.
"It's a public health indicator," says Joëlle Le Moal, an epidemiologist at the French Institute for Public Health Surveillance near Paris.
Proponents say that exposure to pesticides, endocrine-disrupting chemicals like Bisphenol A and lifestyle habits like sitting for too long contribute to the proposed sperm crisis. And there is increasing evidence that sperm count, like other health conditions, may be influenced by what happens to people early in life, even in the womb.
"If our gametes, male or female, are not produced in the right manner, it could impact the next generation's health," says Dr. Le Moal.
In general, men produce upward of 60 million sperm per milliliter of semen. As long as the count is roughly greater than 40 million per ml, men are considered fertile and have the same chance of getting their partners pregnant as someone who produces a higher count.
But below that threshold and particularly under about 20 million per ml, their ability to help conceive drops. It may take a couple longer than a year to conceive—a problem known as subfertility—or they may not succeed at all.
Not everyone in the field agrees a sperm crisis exists. Critics say that sperm concentration in a population is incredibly difficult to measure for a number of reasons, including the primary one of how to find men representative of the population. Tracking men who show up at a fertility clinic, for instance, would skew the results one way. Neither do sperm donors accurately stand for all the men in a region. Some recent studies have examined young men applying for military service, though the participation rate in the research tends to be low.
In addition, there could be errors in measurement because of any number of immediate environmental factors, including the length of abstinence, scrotal temperature and time of year.
Stefan Schlatt, director of the Center for Reproductive Medicine and Andrology at the University of Münster in Germany, says he doesn't believe there is a sperm crisis world-wide. Even if sperm count decreased over time, it's unclear how many more men fall into the category of subfertile or infertile, Dr. Schlatt says.
But coordinated studies across Northern Europe found that as many as 1 in 5 young men have sperm counts low enough to affect fertility, according to Richard Sharpe, a male reproductive health specialist at the University of Edinburgh.
The key may be Sertoli cells, which support the testes' germ cells during their 10-week development into becoming sperm. Each Sertoli cell can only support a certain number of germ cells that produce sperm, so the number of Sertoli cells essentially caps a man's maximum production of sperm.
Sertoli cells appear to proliferate in the six months before and after a male child's birth. While outside factors can decrease sperm production, there don't appear to be any that can increase the production above the limit set by the number of Sertoli cells, Dr. Sharpe says.
In Dr. Le Moal's study, which she presented last week, she and her colleagues examined data collected by a professional organization in charge of artificial reproductive technique statistics in France, known as Fivnat. The sample consisted of over 26,000 male partners of women who were known to be infertile and going through infertility treatments at clinics around the country from 1989 to 2005. The men weren't known to be infertile, and therefore were considered a more representative sample than studies that focused on men who were seeking treatment for infertility.
The men's semen quality was tested two times in separate laboratories and measured for sperm number, vitality and shape. The researchers averaged the two samples to get a more reliable figure to take into account age and season. In addition, the method of measurement, especially for the sperm's number and vitality didn't change substantially across France during this period, which is a common criticism of such long-term studies, Dr. Le Moal says.
They found that during the 17-year study period, men's sperm concentration decreased by nearly a third, or 1.9% a year. In 1989, the average level of sperm for a 35-year-old man was 73.6 million per ml, well above the subfertile threshold, while in 2005 it had dropped to 49.4 million per ml. The results were published in December in the journal Human Reproduction.
Accumulating evidence suggests that early life influences make a difference. Some researchers say that there is a vulnerable period, perhaps between eight and 14 weeks of gestation, in which influences are irreversible. One of the most robust links with decreased sperm count is maternal smoking during pregnancy.
In a study known as Raine of nearly 2,000 males enrolled as babies and now in their early 20s, the size of the testicles was related to total sperm count, and sperm count was more likely to be lower in boys who were born small for gestational age, who were overweight or underweight in childhood or whose mothers smoked during pregnancy.
The male's own current marijuana use was also linked to lower sperm count, according to data presented at the conference by Roger Hart, a professor of reproductive medicine at the University of Western Australia.
Some factors that impact sperm may be reversible. Others have found that how much belly fat men have, even those not overweight, is linked with testosterone levels in young men.
"By adopting a healthy, balanced diet and lifestyle in pregnancy, you can give your developing baby the very best start in life which will minimize the risk of future diseases," Edinburgh's Dr. Sharpe says. "If it's a boy, then this will also make sure that his sperm count, and thus his fertility, is not compromised in any way."
