Kim Painter|USA TODAY
Boxers or briefs? Men who choose boxers have higher sperm counts than those who favor briefs, according to the largest study ever to look at a long-suspected link between tight underwear and lower sperm production.
The study of 656 men, published Wednesday in the journal Human Reproduction, adds to evidence that underwear choices really do matter. The likely reason: Tight undies slightly raise testicl* temperatures, and that’s bad for sperm.
The difference might be enough to affect fertility in some men who start out with borderline sperm numbers, said the study’s lead author, Jorge Chavarro, an associate professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health.
Those men“might benefit quite a bit from changing the type of underwear they wear,” he said.
The study involved men ages 18 to 56 visiting a fertility clinic at Massachusetts General Hospital in Boston. Researchers collected sem*n and blood samples and asked the men what kind of underwear they usually wore. Just more than half said they mostly wore boxers; the rest wore briefs, bikinis or other tight styles.
The researchers found that men favoring all sorts of underwear produced similar amounts of fluid in an ejacul*tionbut that boxer wearers had 25 percenthigher concentrations of sperm, resulting in total sperm counts that were 17 percenthigher. The boxer wearers also produced 33 percentmore swimming sperm – the ones with the best odds of reaching and fertilizing eggs.
It should be noted that, despite being in a fertility clinic, the men in both underwear groups had adequate average sperm counts, Chavarro said.
And, he said, the sperm produced by brief wearers looked just as healthy as the sperm produced by boxer wearers.
Blood tests also showed no association between underwear choices and testosterone levels.
There was a difference in another hormone, known as FSH, or follicle stimulating hormone. That hormone, produced by the pituitary gland at the base of the brain, stimulates sperm production in men. FSH levels were higher among the briefs wearers, suggesting that the glandwas working to compensate for relatively low sperm numbers, Chavarro said.
Because of the way the study was designed, it cannot prove that tight underwear caused the lower sperm counts. But the researchers accounted for other factors that might matter, such as obesity, smoking and hot tub use, all of which can lower counts.
Some, but not all, previous studies have suggested links between tight underwear and impaired sperm production. In one study, nine healthy volunteers had better sperm quality when they wore boxers for six months than when they wore briefs for six months.
Other studies have linked heat exposure with lower sperm counts. And biology suggests sperm like it cool: It seems to be the reason the scrotum exists outside the warmer body cavity, Chavarro said.
The new study “is very rigorous,” but more research is needed to establish a link firm enough to change the way doctors advise men on fertility, said Michael Eisenberg, an associate professor of urology at Stanford University School of Medicine.
Eisenberg, who was not involved in the study, said that right now “I generally counsel men that they can choose the type of underwear that is most comfortable. But if they do feel warm or constricted, then another type may make sense.”
Chavarro said he sees switching underwear as an easy, harmless thing any man could do if he is trying to become a father.
“If they don’t like it, they can switch back afterward,” he said. “For most men, most of the time, it doesn’t make a difference, because most men most of the time are not trying to get someone pregnant.”
