(Reuters Health) - Men who wear boxers most of the time may be more fertile than those who opt for jockeys and briefs, a new study suggests.

Among the male partners of couples seeking infertility treatment, researchers determined that tight-fitting underwear was linked to lower sperm counts and sperm concentration and higher levels of a hormone that can indicate trouble in the testicles, according to the report published in Human Reproduction.

The effect wasn’t huge, but, “if I were getting ready to start trying for a baby, I’d go shopping for boxers about three months ahead of time,” said senior study author Dr. Jorge Chavarro of the Harvard T.H. Chan School of Public Health in Boston.

“As a society, traditionally, fertility has been thought of as a female issue. But when you actually look at problems you have to remember it’s a team project. And you can identify problems with the man in about half of the couples and in about a third, it’s exclusively the man.”

Why three months before trying to conceive? While sperm are continuously being produced, it takes about three months for them to become mature, Chavarro said.

The researchers analyzed data from 646 men who were members of couples seeking treatment at the Fertility Clinic at Massachusetts General Hospital between 2000 and 2017. The men, all between the ages of 32 and 39, had also completed a survey that included questions about the style of underwear they had been wearing for the previous three months. Options were: boxers, jockeys, bikini, briefs and “other.”

After adjusting for other factors like age, weight and the time of day the sample was collected, researchers found that men who wore boxers had a 25 percent higher concentration of sperm and 17 percent higher sperm count than those who favored other underwear styles.

Blood samples collected from 304 of the participants also showed that men who wore boxers had 14 percent lower levels of a substance called follicle stimulating hormone, or FSH. The hormone is produced by the brain to tell the testes to make more sperm, Chavarro said. “Higher FSH stimulates sperm production in men, so this suggests the testes were trying to tell the brain there’s a need for rescuing and compensation.”

While men who wore briefs had millions fewer sperm, that wouldn’t be a problem in a man with a normal sperm count, Chavarro said. “We’re talking in the neighborhood of about 20 million sperm difference,” he said. “So if you’re going from 150 million (a normal sperm count) to 130 million it’s probably not going to make a difference. But if you’re going from 50 million to 30 million or from 30 million to 10 million it might. And unless you’re in a fertility clinic you don’t know what your sperm count is.”

The reason briefs may impact sperm counts is that they keep the testes close to the body, Chavarro said. “We’ve known for a while that spermatogenesis is very sensitive to temperature. That’s the main reason they reside outside the abdominal cavity. The testes of men wearing tighter fitting underwear would be exposed to higher temperatures than those wearing looser fitting ones.”

There was some good news: the type of underwear didn’t affect the sperms’ structure or their ability to swim forward.

The study wasn’t designed to prove whether or how snug underwear might affect sperm production. And the authors acknowledge that they lacked information about other factors, like the men’s preferred pants styles and materials, that might influence the results.

Men trying to get their partners pregnant may not have to throw out all their briefs, said Dr. Harry Fisch, a clinical professor of urology and reproductive medicine at the Weill Cornell Medical College in New York City.

“If you’re just walking along in an air conditioned room, it’s probably not going to make much difference between boxers and briefs,” said Fisch, who was not involved in the study. “But if you’re exercising and wearing tighter underwear, you’re probably putting your testicles at risk for heat.”

Heat is the only factor that has consistently been found to affect sperm production, Fisch noted. “That’s why you see seasonal variations in sperm count,” he added. “It’s lower in hotter seasons. That’s also why we tell men not go into Jacuzzis.”

And while the underwear effect is not “dramatic,” Fisch said. “If you’re trying to have a baby you want to be in the best shape of your life and have less adverse things happening.”

SOURCE: bit.ly/2OjsjqI Human Reproduction, online August 8, 2018.