The study found men who wear boxer shorts have higher sperm concentrations than men who wear tighter fitting underwear including very skimpy briefs dubbed a bikini. The simultaneous presence of lower sperm counts and higher FSH among men wearing tight-fitting underwear suggests the presence of a compensatory mechanism.
Unlike similar studies, this was the first to have a narrow focus on semen quality and include information on a variety of indicators of testicular function, such as reproductive hormones and sperm DNA damage.
Researchers, however, also say that the findings of the studies can not be generalised as the study focuses only on men attending the fertility centre. FSH is a hormone that is known to play a role in male fertility. Vij said. "We always struggle in our field to make that leap".
The new findings add to a growing body of knowledge on the subject.
Despite this, studies on underwear choice and its relation with scrotal temperature and male testicular function have been inconsistent.
There was some good news: the type of underwear didn't affect the sperms' structure or their ability to swim forward.
The men, who were between the ages of 32 and 39, completed a survey that included questions about the style of underwear they wore in the previous three months.
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The explanation behind this process seems to be that when sperms gets damaged by a tight fitting underwear, the brain responds by producing more FSH to try and rectify the situation.
But men who wore boxers had 25 percent higher sperm concentrations, and 33 percent more swimming sperm in a single ejaculate, on average, than men who wore tighter underwear.
After accounting for the men's activity, smoking, body mass index and even if the men enjoyed hot baths, the researchers found consistent differences in their semen samples. And interestingly enough, the boxer men also had lower levels of follicle stimulating hormone (FSH), which helps regulate fertility. Although, the hypothesis needs confirmation by further research.
"It may not be possible to generalize out findings to men from the general population", said lead researcher Lidia Mínguez-Alarcón.
"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. In 2016, Eisenberg co-authored a study of 500 men that found that, no matter what type of underwear the men wore, the time it took couples to become pregnant was about the same.
A number of small-scale studies previously have linked tighter underwear to lower sperm counts, Chavarro said.
"It is a simple measure to wear boxer shorts when there is uncertainty regarding fertility", he commented.