USTC study of 21,221 and 5,615 men links tight underwear to lower semen quality

Researchers led by urologist Xue-Chun Hu from the University of Science and Technology of China (USTC) compared semen quality in 21,221 men who reported wearing loose underwear and 5,615 men who wore tight underwear, ScienceAlert reports. All participants were seeking fertility assessments at The First Affiliated Hospital of USTC, and their median age was 30. The results were published in the Asian Journal of Andrology. sciencealert.comScienceAlertsciencealert.comOpen source ↗

Semen measures and underwear type

Participants reported which underwear they had worn most often in the previous 3 months: "loose (boxer)" or "tight (Y-front, thongs, and boxer briefs)". After adjustment for other health factors, tight underwear was associated with modest reductions in semen quality and quantity. sciencealert.comScienceAlertsciencealert.comOpen source ↗

Compared with loose underwear, tight underwear was linked to 18 percent higher odds of lower semen volume and 12 percent higher odds of fewer motile sperm. The odds of fewer normally shaped sperm were 48 percent higher, and the odds of more sperm with DNA fragmentation were 108 percent higher. sciencealert.comScienceAlertsciencealert.comOpen source ↗

High sperm DNA fragmentation was found in 10.7 percent of men in tight underwear and 5.8 percent of men in loose underwear. Most men in both groups therefore did not have abnormally high levels. In an exploratory secondary analysis, erectile dysfunction and premature ejaculation were more prevalent in the tight underwear group. The authors describe these findings as preliminary. sciencealert.comScienceAlertsciencealert.comOpen source ↗

Heat hypothesis and study limits

The authors suggest scrotal heat stress may impair spermatogenesis. Sperm cells develop best at approximately 2–3 °C below core body temperature, and tight briefs reduce breathability and can raise scrotal temperature by up to 1 °C compared with looser underwear. The authors also hypothesize that prolonged use of tight underwear can reduce testosterone secretion. sciencealert.comScienceAlertsciencealert.comOpen source ↗

The study cannot prove that tight underwear reduces sperm quality or quantity, or that looser underwear helps conception. Participants were men seeking help for fertility problems, so the results cannot be generalized to the whole population, as biomedical scientist Tim Moss noted in 2022. sciencealert.comScienceAlertsciencealert.comOpen source ↗

A 2018 Harvard study of 656 men found higher sperm concentrations and total sperm counts in those who most often wore boxers. The new analysis is more than 40 times larger. A 2022 analysis of 3,041 couples in the US or Canada found little evidence that the probability of conception is tied to scrotal heat, including tight-fitting underwear. sciencealert.comScienceAlertsciencealert.comOpen source ↗