If you’re packing a pair of testicles, you’ve likely heard that keeping your scrotum and its precious cargo from getting too overheated is essential for maintaining healthy sperm. This proven link between temperature and fertility is often the linchpin of pro-boxer arguments presented during debates with briefs wearers. And now, a new study by Harvard TH Chan School of Public Health adds more fuel to the long-running underwear superiority war by confirming that tightly fitting gonad garments are associated with reduced sperm concentration and counts using the largest-ever cohort of reproductive age men. Though the physiological mechanisms behind this pattern remain mysterious, the authors also discovered that men who wore briefs, "jockeys" (tight but long undies), or 'bikinis" (tight and very short undies) had lower blood levels of follicle-stimulating hormone (FSH) – a chemical messenger involved in spermatogenesis that is produced in the brain’s pituitary gland. "An important strength of this study is that we were able to investigate the potential relationship between the type of underwear worn and indicators of testicular function such as reproductive hormone levels and DNA damage, which were missing in all previous studies on the topic,” Dr Lidia Mínguez-Alarcón, first author of the study published in Human Reproduction, said in a statement. She and her colleagues note that numerous investigations have tried to directly connect temperature differences from types of underwear to reproductive health, but the results have been inconsistent. “Because of this, we were able to find a potential compensatory mechanism whereby decreased sperm production relating to the type of underwear signals to the hypothalamus to increase secretion of gonadotropin, [hormones that act] on the testes and that is reflected by the increased levels of FSH, to try to increase sperm production.” The team’s data was drawn from analyses of 1,186 semen samples from 656 men who, alongside their partners, were seeking fertility treatment at Massachusetts General Hospital between 2000 and 2017. Of this group, 304 men also had a one-time blood sample collected so that physicians could assess levels of FSH, estradiol (a type of estrogen), and luteinizing hormone (the other gonadotropin hormone involved in reproductive cell development). Overall, men who wore boxers – 53 percent of the group – had 25 percent higher sperm concentrations, 17 percent higher sperm counts, and 14 percent lower FSH levels than men who wore tighter undergarments, after adjusting for several factors known to influence sperm: age, BMI, smoking history, and the man’s self-reported time window between the sample’s collection and his last ejaculation. Because circadian rhythms are known to change the quality of sperm over the course of a day, the authors concede that their results were unavoidably impacted by the fact that 67 percent of participants only submitted one sample. Similarly, circadian variation in hormone production and the finding that men who wear boxers are more likely to take hot baths and lounge in Jacuzzis – an amusing revelation that possibly merits its own study – may have confounded the data, although the authors tried to adjust the statistics for these factors. Moreover, the findings can’t be extrapolated out to the general public because all the subjects were patients at a fertility clinic. Moving forward, Dr Mínguez-Alarcón hopes to confirm the molecular mechanism behind hormone levels and testicular damage from underwear and to see if this pattern also emerges in more diverse groups of men. However: “At this point, we would recommend wearing boxers or looser underwear compared to tighter underwear,” she told IFLScience, “since it is a modifiable lifestyle that could easily improve the semen quality of the men. Source
