For men, the choice to wear boxers or briefs might be a decision based on comfort, but a new Harvard study suggests there could be another reason to ditch the tighty-whities, particularly if you're trying for a baby.
The research, published in the journal Human Reproduction, found that men who wear boxer shorts have higher sperm concentrations than those who wear more tight-fitting underwear.
"These results point to a relatively easy change that men can make when they and their partners are seeking to become pregnant," said lead author Dr Lidia Mínguez-Alarcón.
In the largest study of its kind, 656 male partners of couples seeking infertility treatment at the Massachusetts General Hospital, between 2000 and 2017, provided semen and blood samples. Participants were also questioned about the type of underwear they wore most frequently in the three months prior: 'boxers', 'jockeys', 'bikinis', or 'briefs.'
Why the focus on underwear? While previous studies have found that "elevated scrotal temperatures" has a negative impact on testicular function, research into the type of jocks men wear (as a way to determine scrotal temperature), and male testicular function has been inconsistent. A 2012 paper published in Human Reproduction, found not wearing loose underwear was related to low-motile sperm concentration. In contrast, a 2016 study published in Andrology found that male underwear choice was associated with few differences in sperm, and had no association with time-to-pregnancy.
For Dr Mínguez-Alarcón and her team, however, the association between underwear choice and sperm quality was clear.
Men who primarily wore boxers had 25 per cent higher sperm concentration, 17 per cent higher total sperm count, 33 per cent more "swimming sperm" in a single ejaculate and 14 per cent lower follicle-stimulating hormone (FSH), (which stimulates sperm production), than those who wore more tight-fitting undies. This occurred even after the researchers took into consideration other factors that might have affected the results including BMI, physical activity, taking hot baths, spending time in jacuzzis and smoking.
The greatest difference in sperm concentration was found between men wearing boxer shorts most frequently and men wearing jockeys or briefs.
"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," says Dr Mínguez-Alarcón. "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, a hormone that acts on the testes and that is reflected by the increased levels of FSH, to try to increase sperm production."
In other words, the researchers believe that FSH, which stimulates sperm production, "kicks into gear" when it needs to compensate for testicular damage from increasing scrotal temperatures and decreasing sperm counts and concentration. (And that's why it's lower in men who wears boxers.)
"Beyond providing additional evidence that underwear choices may impact fertility, our study provides evidence, for the first time, that a seemingly random lifestyle choice could have profound impacts on hormone production in men at both the level of the testis and the brain," said co-author Jorge Chavarro.
It's important to note that the study has one clear limitation. As it only included men from couples who were actively seeking fertility treatment, the findings may not be applicable to men from the general population. That said, the researchers note that the men tended to have good semen quality when compared to the World Health Organization's reference standards.
In addition, the study doesn't prove that type of underwear causes differences in semen quality and levels of the sperm producing hormone, only that there's a link between the two. According to the researchers, other factors that might affect "scrotal heat" including wearing skinny jeans or different types of underwear fabric, (which weren't part of the study) could also impact the results.
Nevertheless, according to Dr Mínguez-Alarcón, the take home message is clear: "Since men can modify the type of underwear they choose to wear, these results may be useful to improve men's testicular function."