Men who wear boxer shorts tend to have higher sperm counts and better quality sperm than those who wear tight-fitting jockeys and briefs, Harvard University researchers have found.
“These results point to a relatively easy change that men can make when they and their partners are seeking to become pregnant,” said Lidia Mínguez-Alarcón, the study’s lead author and a research scientist at the Harvard T.H. Chan School of Public Health, in a released statement.
The study, published earlier this month in the journal Human Reproduction, is the latest in a string of epidemiological studies that have looked at whether there is a link between men’s underwear choices and sperm quality. The results of those studies have been inconsistent.
Still, other research has shown that when the temperature of the scrotum becomes elevated, sperm production is reduced. So the idea of a link between tighter underwear — which can raise the temperature of the scrotum — and sperm production is not far-fetched.
Furthermore, if such a link is confirmed, it will have practical implications for couples having difficulty conceiving. After all, choice of underwear is an infertility risk factor that’s easy to modify.
As background information in the study points out, male infertility is on the rise. The United States and other developed countries have seen a decrease in sperm counts during both the 20th and 21st centuries. Some studies have also reported a decrease in testosterone levels among men.
Many factors are believed to be involved, including increased exposure to endocrine-disrupting chemicals, higher rates of obesity and less healthful diets. But elevated scrotal temperatures are also on that list.
The current study is the largest one to date to look at underwear and semen quality. For the study, Mínguez-Alarcón and her colleagues analyzed semen samples collected from 656 men (median age: 35) who were part of couples seeking treatment at a Massachusetts fertility clinic between 2000 and 2017.
The men also filled out a questionnaire in which they were asked what style of underwear they had most frequently worn during the previous three months. The options were boxers, jockeys, bikinis, briefs or “other.”
More than half — 53 percent — of the participants said they primarily wore boxers. When the researchers analyzed the semen samples, they found some significant differences in the samples, based on the kind of underwear worn.
When the sperm of the boxer-wearers was compared to that of the men who wore all the other types of underwear, it was found to have, on average:
- a 25 percent higher sperm concentration (the amount of sperm found in a milliliter of the sperm sample)
- a 17 percent higher total sperm count (the amount of sperm found in the entire sample)
- a 33 percent total motile count (the ability of the sperm to move through the female reproductive system and fertilize an egg)
The most significant difference, say the researchers, was between the men who wore boxers and those who wore jockeys and briefs.
The researchers also analyzed blood samples taken from 304 of the study’s participants for levels of follicle stimulating hormone (FSH), which is known to play a role in male fertility. The FSH levels of the men who wore boxers was 14 percent lower than that of the men who wore the other underwear.
This finding suggests, say the researchers, that men’s bodies have some kind of mechanism that compensates for the lower sperm levels that result from higher scrotal temperatures caused by wearing tight-fitting underwear — a mechanism that sends signals to the brain to increase FSH production.
Limitations and implications
This study comes with several caveats. Most notably, the study is observational, which means it shows only an association between tight-fitting underwear and lower sperm count and motility, not a cause-and-effect. Although the researchers adjusted the study’s data to account for several factors that can affect sperm, such as age and weight, other factors not adjusted for — such as the type of trousers worn by the men — might also explain the results.
Another limitation is that study included only men in couples that had sought treatment from a fertility clinic, so they may not be representative of men in the general population.
Also, as Jorge Chavarro, the study’s senior author and an associate professor of nutrition and epidemiology at Harvard, pointed out to National Public Radio reporter Paul Chisholm, the average sperm count of the men in the study who wore tight-fitting underwear instead of boxers was still within healthy levels.
“For most men, it probably doesn’t make a lot of difference,” he said. “The men who are most likely to benefit are the men who are on the border — who have relatively low sperm count.”
FMI: The article can be read in full on Human Reproduction’s website.