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Fans of US sitcom Seinfeld may recall a 1994 episode in which Kramer is advised to switch from wearing tight-fitting underwear to boxer shorts, in order to improve his low sperm count. But does the science stack up? According to new research published in the journal , it鈥檚 even more accurate than originally thought.

The new study is the first to go beyond the traditional, narrow focus on semen quality and include information on a variety of indicators of testicular function, such as reproductive hormones and sperm DNA damage. These can help in understanding how choices of underwear affect the key regulator of sexual development and reproduction, the hypothalamic-pituitary-gonadal axis. It also includes a larger number of men than previous studies, with 656 participants.

Led by Dr Lidia M铆nguez-Alarc贸n, a research scientist at , scientists recruited the male partners of couples who were seeking infertility treatment at the between 2000 and 2017. Each of them provided a semen sample and blood sample on the same day, and answered a questionnaire that asked about the style of underwear they wore most frequently in the preceding three months. Among the 656 men, 53% (345) reported that they usually wore boxer shorts; these participants tended to be younger, slimmer and more likely to take hot baths or Jacuzzis than the men who wore other, more tightly fitting underwear.

The researchers found that men who wear boxer shorts had a statistically significant 25% higher sperm concentration, 17% higher total sperm count and 33% more swimming sperm in a single ejaculate than men who wore briefs, 鈥榖ikinis鈥� (very brief briefs), 鈥榡ockeys鈥� (underwear that finishes just above the knee) or other tight-fitting underwear, after adjusting for factors such as BMI, physical activity, hot baths and Jacuzzis, smoking and the year the sample was taken. In addition, more sperm were correctly shaped, although this result was not statistically significant.

They also found that boxer shorts-wearing men had 14% lower levels of follicle-stimulating hormone (FSH), compared to men who wore tight-fitting underwear. FSH stimulates sperm production, with the researchers suggesting that it kicks into gear when it needs to compensate for testicular damage from increasing scrotal temperatures and decreasing sperm counts and concentration.

There were no significant differences in other reproductive hormones or damage to the DNA of the sperm. When the researchers adjusted their results to take account of reproductive hormones, apart from FSH, they found the adverse effect of tighter underwear on sperm quality remained. However, when they adjusted for FSH levels, the effect was no longer significant. This suggested to them that it was the damage to sperm concentrations and total sperm count associated with wearing tighter underwear that could be responsible for the changes in FSH levels.

鈥淎n 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,鈥� said Dr M铆nguez-Alarc贸n. 鈥淏ecause 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. This hypothesis requires confirmation by further research.鈥�

The researchers point out that it may not be possible to generalise the findings of their study to all men, as the study focused on men attending a fertility centre. However, the men in this study tended to have good semen quality when compared to the 鈥檚 reference standards. It is also not possible to prove from these findings that the type of underwear causes the difference in semen quality and FSH levels 鈥� only that that there is an association between them. Other factors that might affect scrotal heat, such as type of trousers (eg, skinny jeans) and underwear fabric, could also affect the results.

鈥淪ince men can modify the type of underwear they choose to wear, these results may be useful to improve men鈥檚 testicular function,鈥� Dr M铆nguez-Alarc贸n concluded.

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