They found that tight-fitting underwear is indeed associated with reduced sperm concentration and can result in lower blood levels of follicle-stimulating hormone (FSH).
Dr Lidia Minguez-Alarcon, the first author on the study said in a statement:
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.
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[s], [hormones that act] on the testes and that is reflected by the increased levels of FSH, to try to increase sperm production.
The experts conducted this study by analysing 1,186 sperm samples from 656 different men who were seeking fertility treatment at the Massachusetts General Hospital between the years 2000 and 2017. In this number, 304 men also had a blood sample taken so that physicians could study their levels of FSH, estradiol (a form of estrogen) and luteinizing hormone (a hormone used in reproductive cell development).
They found that the men who wore boxers, which made up 53 per cent of the group, had a higher sperm concentration of 25 per cent, a higher sperm count of 25 per cent and lower FSH levels of 14 per cent than those who worse tighter items of underwear. Factors that could influence the sperm like age, BMI, smoking history and reported time between the sample and last ejaculation were all adjusted accordingly.
However, they concede that their findings might be affected by the fact that only 67 per cent of the participants submitted only one sample and that circadian rhythm throughout the day can change the quality of sperm. Circadian variation can also change hormone production, as can a hot bath which may alter the data but the authors do say that they tried to accommodate for these factors.
Furthermore, these results can not be considered to represent the wider public as all of the samples were taken from men who were part of a fertility clinic.
Dr. Minguez-Alarcon added that in future studies they hope to look at the molecular mechanism in hormone levels and testicular damage from underwear to see if patterns emerge.