Prof Allan Pacey, professor of andrology, from the University of Sheffield, who was not involved in the study, said the different levels of FSH among men with different types of underwear "suggests that the tight-pant wearers have evidence of testicular damage". The study followed the men through their time in fertility treatment and monitored changes in their sperm, but it did not ask about their underwear choice again after the initial questionnaire. Sperm cells lose their mobility and viability at 37 degrees Celsius, so keeping them external to the abdomen allows them to stay cooler.
According to Chavarro, this study was just a proof of what was speculated from earlier smaller studies.
The groundbreaking study is the largest of its kind to look into the truth of a long-held suspicion that tighter trousers such as Y-fronts or briefs affect a man's ability to father children.
The men were aged between 32 and 39 years. It was found that these men had 25% higher sperm concentration than the men who mostly wore tighter-fitting underwear and their total sperm count was 17% higher.
The study, published in the journal Human Reproduction, also found that a hormone from the brain that tells the testicle to make sperm, called follicle stimulating hormone (FSH), was 14% lower in wearers of looser underwear.
The team of USA researchers examined data on 656 men who were part of couples seeking treatment at a fertility centre in America between 2000 and 2017.
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Every man provided semen and blood samples.
Anything that prevents this cooling is likely to impair both sperm output and quality.
Well, if one is wondering how does the type of underwear affect sperm concentration then here is the answer.
"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". The cause behind such fall is unknown say experts.
'I would also question the use of underwear type as a scientific way of measuring scrotal heat. The scientists meant to study the link between all these factors to figure out whether there was a scientific explanation for the correlations.
Lidia Mínguez-Alarcón, author and a research scientist at Harvard's T.H. Chan School of Public Health also said that this study provides science behind the recommendation for loose underwear to men who have fertility problems.
Firty-three percent of the men said they usually wore boxers.