Men who want to boost their sperm counts may want to swap their tight-fitting underpants for more roomy boxers which are better suited to keeping testicles cool, scientists say.

A major study into the impact of underwear on sperm quality found that men who favoured airy boxer shorts made significantly more sperm than those who tucked themselves into more restrictive items of underwear.

In tests on 656 men, those who mostly wore boxers had 25% more sperm per millilitre, and 17% more sperm in each ejaculate, than men who opted for jockeys, briefs, or even smaller smalls which the researchers classed as “bikinis”.

“The results from this study are very practical,” said Lidia Mínguez-Alarcón who worked on the study at the Harvard T H Chan School of Public Health in Boston. “Men could improve their sperm production by easily changing their type of underwear.”

The report builds on three decades of research that have sought to tackle the question of whether a man’s choice of underwear has an impact on how well his testes function. There is now a fair consensus that tight underpants may lead to “scrotal heating” which harms the Sertoli cells that make sperm in the testes.

While the present study did not investigate the temperature of men’s testicles, the importance of keeping the male gonads cool has been apparent since at least the 1920s. Scientists now know that the best conditions for sperm production occur in free-dangling testes that are 3 to 4C cooler than the normal body temperature of 37C.

“It has been shown that high scrotal temperatures alter spermatogenesis,” said Mínguez-Alarcón, referring to the 90-day job that is sperm manufacture. “Men wearing tighter underwear may have higher scrotal temperatures compared to men wearing looser underwear.”

Writing in the journal Human Reproduction, the US researchers describe how they quizzed the male partners of couples seeking fertility treatment about their preferred choice of underwear. More than half, or 345 men, said they mostly wore boxer shorts, with 311 men favouring more snug items of underwear.

After accounting for variations in physical activity, body mass index, smoking and whether or not the men enjoyed jacuzzis or hot baths – all of which affect sperm – the scientists found consistent differences in semen samples taken from the men. Those who favoured boxer shorts had on average 64.7m sperm per millilitre and 168m sperm per ejaculate. The figures compared to an average of 51.9m sperm per millilitre and 138m sperm per ejaculate among men who sported closer-fitting underwear.

While the differences are marked, none of the men in the study can be considered to have a clinically low sperm counts, for which the cut-off is 15m sperm per millilitre and 39m per ejaculate. So while tight pants may reduce sperm counts, it is unlikely they would make a healthy man infertile.

Further tests on a subset of the men showed that those who wore tighter underwear had on average 14% higher levels of a substance called follicle-stimulating hormone in their blood. The rise in the hormone, which is made by the pituitary gland to drive sperm production, is a sign that the body is trying to restore normal sperm manufacture in testicles that are impaired by overheating, the scientists believe

The study does not prove that tight underwear causes sperm counts to fall. And because the men were asked to recall their usual underwear choice, rather than having the information recorded day by day, there is room for error in the results. Furthermore, all of the men were in couples seeking fertility treatment, so it is unclear whether the findings reflect what would be seen in healthy men in the wider population. Other factors could also matter, such as how long men spend sitting down each day.

Allan Pacey, a professor of andrology at Sheffield University, has published two large studies on the factors that affect sperm quality and found similar links to tight underwear. He said UK guidelines drawn up by the National Institute for Health and Care Excellence (Nice) in 2004 are still sound. They recommend that men seeking fertility treatment are told about the link between scrotal temperature and reduced semen quality, but are warned that it is unclear whether underwear choice actually affects fertility.

Prof Lee Smith, who studies testicular function at the University of Newcastle in Australia, said: “This is an interesting study further highlighting the links between raised scrotal temperature and an increased risk of slightly reducing sperm count, however the study is a correlation based on a self-reporting survey and additional research is required to unequivocally identify a causal link.”