Cutting back on alcohol, stopping smoking and losing weight has little effect on sperm production, according to research.
The study findings fly in the face of guidelines from the National Institute for Health and Clinical Excellence which says that doctors should warn infertile men about the dangers of smoking, alcohol consumption and recreational drug use, as well as the risks of being overweight and wearing tight underwear.
Researchers from the Universities of Manchester and Sheffield say the current guidelines mean that couples may delay getting fertility treatment as they try to make lifestyle changes which will have little impact on male fertility.
More than 2,200 men from 14 fertility clinics around the United Kingdom completed detailed questionnaires about their lifestyle.
This allowed the researchers to compare the lifestyles of 939 men who ejaculated low numbers of swimming sperm with those of 1,310 men with levels within the normal range.
Recreational drug use, smoking, alcohol consumption and body mass index (BMI) had little effect on sperm production, the study found.
However, other factors did appear to affect the numbers of swimming sperm. Wearing boxer shorts rather than tight underwear and a previous successful conception was linked to higher numbers of swimming sperm.
But men with low levels of sperm were 2.5 times more likley to have had testicular surgery, 1.3 times likely to be in manual work and twice as likley to be of black ethnicity.
Dr Andrew Povey, from the University of Manchester said: "Despite lifestyle choices being important for other aspects of our health, our results suggest that many lifestyle choices probably have little influence on how many swimming sperm they ejaculate.
"This potentially overturns much of the current advice given to men about how they might improve their fertility and suggests that many common lifestyle risks may not be as important as we previously thought.
"Delaying fertility treatment then for these couples so that they can make changes to their lifestyles, for which there is little evidence of effectiveness, is unlikely to improve their chances of a conception and, indeed, might be prejudicial for couples with little time left to lose."
Dr Allan Pacey from the University of Sheffield added: "In spite of our results, it's important that men continue to follow sensible health advice and watch their weight, stop smoking and drink alcohol within sensible limits. But there is no need for them to become monks just because they want to be a dad.
"Although if they are a fan of tight Y-fronts, then switching underpants to something a bit looser for a few months might be a good idea!"