Smoking and drinking has 'little effect' on sperm counts
Lifestyle advice given to tackle male infertility may be futile and could delay other options, according to researchers in the UK.
Their study in the journal Human Reproduction said smoking, alcohol consumption and being obese did not affect semen quality.
However, they warned that avoiding them was still "good health advice".
Wearing boxer shorts rather than tighter underwear was linked to higher sperm levels.
Advice for doctors by the National Institute for Health and Clinical Excellence says men should be warned about the impact of smoking, drinking and taking recreational drugs on their sperm.
A study by researchers at the Universities of Sheffield and Manchester compared the lifestyles of 939 men with poor sperm quality with 1,310 men with normal sperm quality.
The study showed there was little difference in the number of mobile sperm between patients who never smoked and those who had a 20-a-day habit.
There was "little evidence" that recreational drug use, a high BMI or excessive alcohol consumption affected sperm quality.
Dr Andrew Povey, from the University of Manchester, said there was these lifestyle choices were hugely important for wider health but "probably have little influence" on male fertility.
He said: "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."
Wearing boxer shorts was associated with higher-quality sperm.
Dr Allan Pacey from the University of Sheffield said: "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."
There are other measures of fertility, such as the size and shape of the sperm or the quality of the sperms' DNA, which were not considered in the study.
The National Institute for Health and Clinical Excellence is reviewing the evidence.
A NICE spokesperson said: "The draft update of our fertility guideline is currently open for consultation.
"However, until the update of this guideline is published later this year, the NHS should continue to follow the recommendations in the current fertility guideline."