Drinking, smoking or being fat make little difference to how well a man's sperm cells can swim, a new study has found.
The peer-reviewed findings to be published on Wednesday in the medical journal Human Reproduction fly in the face of the common advice that men having fertility problems should stick to a healthier lifestyle.
The study looked at sperm samples from 2,249 British men recruited from fertility clinics and andrology labs.
The researchers found that men who wore briefs rather than boxer shorts, had testes surgery, or did manual work that would expose them to chemicals, were more likely to have a low motile sperm count.
However, "no relation was found to consumption of alcohol, use of tobacco or recreational drugs or high body mass index," the article says.
In fact, having very low body mass index appeared to have a negative impact on sperm quality, although that sample size was too minute to be meaningful, the study said.
One of the researchers said the results suggest that men shouldn't wait until they have developed a healthier lifestyle before they try to procreate, especially if age is a crucial factor for their spouses.
"Success in fertility treatment is an issue for the couple and not simply the male partner … the prime issue in delaying treatment is that success drops dramatically with the age of the female partner," said Nicola Cherry, an epidemiologist at the University of Alberta's faculty of medicine.
Guidelines from the Public Health Agency of Canada and the Guide to Infertility booklet produced by Assisted Human Reproduction Canada both say that smoking and alcohol consumption decrease success rates.
"Perhaps where fertility treatment is prohibitively expensive it seems better to suggest a healthy lifestyle rather than just to advise, 'keep trying,'" Dr. Cherry said.
The study also found that black men were more likely to have low numbers of swimming sperm, but with a sample size of only 48, the link wasn't strong, she said.
Watching one's weight and limiting consumption of alcohol and tobacco is still a good idea for general health reasons, the researchers said.
However, the study's findings are a reminder that would-be fathers need not be anxious about their lifestyle and live like monks, said another member of the research team, Allan Pacey, a senior lecturer in andrology at the University of Sheffield.
"Try and be healthy – there are many reasons to do that. But don't beat yourself up for the odd indulgence," Dr. Pacey said.