Men who drink a pint of beer a day double their chances of becoming a father – just don't follow it up with a cup of coffee!
Researchers from the Massachusetts General Hospital in Boston studied 105 men whose wives and girlfriends were having IVF.
The men, who had an average age of 37, filled in an extensive dietary questionnaire that included questions about alcohol intake and caffeinated drinks.
The amount of alcohol consumed by the would-be fathers was relatively moderate, with the biggest drinkers downing just under three units a day on average. A pint of Stella Artois, for example, has 2.7 units.
The tests showed that there was no link between the amount of caffeine or alcohol they took in and the quality of their sperm.
But those who had the most alcohol had a 57 per cent chance that a session of IVF would result in a baby being born.
This was twice the 28 per cent success rate of those who drank the least.
When the researchers looked at caffeine intake, they found that birth rates dropped from 52 per cent among those who had very little caffeine to just 19 per cent for those who had the most.
The damage was done above 265mg of caffeine a day. A mug of filter coffee contains 140mg.
The American Society for Reproductive Medicine's annual conference in Honolulu heard that the results could not be explained away by factors such as the men's age or weight.
It should be pointed out that men whose partners weren't having IVF weren't featured in the study.
Researchers were unsure why caffeine and alcohol had such a noticeable effect on birth rates when it seemed to have no impact on the quality of the prospective fathers' sperm.
Researcher Anatte Karmon suggested that caffeine could damage sperm in a way that is not currently picked up by tests.
She called the increase in pregnancy rates 'very impressive', but said more tests would be needed to confirm the finding – and pointed out that none of the men were heavy drinkers.
In a separate study, doctors found that vegetarians and vegans had poorer quality sperm and lower sperm counts than others.
The Reproductive Medicine conference heard that soy products eaten as meat and dairy substitutes could be the problem.
Dr Eliza Orzylowska, from the Loma Linda University Medical Centre in California, said: "We found diet significantly affects sperm quality. Vegetarian and vegan diets were associated with much lower sperm counts.
"It's hard to tell people not to be vegetarians if they are trying to conceive, but I would caution against soy."