If you're trying to become a father, it's time to turn down the bacon, burgers and beers.
Men who eat diets high in saturated fat have the lowest sperm counts, and it's of poorer quality, scientists have found.
But men who eat more 'good' fats - link the omega-3 fatty acids found in fish and some plant oils - have better quality sperm.
The study is the largest to investigate the effect of specific dietary fats on male fertility.
It comes amid concern that both the quantity and quality of sperm appears to be declining in Western countries, with some studies showing average sperm counts have fallen by more than half over the past 60 years.
Estimates suggest around 30 per cent of men in couples seeking IVF treatment are what is called sub-fertile and two per cent are totally infertile.
American researchers studied 99 men attending a fertility clinic, questioning them about their diet, analysing semen samples and measuring the fatty acid levels of 23 of the participants.
They then divided the men into three groups according to how much fat they consumed.
The findings showed those with the fattiest diets had a 43 per cent lower total sperm count and 38 per cent lower sperm concentration than those eating the least saturated fat. Total sperm count is the number of sperm in a sample and sperm concentration is the number of sperm per unit volume of semen.
Men consuming the most omega-3 fatty acids also had about two per cent more correctly-formed sperm than men with the lowest intake, according to the study, which was published in the journal Human Reproduction.
Previous studies have suggested a link between obesity and male infertility but this latest study took account of the fact that 71 per cent of the men were overweight or obese.
Lead scientist Professor Jill Attaman, of Harvard Medical School, Boston, said: "If men make changes to their diets so as to reduce the amount of saturated fat they eat and increase their omega-3 intake, then this may not only improve their general health but could improve their reproductive health too."
The researchers did not say whether the men had fathered children.
Professor Richard Sharpe, of the MRC Centre for Reproductive Health, University of Edinburgh, said: "Most evidence points towards low sperm counts being due to impaired testis development whilst the men were babies in the womb but the current study suggests that consumption by men of a 'Western' style diet, high in saturated fats, might also lower their sperm counts.
"This study shows an association between eating a high (saturated) fat diet and lower sperm concentration but it does not show that one causes the other."
He said the existing consensus that obesity has a small effect on male fertility might have to be re-visited.
"In the present study most men were overweight but lower sperm concentration was found only in those who had higher fat intakes.
"This raises the possibility that previous studies missed associations because they grouped men just according to their weight," he added.
Leading British fertility expert Dr Allan Pacey, of the University of Sheffield, said: "Currently we have few ways of improving sperm quality for men who are sub-fertile and so men wishing to improve their chances of conception should try to be as healthy as possible.
"That includes eating sensibly, stopping smoking and, if they are overweight, trying to bring their BMI (body mass index) into the normal range."
- DAILY MAIL