Young men who eat a mainly "Western" diet of pizza, chips and burgers may end up with drastically lower sperm counts.
A study of almost 3 000 men aged 18 to 20 found those with a diet packed with processed meat, pizza, sugary drinks and snacks have worse-quality sperm than those who eat healthily – and suggests some of the damage may be permanent.
Most previous research on diet and fertility has focused on older couples trying to get pregnant, rather than the effects on the sperm of healthy young men. This study found vegetarians have higher sperm counts, as do those whose diets include fish and whole grains, making them more fertile.
Allan Pacey, a professor of andrology – the study of diseases and conditions specific to men – at the University of Sheffield, said: "This just shows the power of diet to the way that testicles function.
"It’s almost certain that this is down to an effect that those with the better diets are taking more antioxidants. With pizza, chips and red meat we know that the antioxidant stress goes up, and that is bad for sperm."
Men’s diets have worsened over the past few decades, coinciding with a fall in the average sperm count of almost 60%. When couples struggle to have a baby, in a third of cases it is because of sperm problems that some men can reverse by improving their lifestyle.
The findings, presented at the annual meeting of the European Society Of Human Reproduction And Embryology in Vienna, come from 2 935 men who took a medical exam to determine their fitness for military service.
While a normal sperm count contains about 39 million sperm – according to the World Health Organisation – researchers from the Harvard TH Chan School of Public Health in the US found that men with a highly Western diet produced 25.6 million fewer sperm than the healthiest eaters.
The findings also suggest that an unhealthy diet could harm or reduce the amount of cells which produce sperm, which may cause permanent damage as they are not believed to regenerate.
Dr Jorge Chavarro, who led the Harvard study, said: "This is the largest study to have found this in young men.
"You would be surprised to see how sensitive they are to things that might affect sperm count, because it’s a perceived measure of masculinity."