Young men who eat pizza, chips and burgers may end up with drastically lower sperm counts.
A study of almost 3,000 men, aged only 18 to 20, found those with a 'Western diet', including processed meat, pizza, sugary drinks and snacks, have worse quality sperm than those who eat healthily.
Vegetarian men have higher sperm counts, as do those whose diets include fish and whole grains, making them more fertile.
Men's diets have worsened over the past few decades, coinciding with a fall in the average sperm count of almost 60 per cent, which some experts have warned could lead to the extinction of the human race.
A study of almost 3,000 men, aged only 18 to 20, found those with a 'Western diet', including processed meat, pizza, sugary drinks and snacks, have worse quality sperm than those who eat healthily, and some of the damage may be permanent
When couples struggle to have a baby, in a third of cases it is because of problems with sperm, which some men could reverse by improving their lifestyle.
Researchers led by scientists from the Harvard TH Chan School of Public Health compared the sperm counts of healthy young men eating western diets with those who were vegetarian and those who ate a 'prudent' diet high in fish, chicken and vegetables or a traditional Scandinavian diet including whole grains and dairy.
Men with a highly western diet produced 25.6 million fewer sperm than men whose diet was least like this.
A normal sperm count contains 39 million or more sperm, according to the World Health Organisation.
Dr Jorge Chavarro, who led the study from Harvard, said: 'We are seeing lower sperm counts and a few men fall below the WHO lower reference limits. This is the largest study to have found this in young men.
'You would be surprised to see how sensitive young men are to things that might affect sperm count,
