Eating a diet of junk food reduces the fertility of men aged 18 to 20, a study suggests.
The sperm counts of those who ate pizzas, burgers and chips were 26 million lower than those on a “healthy” diet of fish, chicken, vegetables, fruit and water.
A normal count is 39 million or more.
US scientists analysed samples from 3,000 men with an average age of 19 joining the Danish military.
The findings suggested red meat, bread and sugary drinks lowered their sperm counts. The hormone levels in men who ate more junk food suggested the cells which produced the sperm were permanently damaged.
Author Dr Jorge Chavarro, of Harvard University, said: “You would be surprised to see how sensitive young men are to things that might affect sperm count, a perceived measure of masculinity.”
It is the first study to suggest diet can damage fertility at such a young age.
Men on a poor “Western” diet of junk food had the lowest sperm counts, followed by those on a Scandinavian diet, featuring processed meats, fish and diary, then those on a vegetarian diet.
These were compared to people with a diet high in fruit, vegetables and fish.
Fertility expert Prof Allan Pacey, of Sheffield University, said: “With pizza, chips and red meat we know antioxidant stress goes up and that is bad for sperm.”
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