Male fertility is being irreversibly damaged by a diet of western junk food by the time men reach 18, a study has found.
A groundbreaking investigation has established that teenagers who favour high-fat and processed foods like pizzas, chips and snacks are killing off sperm-producing cells that can never be replaced.
It showed that a diet dominated by fish, chicken, vegetables and fruit is best is for protecting those cells and ensuring healthy levels of sperm.
Sperm counts have been declining in recent decades and various studies have pointed to poor diet as a significant cause.
However, they have tended to focus on older men - those in their late 20s, 30s and 40s who are struggling to conceive.
By contrast, the new Harvard University investigation shows for the first time that poor diet while growing up may reduce a man’s chances of fathering children for the rest of his life.
Experts believe processed food starved of antioxidants places sperm-producing cells under “oxidative stress”, ultimately killing them.
Professor Allan Pacey from the University of Sheffield, a leading authority on sperm, said: “This just shows the power of diet to the way that testicles function.
“The concern would be that poor diet younger in life makes a change that sticks with you.”
The Harvard researchers accessed the data of nearly 3,000 men who underwent a medical examination upon starting national service in the Danish armed forces.
The average age was 19.
From the responses to a dietary questionnaire, four types of diet were identified: a “Western” diet characterised by red meat, processed meat, fatty and sugary food and drink; a “prudent” diet comprising mainly chicken, fish, vegetables and fruit; a “Smørrebrød” diet cold processed meats, whole grains, mayonnaise, cold fish, condiments, and dairy; and a traditional vegetarian diet, involving lots of vegetables, soya milk and eggs.
Sperm health, as measured by concentration, volume and motility, was best in those following the prudent diet, followed by the vegetarian and then Smørrebrød diets, with those adhering to a western diet yielding the worse readings.
The scientists also conducted hormonal tests indicating the health of sperm-producing Sertoli cells, again finding that these were depleted in the young men who favoured junk food.
While a person can boost their sperm health over time by improving their diet, it is thought they cannot recover Sertoli cells killed by oxidative stress.
That has implications for the amount of sperm they can produce at any one time.
Dr Jorge Chavarro, who led the research, said the perceived threat to young men’s masculinity may prompt them into eating healthier food.
“You would be surprised to see how sensitive young men are to things that might affect sperm count, because it’s a perceived measure of masculinity,” he said.
The research is being presented at the European Society for Human Reproduction and Embryology (ESHRE) annual conference in Vienna.
It came as scientists called for men with a poor diet to be screened for DNA damage to their sperm.
DNA damage is a factor in up to half of miscarriages.
Meanwhile Dr Roy Farquharson, an NHS consultant and president of ESHRE, said: “Most men think they’re invincible until their first big health event occurs, which is often either miscarriage or infertility.
“Then their partner will quite rightly ask them, ‘what are you doing?’ Because the sperm is just as important as the egg.”
He said improvements in lifestyle can begin to benefit sperm quality within two to three months.
The new research follows a study presented last October which indicated that men produce the strongest sperm in spring and autumn, which could be linked to hours of daylight during those months and the fact men tend to exercise more.