experts believe processed food starved of antioxidants places sperm-producing cells under "oxidative stress", ultimately killing them.
Professor Allan Pacey of 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 make that poor diet younger in life make a change that sticks with you."
The Harvard researchers accessed the data of nearly 3,000 men who underwent a medical examination.
The average age was 19.
From the responses to a dietary questionnaire, four types of diets identified: a "Western" diet characterized by red meat, processed meat, fatty and sugary food and drink; a "prudent" dietary chicken, fish, vegetables and fruit; a "smørrebrød" diet cold processed meats, whole grains, mayonnaise, cold fish, condiments, and dairy; and a traditional vegetarian diet, containing lots of vegetables, soya milk and eggs.
Sperm health, as measured by concentration, volume and motility, which is 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 The sperm-producing Sertoli cells, again found that these were depleted in the young men who favored junk food.
Sertoli cells killed by oxidative stress. While a person can boost their sperm.
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 people's masculinity may prompt them into eating healthier food.
"It would be difficult to 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 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, to 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 you, '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 most recent sperm in spring and autumn, which could be linked to the daylight during those months and the fact to tend to exercise more.
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