Sperm counts in Kiwi men have halved over the past 40 years, and now scientists think they know why.
New research has linked fatty Western diets to plummeting fertility.
Scientists in the US and Europe looked at the health and diets of nearly 3000 Danish men who enlisted for military service over a 10-year period.
They divided the men into four groups, based on their dietary patterns:
- Western, "characterized by intake of pizza, chips, processed and red meats, snacks, refined grains, high-energy drinks, and sweets";
- prudent, "fish, chicken, vegetables, fruit, and water";
- smørrebrød, "cold processed meats, whole grains, mayonnaise, cold fish, condiments, and dairy"
- and vegetarian, "vegetables, soy milk, and eggs".
The results were startling - men with a predominantly Western diet had a sperm count on average 25.6 million fewer than those eating prudently. Vegetarian came in second for sperm count, and smørrebrød third.
The range for men eating Western-style was between 8 and 42 million - below 15 is considered low. Even the worst sperm count for a man on the prudent diet was well above this, at 22 million. Having a low sperm count can make it hard for men's partners to get pregnant.
"A number of studies have examined the role of diet on semen quality but data on how adherence to different diet patterns influences testicular function is scarce," the study, unveiled last week at the European Society of Human Reproduction and Embryology, said.
"Our findings support the growing evidence that adhering to generally healthy diet patterns, including local variations, is associated to higher sperm counts and more favorable markers of sperm function."
Western diets appear to "be responsible for the known trend that has been recorded over the recent past of progressively decreasing sperm counts", fertility researcher Charles Lindemann at Oakland University told CNN.
He said it could be that mineral deficiencies were to blame. Previous research has also linked obesity to declining sperm counts - and there's little doubt the Western diet contributes to obesity.
Other ways sperm count can be impacted include smoking, catching an STI and getting old. Other claims - such as wearing briefs instead of boxers or using a laptop can reduce your sperm count - have not been proved.