(CNN)If sperm was an animal, science might worry that it's heading toward extinction in Western nations.
Total sperm count in North America, Europe, Australia and New Zealand dropped by up to 60% in the 38 years between 1973 and 2011, research found -- an acceleration of a trend that began in the 1940s. More recent studies show the trend is continuing.
At the same time, studies show a concurrent decline in testosterone levels -- the hormone needed to build a man's muscle and bone mass and boost his sex drive. ame
Why? No one knows for sure. Debate rages about the role of radiation, air pollution and chemicals in our food, clothes and water. Smoking, alcohol consumption and obesity likely all play a role.
So could the lower nutritional quality of the typical Western diet, according to a new study published Friday in JAMA Network Open.
"This study is the largest study to date to examine the diet pattern with men's testicular function," said study author Feiby Nassan, a research fellow at Harvard's T.H. Chan School of Public Health.
The study found that, on average, men who typically ate a Westernized diet of pizza, snacks, sweets and processed foods produced around 68 million fewer sperm upon ejaculation than men who ate a more healthy, balanced diet.
A man is considered to have a low sperm count if he has less than 39 million sperm per ejaculation or fewer than 15 million sperm per milliliter. A low sperm count can negatively impact a man's ability to get a partner pregnant, and it can be a key marker for overall male health.
"Fertility is not just important to make babies," Nassan said, adding that new research recently shows fertility is related to a man's general health and life expectancy.
A huge difference
The study looked at 2,935 Danish men of normal weight -- with a median age of 19 -- who were undergoing a physical to determine their fitness for military service (something all men in Denmark have to do after they turn 18).
Blood and semen samples were taken, and the men completed a questionnaire that asked how often they had eaten 136 food items in the prior three months.
The study looked at four food patterns:
- The "prudent," healthy pattern, in which fish, chicken, vegetables, fruit and water were mostly consumed.
- The "open-sandwich pattern," a more typically Danish diet with a greater intake of cold, processed meats, whole-grain breads, mayonnaise, cold fish, condiments and dai