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.
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 Urology.
"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.
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 dairy.
- The vegetarian-like pattern, with a high intake of vegetables, soy milk, and eggs, with little to no red meat or chicken.
- And the "unhealthy" Western pattern, with more pizza, snacks, french fries, sweets, sugar-sweetened drinks, processed and red meat, snacks and highly processed grains.
Men who closely followed the prudent pattern of eating -- characterized by lots of fish, chicken, vegetables, fruit, and water -- were associated with the highest sperm counts. This was followed by the semi-vegetarian and then the "smørrebrød," or Danish, eating style.
"The median sperm count of men who had the highest adherence to the 'prudent' pattern was 68 million higher than men who had the highest adherence to the 'Western' pattern," Nassan said, with 95 percent confidence intervals of 43 and 93.
In addition, the median sperm count of men who had the highest adherence to the vegetarian-like pattern was nearly 33 million higher than men who mostly ate the less nutritious Western diet.
CNN wrote about the study's preliminary results last year. At the time, Charles Lindemann, a professor emeritus and researcher at Michigan's Oakland University who did not participate in the research, said the study's findings "could be an important clue if it holds up to scrutiny."
A diet high in processed foods, he said, "may be responsible for the known trend that has been recorded over the recent past of progressively decreasing sperm counts."
Nassan said her findings show eating seafood, poultry, nuts, whole grains, fruits, and vegetables give the body the antioxidants and omega-3 fatty acids " essential for good sperm production."
"Changing diet pattern may be a simple and inexpensive change" to protect a man's testicular function, Nassan said.
"I believe that it is not only 'you are what you eat' but it is also 'your sperm is what you eat.' "
"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.