Gentlemen, you might want to think twice about eating that Big Mac. Diets that contain high amounts of processed foods not only deplete men’s sperm counts, but also cause irreversible damage to the little swimmers, according to a new Harvard study.

Healthy men typically have at least 39 million sperm per ejaculate, according to the World Health Organization. Men who ate a “Western diet”—one filled with pizza, chips, refined grains, high-energy drinks, and sweets—had a sperm count that was 25.6 million lower, the study found.

The highest sperm count was found in people who ate mostly fish, chicken, vegetables, fruit, and, water; vegetarians had the second-highest sperm count.

“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,” the researchers wrote in their study, which was presented at the annual meeting of the European Society of Human Reproduction and Embryology.

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For the study, the medical tests of 3,000 Danish men ages 18 to 20 were examined. Not only did those with a poor diet have a lower sperm count, researchers found, but the food seemed to cause irreparable damage to their sperm health. Meaning, even if they ate healthier later on in life, their sperm counts were still much lower than clean eaters. 

In recent years, the issue of male infertility has been gaining steam in headlines and studies worldwide. The Centers for Disease Control and Prevention reports that 35 percent of couples have problems with infertility, and in 8 percent of those cases the man is the only factor.

Additionally, according to the CDC, 9 percent of men aged 25 to 44 have sought treatment for infertility. Low sperm count could be the result of genetics or a disease, but much research has also found that external factors are adding to the problem.