The secret to stronger swimmers may be living in your gut. 

A growing body of research suggests that an industrial chemical found in plastic food containers and other everyday products may be taking a toll on male fertility, lowering sperm counts, slowing their movement and raising the risk of abnormalities.

Now, scientists say a dietary supplement developed in Japan may help blunt some of that damage — and it’s inspired by something typically flushed away without a second thought.

The new study, led by scientists at Osaka Metropolitan University, focused on countering the effects of Bisphenol A, or BPA.

The man-made chemical has been used since the 1950s to make certain plastics and resins stronger, more durable and heat-resistant, including those used in food packaging and the inner coatings of metal cans.

But BPA doesn’t always stay locked inside those materials. Studies show it can leach into food and drinks from the containers they’re stored in, quietly making its way into the human body.

In fact, government scientists estimate that more than 90% of Americans have detectable levels in their system.

Over time, exposure to BPA has been linked to a range of health concerns, including developmental issues and problems with reproduction.

To better understand the impact, the researchers exposed rodents to the chemical in a lab. They found it triggered oxidative stress — or an imbalance between the body’s natural antioxidant defenses and unstable, highly reactive molecules known as free radicals that can damage healthy cells. 

The effects on the rodents’ sperm showed up quickly. Within just one week of BPA exposure, the scientists observed a spike in free radical production in the animals’ swimmers. 

By eight weeks, the damage was more pronounced. The sperm weren’t moving as well across several key measures, including speed and the subtle “head movements” that help guide them toward an egg and play a role in fertilization.

Because free radicals can be neutralized by antioxidants, the research team began looking for compounds that might help reduce the oxidative stress triggered by BPA in sperm.

That search led them to a newer category of supplements known as paraprobiotics, with one in particular standing out: FK-23.

FK-23 is derived from a type of bacteria known as Enterococcus faecalis that naturally lives in the human digestive tract and is commonly found in fecal matter.

While in the gut, it’s normally harmless and helps support digestion and the immune system. But if it escapes the intestines or overgrows, it can cause infections in other parts of the body.

That’s why scientists don’t use live bacteria when making FK-23. Instead, the microbes are grown in a lab and then killed with heat.

This process prevents them from reproducing or causing infection while preserving their structural components, which are believed to trigger beneficial effects in the body.

Researchers believe it may work through what is known as the gut–immune axis, a two-way communication system between the digestive tract and the rest of the body that can influence inflammation and oxidative stress.

To test its effects on sperm already damaged by BPA, the researchers gave the rodents FK-23. They found that their sperm movement improved despite continued exposure to the chemical, while signs of cellular stress in their reproductive tissue were significantly reduced.

“These findings suggest that components derived from lactic acid bacteria exert protective effects against reproductive toxicity caused by environmental chemicals,” Visiting Professor Yukiko Minamiyama of the Graduate School of Medicine at OMU, who led the research team, said in a press release. 

Looking ahead, the researchers said more work is needed to figure out exactly how the gut is driving these effects, and to confirm whether the same results happen in people.

If they do, Minamiyama said it could “pave the way” for using the dietary supplement as a way to support reproductive health.

The study findings come as BPA’s presence in consumer products has declined over the past decade.

In the US, the Food and Drug Administration prohibits its use in baby bottles and sippy cups, and many manufacturers have switched to “BPA-free” plastics.

However, BPA is still allowed in other food-contact materials and certain consumer goods. The FDA maintains that, based on current evidence, the small amounts that can leach from these products are not considered harmful.

Other parts of the world have taken a stricter approach. In the European Union, for example, BPA is banned from use in food containers.