A mouse study suggests that too much zinc may raise the risk of C. diff, a diarrhea-causing infection that's rising among young, healthy adults.

At the first sign of sniffles, your instinct may be to load up on zinc. But that might not be the best idea for everyone, and could be downright risky for some, according to researchers at Vanderbilt University.

In a study on mice, they found that excess amounts of zinc may increase susceptibility to a bacterium that causes diarrhea and inflammation of the colon. The bacteria, Clostridium difficile (C. diff), is the most common cause of infections picked up in hospitals, and cases are on the rise among healthy, non-hospitalized people, as well.

C. diff often occurs in people who have been taking antibiotics. Along with whatever infection the drugs are prescribed for, the antibiotics also kill much of the healthy bacteria in the gut. When the beneficial gut bacteria are gone, it allows C. diff to take hold, according to study co-author Eric Skaar, PhD, professor of pathology, microbiology and immunology. To mimic these conditions in an animal study, Skaar and his colleagues focused their research on mice exposed to C. diff while being given antibiotics.

They found that mice who were fed a high-zinc diet experienced changes in their gut microbiome that made them more vulnerable to C. diff infection at lower doses of antibiotics, compared to mice on a normal diet. And when the mice did become infected, those in the high-zinc group experienced more severe symptoms and were more likely to die. (The high-zinc diet was designed to mimic levels humans might experience if they got plenty of zinc from food but also took a supplement for many weeks, for example, or if they took a zinc lozenge and a multivitamin every day.)

The authors published their findings today in the journal Nature Medicine, concluding that too much dietary zinc “reduces the threshold of antibiotics that are needed to convert a resistant microbial community to one that is sensitive to C. diff.”

In humans, C. diff most commonly affects older adults in hospitals or long-term care centers—but studies have shown that infection rates are increasing among young, healthy adults who aren’t on antibiotics and haven’t spent time in medical facilities.

Skaar says his findings may help to explain this increase. About half a million Americans get sick every year from C. diff, and infections in recent years have become more severe and harder to treat. In serious cases, symptoms can be life threatening, with complications including inflammation and damage to the colon, dehydration, and kidney failure. Although C. diff can be treated (with more antibiotics), sometimes a fecal transplant of healthy gut bacteria from a donor is used to stop the infection.

Although Skaar’s findings must be replicated in humans, his research does call into question the use of dietary supplements (like multivitamins) and cold remedies (like nasal sprays and lozenges) containing zinc. While zinc supplements are important for people with a diagnosed deficiency, Skaar says, they may not be so helpful for everyone else.

“Our study suggests that megadosing of zinc may be a dangerous practice, particularly for those who already obtain plenty of zinc from their diet or are at risk for C. diff infection,” Skaar told Health. (Risk factors for C. diff include a serious underlying illness or a compromised immune system, gastrointestinal surgery, and the use of proton pump inhibitors, according to the U.S. Centers for Disease Control and Prevention.)

And while zinc is important for fighting off infection, studies on whether zinc products actually reduce the duration of colds have been contradictory, he says. As for multivitamins—another common source of zinc—many experts no longer recommend them after several studies with disappointing results.

“I recommend that people try to get their nutrients from food unless they have a known deficiency,” Skaar says.

The findings may also have implications for livestock producers, who often use high doses of antibiotics, vitamins, and minerals to grow bigger animals. “It’s possible that zinc supplementation of livestock is leading to animals that are more susceptible to colonization with C. diff, and that might be a way that C. diff is then passed to people,” Skaar says.