Could this household chemical mess with your body's microbiome?

Some scientists are raising concerns about a common antimicrobial chemical found in household products millions of people use every day, and what it might do to the healthy balance of bacteria in the human body.

Triclosan is used in many brands of toothpastes, antibacterial soaps, and disinfecting wipes and gels. It's infused into everything from cosmetics to plastics to kitchen utensils, and even added to toys to make them germ-resistant. It's so widespread that a 2008 study found traces of triclosan in urine samples of 75 percent of the population.

What's the significance for our health? The long-term verdict is still out. "Triclosan is not known to be hazardous to humans," the FDA states. But it adds, "In light of questions raised by recent animal studies of triclosan, FDA is reviewing all of the available evidence on this ingredient's safety in consumer products." So far the main concerns have focused on bacterial resistance and possible impact on the body's endocrine system (the hormones that affect growth, reproduction and other functions).

Some animal studies have suggested another potential issue: absorbing triclosan could be doing silent damage to the microbiome -- the communities of billions of microbes in each of our bodies that work together to play a vital role in human health. A small study in humans, however, found no evidence of harm to the microbiome. A paper published Friday in the journal Science examines the conflicting evidence about triclosan's effect on these important networks of microscopic helpers, and stresses the need for further research.

Given the growing evidence that a person's microbiome can influence everything from the immune system to weight problems to a range of debilitating disorders, the authors say triclosan's role is worth scrutinizing.

"Exposure to antimicrobial compounds can disrupt the community of microorganisms that colonize the human body. Perturbations in the microbiota have been linked to a wide array of diseases and metabolic disorders, including obesity, inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), and behavioral and metabolic disorders," paper authors Alyson L. Yee and Jack A. Gilbert wrote. "However, it remains unclear whether triclosan can disrupt the microbiome to such an extent that it influences health and well-being."

How does triclosan work? At very low concentrations, the chemical can stop microbial growth, but when used in high doses it can kill bacteria completely. The paper's authors cite past research showing it's not certain exactly how resistant bacteria have become to the chemical. More bacterial resistance has been found in the lab, but it is unclear how common that resistance is in a more natural environment.

To examine its effect on the microbiome, a 2016 study published in PLOS One exposed adult zebrafish to high doses of triclosan over a seven-day period. The researchers detected "changes in community structure" in each zebrafish's microbiome as well as an increase in resistance to the chemical. Fish exposed for a shorter period of time -- four days, along with three days of no exposure triclosan -- did not show any change in their microbiome. Yee and Gilbert point out that this could show that the fish in the study either had "resilience to temporary disturbance" in their microbiome or that "changes are reversible."

Another study on minnows in 2015 had similar results: high doses of triclosan altered the fishes' microbiomes, but they recovered after exposure stopped.

Unlike the fish in those studies, which spent extended time fully immersed in triclosan-tainted water, humans tend to use small amounts of the chemical only briefly and then rinse it off.

A 2016 Cornell University study tested lower concentrations of triclosan in humans and found no detectable impact on their microbiomes. Researchers gave seven volunteers triclosan-containing products like toothpaste and dish soap to use for over the course of four months. After this period, the volunteers would switch to equivalent products without the chemical for another four months. For comparison, a second group of people started the experiment with non-triclosan-containing products and then moved on to triclosan-containing products for the last four months of the study.

The researchers found that more triclosan was found in the urine of all participants during their triclosan-using period. That being said "the stool, molar, or incisor microbiomes did not change" during this time.

To fully understand the potential impact of triclosan, the Science authors conclude that more research needs to be done.

"Future research should explore the role of dose, timing, and route of triclosan exposure," Yee and Gilbert wrote. "Humans are exposed to triclosan transiently and in small doses, but the presence of triclosan in surface, ground, and drinking water indicates its potential to persist and accumulate in the environment."

They also raise the possibility that prenatal exposure of developing fetuses, or exposure in newborns, could be more detrimental than what's been seen in adults, and needs further study as well.