The antimicrobial agent triclosan -- widely found in soaps, toothpastes, detergents and other cleansers beloved by germophobes, may promote scarring of the liver and the growth of cancerous liver tumors, says a new study.
That finding -- in male mice fed a daily dose of triclosan in their diet over eight months -- is likely to lead regulators at the Food and Drug Administration and Environmental Protection Agency down new paths as each agency continues its review of the antimicrobial agent's safety.
While triclosan has raised concerns as a possible hormone-disrupting chemical, this is the first study in living animals (as opposed to cell cultures) that suggests it could harm the liver or promote the proliferation of cancerous cells.
The study, published this week in the journal PNAS, found, first, that compared with mice that had no exposure to triclosan, those that had a steady exposure to relatively high concentrations of it were far more likely to develop inflammation and scarring of the liver, a condition that, in humans, raises the risk of developing liver cancer.
Second, the researchers found that where cancerous cells had established a beachhead in a mouse's liver, those steadily exposed to triclosan were more likely to develop liver tumors, and to develop more and bigger tumors, than were mice that did not have the same exposure.
The new findings are worrisome because triclosan, since its use in consumer products began to explode 20 years ago, has become a ubiquitous manufactured chemical: It is listed as one of the seven compounds most frequently detected in American streams, and is widely found in the urine, blood and breast milk of U.S. residents as well as wild animals.
That growing presence comes at a time when Americans' livers are already in greater peril. Excessive alcohol consumption has always been a danger, but the liver may be among the organs endangered by our constant exposure to heterocyclic amines -- the compounds formed by cooking foods at high heats and which are thought to be carcinogenic. And the obesity crisis has made "fatty liver" -- once a rare condition, far more common.
The study suggests that where other environmental exposures may increase the prospect of chronic liver inflammation, triclosan may make cancer -- and more advanced cancers -- a more common outcome.
The authors of the latest study acknowledged that they were unsure how well the triclosan exposure they chose to use in mice would mimic the effects of triclosan in humans. The daily dose they dissolved in the water consumed by mice was close to 60 times the dose, pound-for-pound, that a human might be exposed to in a single gram of toothpaste (much of which goes down the drain). Not only does use of triclosan-containing products vary widely among Americans, but so does Americans' ability to metabolize and expel the compound from our bodies.
"It is challenging ... to translate findings to human health," the authors wrote. But such questions will be central to both the FDA's and EPA's reviews.
And the study does elucidate a possible mechanism by which triclosan may harm the liver -- first by scarring it, which prompts the liver to overproduce replacement cells, causing inflammation and eventually promoting abnormal cell growth. They added that it was "worthwhile exploring the possibility" that triclosan may have a similar effect on other organs.