Tuesday November 18 2014
Let's not find more reasons for poor hand hygiene
“A chemical ingredient of cosmetics, soaps, detergents, shampoos and toothpaste has been found to trigger liver cancer,” reports The Independent. The chemical in question, triclosan, is used in many products as an antibacterial.
Should you be worried if you have just washed your hands? Probably not. The link was found in mice, not humans, and the mice were given a much larger comparable dose than humans are ever likely to be exposed to.
The study found that mice fed high amounts of triclosan daily for six months suffered liver damage and were more susceptible to liver tumours induced by other cancer-causing chemicals.
The findings tell us very little about the potential health effects on people. However, it’s important not to be complacent. Further investigation may be warranted in humans, especially when it comes to topical application, and at lower exposure levels.
The concerns have resulted in an investigation by the US Food and Drug Administration (FDA), which regulates its use in America. The FDA said that it does not have enough safety evidence to recommend any “change to its use in consumer products”. This means that the evidence does not tell us whether or not triclosan is harming people through background exposure. Until further evidence accumulates, we will remain in the dark about this issue.
Where did the story come from?
The study was carried out by researchers from the University of California and was funded by US Public Health Service Grants.
The study was published in the peer-reviewed science journal PNAS.
Generally, the media reported the story accurately. The Independent, for example, took the commendable step of indicating that it was research on mice in their main headline. This prevents any incorrect assumptions that it was on humans. The body of the Independent article also appeared factual and not overly alarmist, discussing the views of different scientists who thought the chemical might pose a risk to humans, and those that thought it was too early to tell.
Conversely, the Daily Express chose to lead with the words "Cancer scare", which was an unnecessary step. The paper also took several paragraphs to explain that only mice were studied.
What kind of research was this?
This was a laboratory study using mice to investigate the potential cancer-promoting properties of triclosan.
Triclosan [5-chloro-2-(2,4-dichlorophenoxy)phenol; TCS] is a synthetic, broad-spectrum antibacterial chemical used in a wide range of consumer products, including soaps, cosmetics, therapeutics and plastics. The general population, the researchers point out, are exposed to triclosan because of its prevalence in a variety of daily care products, as well as through waterborne contamination. They say it is linked to a variety of health and environmental effects, and wanted to investigate the effect on the liver.
Researchers often use mice because, as mammals, they share similar biology with humans. Hence, research on mice can tell us what might happen in humans, without directly experimenting on them. The caveat is that there is no guarantee that results in mice will be replicated in humans as, while similar, the biology of the two organisms is not identical, and the differences can sometimes be crucial.
What did the research involve?
The research involved two groups of mice: one fed a normal diet and the other a diet supplemented with triclosan. After eight months on the diets, the mice were killed and their livers removed and analysed for physiological and genetic signs that the chemical was promoting cancer growth.
In a second experiment, the research team injected two groups of mice with a chemical that causes the development of cancerous liver tumours, to see whether giving triclosan (this time given in their drinking water) influenced the development of the tumours thereafter.
What were the basic results?
Effect of long-term triclosan in the diet on liver biology
Through physiological and genetic analysis, the results suggested that triclosan increases liver cell proliferation, induces liver scarring and reactive oxygen species accumulation. Taken together, the team concluded this was a sign that triclosan damaged the liver cells, implying they may be more likely to become cancerous.
Effect of triclosan after tumour promoting injection
Triclosan-treated mice had a higher tumour number, bigger tumour size and greater tumour incidence than mice given the tumour-promoting injection alone. The number of detectable liver cancers was around 4.5 times higher in triclosan-treated mice than in control mice.
Approximately 25% of mice receiving the tumour promoting injection only exhibited small cancerous nodules, whereas more than 80% of triclosan-treated mice developed tumours. Maximal tumour diameter was also 3.5-fold larger in triclosan-treated mice.
How did the researchers interpret the results?
The study authors acknowledged that, “animal studies require higher chemical concentrations than predicted for human exposure”, but said their study, “demonstrates that TCS [triclosan] acts as a HCC [liver cancer] tumour promoter and that the mechanism of TCS-induced mouse liver pathology [disease] may be relevant to humans.”
This small mouse study raises the prospect that triclosan may have tumour promoting-properties that could be relevant to humans but, on its own, does not provide any conclusive evidence that it does.
Firstly, the findings in this small group of mice need to be replicated by other research teams to ensure they are reliable. This should include the effect of triclosan at different levels of exposure and through different exposure paths, such as through food, water or skin. The latter would be of particular relevance to humans, given that much of our exposure to triclosan is topical (via the skin) rather than oral.
The current mouse study, as the authors acknowledged, “require[d] higher chemical concentrations that predicted for human exposure”. This means the mice were given very high amounts of the chemical relative to what you might expect the average person to be exposed to in real life.
The second issue is that even if the results are found to be reliable in mice, there is no guarantee that the same effects will be the seen in humans, irrespective of exposure levels or exposure route. While humans and mice share many biological mechanisms and similarities as common mammals, their differences can be crucial during disease processes.
At present, we simply don’t know if similar results would be found in people. It would also be unethical to give someone a high dose of something on the premise that you are trying to prove it causes cancer. Therefore, it is likely that large and long-term cohort studies, using natural exposure levels, will give us the best evidence on the potential health effects of triclosan.
As a result, there are many unanswered questions around this research and the potential harms (or lack of) associated with triclosan that may warrant further investigation. This is especially due to its ubiquitous use in a range of both commercial and healthcare products.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.