Updated: 12th January 2020
In March 2018, a conference report by a research group at the National Institute of Environmental Health Sciences (NIEHS, USA), led by Dr Korach, made a lot of noise that reached far beyond the aromatherapy community. The media were full of alarming reports of new evidence of hormone disruption caused by lavender and tea tree essential oils.
The majority of responses from the community were either critical of the research – pointing to alternative explanations for the putative hormone-disrupting effects of essential oils – or neutrally cautious, warning that we should wait for the research to be properly published in a peer-reviewed journal.
In August 2019, the full article by Dr Korach’s group was accepted for publication, and in November it was published in The Journal of Clinical Endocrinology & Metabolism (Ramsey et al. 2019). The article features new case reports of hormone disruption in prepubertal boys and girls who purportedly used cosmetic products containing lavender oil, and evidence of in vitro estrogenic and anti-androgenic activity of several essential oil constituents.
The issue seems to have passed under the radar compared to the buzz caused by the preceding conference report. Nevertheless, this is a good opportunity to review the results and assess the current state of research. The issue is clearly sensitive, so it’s important to put away our biases and stick to the facts.
Estrogens and androgens are sex hormones involved in a wide range of sex-related physiological processes. Their activity is mediated through estrogen and androgen receptors, located inside different types of cells not only of reproductive tissues but also of bone, muscle, brain, liver, and cardiovascular tissue. When an estrogen or androgen hormone binds to the receptor, it triggers a molecular cascade which determines how specific genes are expressed, thereby affecting physiological processes in the body.
It is a fact of Nature that the estrogen receptor is shaped in such a way that not only the body’s estrogens but also a variety of other substances can bind to it and activate the signaling cascade. These molecules are termed xenoestrogens (meaning they are foreign to the body) and can be of natural or synthetic origin.
The natural balance of sex hormones is sex-specific and is tightly regulated during the different phases of development before and after birth. Imbalances due to internal factors or exposure to endocrine-disrupting chemicals can cause a variety of developmental disorders. In prepubertal children, elevated estrogen-mediated activity or depressed androgen-mediated activity can cause premature thelarche (breast growth in prepubertal girls) and gynecomastia (breast growth in boys).
In specific cases, however, targeting sex hormone receptors may have therapeutic value, such as in hormone replacement therapy and the prevention of neurodegenerative diseases.
POTENTIAL ESTROGENIC AND ANTI-ANDROGENIC EFFECTS OF LAVENDER AND TEA TREE OIL: A BRIEF HISTORY OF RESEARCH
The debate started in 2007 when Henley and co-workers published a short article in the highly esteemed New England Journal of Medicine. The authors established a “probably causative” link between the use of cosmetic products that purportedly contained essential oils of lavender (Lavandula angustifolia) or tea tree (Melaleuca alternifolia) and reports of prepubertal gynecomastia in boys.
To test the hypothesis that essential oils caused the symptoms, the authors conducted several in vitro tests with lavender oil and tea tree oil on cells that expressed estrogen or androgen receptors. Using a luciferase reporter assay, they observed maximum estrogenic and anti-androgenic responses at 0.025% and 0.005%, respectively, in both oils (higher concentrations were toxic to the cells).
The estrogenic response was about half that of the positive control estradiol at 1 nM concentration. In quantitative terms, this means that the estrogenic potency of essential oils was in the order of 1-2 million times weaker than estradiol (human estrogen).
In the following editorial correspondence, several researchers and industry representatives were critical of the study (Dean 2007, Kaylan 2007, Kemper et al. 2007, Kurtz 2007). Their common ground was that the effects were weak and, if true, would have been observed long ago considering the global exposure to lavender and tea tree oils.
In another response, Robert Tisserand (2009) pointed out that the products used by the boys with clinical gynecomastia were not specified and verified for essential oil constituents. Moreover, he argued that, given the nature of the products and their supposed use (mainly rinse-off products with typically very low amounts of essential oil), it seemed unlikely that essential oils could have caused the clinical symptoms.
Nielsen (2008) conducted another experiment on tea tree oil, employing a cell proliferation assay. The author confirmed the estrogenic effect caused by the complete oil but did not observe such effects from terpinen-4-ol, α-terpineol and 1,8-cineole, major constituents of the oil known to penetrate human skin. Moreover, earlier in vitro studies did not indicate any estrogenic activity for (+)-limonene, 1,8-cineole or (-)-linalool (Howes et al. 2002, Perry et al. 2001). In contrast, all of those constituents were found to have weak estrogenic or anti-androgenic activity in the new study – see below.
Adding further to the confusion, in a recent study Simões and co-workers (2018) found no estrogenicity for lavender oil but relatively high estrogenicity for palmarosa oil (Cymbopogon martini) and geranium oil (Pelargonium graveolens) – possibly due to geraniol (Howes et al. 2002) – and weak estrogenicity for nine other common essential oils. To test for estrogenicity, the authors employed a cell growth assay, and a luciferase reporter assay in MCF-7 cells similar to that used by Henley and co-workers (2007).
All of these were in vitro tests. In vivo tests of essential oil estrogenicity, on the other hand, are scarce. The vascular response and uterotrophic assays conducted by Howes and co-workers (2002) did not confirm in vitro estrogenicity of geraniol and citral reported in the same study. The authors administered 50-100 μL alcoholic solutions of 1.9 M citral and geraniol (approximately 30% dilution) dermally to mice twice a day for three days, but did not cover the skin, allowing evaporation.
In response to the study by Henley and co-workers (2007), an uterotrophic assay of lavender oil by dermal application (once daily for three consecutive days to rats’ backs covered with patches, at 4% and 20% dilution) did not indicate estrogenic activity (Politano et al. 2013). The doses were presumably equivalent to 6000 and 30 000 times the human maximum daily exposure by cosmetic products, and greater than 5000 and 1 000 000 times the estimated exposure from products used by prepubertal boys, as described by Henley and co-workers.
The uterotrophic assay is the standard in vivo method for evaluating potential estrogenicity based on an increase in uterine weight of exposed animals. It is generally considered a more reliable indicator than in vitro assays.
THE NEW STUDY
So why did Ramsey and co-workers (2019) conduct another set of in vitro experiments?
The study is a follow-up on the previous research on complete oils (Henley et al. 2007), but this time to establish mechanistic links to individual constituents from lavender and tea tree oils.
Here are the main results of the study:
- In addition to the previously known case reports of prepubertal gynecomastia, another three cases of premature thelarche and one case of prepubertal gynecomastia were reported. In all cases, children were exposed to products that purportedly contained lavender oil and clinical conditions resolved several months after discontinuation of their use. The authors leave open the possibility that the conditions were caused by other ingredients in cosmetic products (difficult to trace due to poor labelling). And, the conditions may have regressed spontaneously – in line with their often transient nature – thus coinciding with the withdrawal of cosmetic products.
- Lavender and tea tree oils and eight constituents: (±)-linalool, linalyl acetate, (±)-limonene, α-terpinene, 𝛾-terpinene, 1,8-cineole, terpinen-4-ol and α-terpineol (present in either of these oils) were tested separately for direct receptor activation and receptor-mediated gene expression for both estrogen and androgen receptor. The luciferase reporter assay was used to evaluate direct receptor activation (but on different cells than the studies mentioned above), and for receptor-mediated gene expression, the levels of mRNA were determined. In addition, the authors evaluated whether the constituents could stimulate activation of the estrogen receptor by recruiting its coactivator, SRC-2.
- The whole essential oils caused more pronounced estrogenic and anti-androgenic responses than any of the tested individual constituents at comparable concentrations. All the tested constituents caused some (low) estrogenic or anti-androgenic response, linalool, α-terpineol and terpinen-4-ol showing the strongest overall response. In both whole oils and individual constituents, anti-androgenic responses were more pronounced than estrogenic responses, which may be equally relevant for potential hormone disruption.
- The authors speculate that although the tested constituents caused weak estrogenic and anti-androgenic responses, the clinical conditions seen may have resulted from local exposure by dermal application to the breast area in children with increased susceptibility to endocrine disruption. They further suggest that, due to their lipophilic nature, some constituents could have accumulated in fatty tissues in the breast area and slowly released into the local circulation, thus interacting with the receptors over a prolonged period.
- The authors note endocrine activity in vivo could be more pronounced than the observed in vitro responses due to the ability of some constituents to recruit the estrogen receptor coactivator SRC-2 that is naturally present in the human body. [see Note 1]
- Concerns that plastic laboratory equipment could leach endocrine disruptive chemicals that caused estrogenic and anti-androgenic responses (Carson et al. 2013) were addressed in the study. Additional testing with corn and soybean oils at similar levels to the essential oils resulted in no detectable responses. [see Note 2]
- In conclusion, the authors do not recommend avoiding products that contain lavender or tea tree oil with children, but they do suggest their discontinuation in cases where these products are “suspected to be a possible cause for idiopathic premature thelarche or prepubertal gynecomastia”.
Note 1: Many other factors affect estrogenicity in vivo, such as the ability to pass biological barriers, hepatic metabolism, and other toxicokinetic parameters. For example, trans-anethole shows very weak estrogenicity in vitro (Howes et al. 2002, Tabanca et al. 2004) but seems to be more estrogenic in vivo (Türkyılmaz et al. 2008), possibly due to biotransformation to a more potent metabolite in the liver (Nakagawa and Suzuki 2003). Currently, trans-anethole is the only essential oil constituent contraindicated for its estrogenic activity during pregnancy, breastfeeding, endometriosis and estrogen-dependent cancer (Tisserand and Young 2014). In contrast, benzyl-butyl phthalate is estrogenic in vitro but has no estrogenic activity in vivo due to rapid hydrolysation to inactive metabolites (Conley et al. 2016).
Note 2: The use of fatty plant oils was likely based on the assumption that they have chemical properties comparable to essential oils, which may not be the case. Essential oil constituents have specific chemical properties that can make them more reactive and better able to dissolve certain components than fatty plant oils. Indeed, tea tree and lavender oils dissolve styrene trimers from multiwell polystyrene microplates at concentrations used in the study (Carson, unpublished data), and plastic leachates can affect estrogenic responses of MCF-7 cells (Ishikawa et al. 2001). The leaching plasticisers seem to be a wider problem (Olivieri et al. 2012) and could potentially confound the results from a variety of in vitro studies on essential oils that involve sensitive biological responses.
WHAT WE CAN MAKE OF THE RESEARCH SO FAR?
The implications of the study potentially concern many popular essential oils, countless cosmetic products, and even foods that contain some constituents in significant amounts. Should we rethink the way we use essential oils?
Let’s see what we can make of the studies so far.
Does the latest study by Ramsey and co-workers (2019) prove that lavender and tea tree oils and their constituents have estrogenic and anti-androgenic properties?
Although the results from the available in vitro studies are inconsistent and even contradictory, the new research by Ramsey and co-workers appears to be the most thorough mechanistic study to date. The results clearly indicate weak estrogenic and anti-androgenic responses of cells treated with the essential oils and some of their major constituents. However, the possibility of plastic leachates confounding the results cannot be excluded at the moment.
Does this mean that essential oils were responsible for the reported cases of endocrine disruption?
No. The ability of lavender and tea tree oil constituents to activate estrogen receptors or antagonise androgen receptors does not mean that the oils must have been responsible for the endocrine disruption in children. Other, more potent hormone-disrupting chemicals in consumer products could have caused the conditions, or they may have appeared and regressed spontaneously. Transient thelarche is a common condition in prepubertal girls (Lindhardt Johansen et al. 2017) while prepubertal gynecomastia is rare and prompts evaluation for the possible endocrine disorder (Ma and Geffner 2008).
It is theoretically possible to extrapolate findings from in vitro assays to in vivo settings by calculating the relative potency of a test compound to a reference compound such as estradiol and scaling the value to its in vivo potency. However, it has been estimated from available research databases that such predictions are in agreement, on average, in about 50% of cases, with a large proportion of false-positive results (Conley et al. 2016). While in vitro assays are useful for screening and establishing mechanistic links – which is what the new study was indeed designed to do – the results on their own are not directly applicable to a real-life situation.
Does the uterotrophic assay conducted by Politano and co-workers (2013) prove that lavender oil was not responsible for the reported cases of endocrine disruption?
No. The study relied on the assumption that the dose provided by dermal administration to rats was much higher than pragmatic maximum human exposure and should suffice to confirm any potential estrogenicity. Although the uterine weight of the test groups was not statistically different compared to control, a slight dose-dependent increase in uterine weight can nevertheless be observed from the available data, with medium to high effect size for the higher dose group.
While the study provides evidence that the potential estrogenicity in vivo is at most very weak, it does not exclude the possibility of endocrine disruption in some human individuals. Further testing is necessary, especially in the light of new research.
THE WAY FORWARD
Just when the endocrine debate in aromatherapy seemed to be resolved, the new study has left us with more questions than answers. The authors’ conclusion seems reasonable as the current evidence is not substantial enough to contraindicate the use of lavender and tea tree oils in children, but more research is needed.
A feasible approach to tackle the endocrine mystery would be to conduct a systematic in vivo study, with uniform methodology and defined doses, on whole oils as well as single constituents that have shown some estrogenic or anti-androgenic activity in in vitro assays. Positive matches from in silico screenings of constituents (Vegelj 2018) and known metabolites could be considered for testing as well. Such an approach may reveal more precise and biologically relevant results, applicable to a broader range of essential oils and products that contain essential oil constituents.
Epidemiological studies may complement the findings, provided there is a sufficient number of well-documented case reports, to help further establish potential links, risks and safety measures.
However, before pointing the finger at essential oils, there are many other estrogenic substances (natural and synthetic) we are exposed to on a daily basis through food, drink, personal care products, plastic toys, and industrial packaging. An additional factor that needs to be taken into account is that there might be individuals with increased genetic susceptibility to endocrine disruption, mediated by genetic polymorphisms (Stenz et al. 2019).
Since no clinical trials can be conducted due to ethical issues, we can only draw data from laboratory studies, observational studies and individual case reports. In complex cases like this, it’s important not to focus on a single line of evidence but look for multi-level explanations.
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