If you’re reading this, you most likely use essential oils. You know your antimicrobial, anxiolytic and anti-inflammatory oils, and probably diffuse your comfort oils to bright up the mood. You might also have some overview of the chemistry of essential oils and how to use them safely.
But have you ever wondered how essential oils work? How they can affect both the body and the mind, and are there different mechanisms at work? Is it important to understand how they work in the first place?
That depends, of course, on how you’re involved in aromatherapy. Understanding the principles of how essential oils work can undoubtedly give you a more in-depth perspective on the ways you can use them. On another note, it enables you to safely bypass nonsensical ideas, such as the functional group approach which only imbues aromatherapy with unnecessary confusion.
Throughout the major part of its history, aromatherapy has been based on the traditional use of aromatic plants. In the second half of the 19th century, empirical research on essential oils took place. Their chemistry and biological properties began to be studied more systematically. In the 1890s, pinene, limonene and terpinolene were among the first chemically characterised terpenes
By the time René-Maurice Gattefossé entered the scene, major constituents were known – Gattefossé mentions over 40 in his book Aromatherapy (1937/1993) – along with their structure, or at least molecular formula. Knowledge of the biological properties of essential oils in that time was a mix of herbal medicine, individual case reports and early experiments.
In subsequent decades, more sophisticated experimental methods were developed: various in vitro assays and animal models for testing biological effects in vivo. However, despite the accumulating knowledge what essential oils do, nothing was known about how they work.
It was not until the late 1980s when pharmacodynamic research on the constituents of essential oils started to emerge, focussing on molecular mechanisms. Of those early studies, some of the more salient were elucidation of the mechanism for the spasmolytic activity of menthol and peppermint oil (Hawthorn et al 1988) and the finding of a connection between linalool and glutamatergic system (Elisabetsky et al 1995), which explains an integral part of how lavender or other linalool-rich oils exert their generally calming effect.
Indeed, it took roughly a century from the early days of elucidating the molecular structure of the first constituents to the beginnings of unravelling the mysteries of their biological activity.
Currently, in times when the discovery of new synthetic drugs has been declining, we’re witnessing an explosion of natural products research, of which plant volatiles and essential oils play a significant role.
So, how do essential oils work? It has become widely accepted that they can act via pharmacological and psychological mechanisms. These are the two fundamental mechanisms, as they are mediated respectively by the body and the mind. We’ll look separately into each, although in practice they often overlap and interact (which is a good thing!), and it may be difficult to distinguish between them. Note that this is not a “this oil does this and that oil does that” type of article but more like a bird’s eye overview.
Effects of essential oils on the body are usually termed pharmacological because they are subject to pharmacokinetic parameters of absorption, distribution, metabolism and elimination. Pharmacokinetic parameters determine bioavailability and affect the toxicity of constituents.
Bioavailability is the crucial pharmacological parameter, and it must be sufficient to produce observable pharmacological effects. If you take a few whiffs of essential oil from the bottle, the concentration of bioactive constituents in your blood will be too low to reach pharmacologically relevant levels. However, the issue of bioavailability is complex, and it will be addressed in another post. Here, we’re only concerned with the general principles.
If there’s one thing you should know to understand the difference between pharmacological and psychological mechanism, it’s substance specificity. Pharmacological effects are substance-specific, which means that a given constituent will cause similar effects (yet not necessarily to the same degree) across the different subjects, by acting on the same molecular targets in a dose-dependent manner.
After entering the body via inhalation, topical application or internally, the constituents can bind to various proteins, such as ion channels, receptors, enzymes, carrier proteins, and proteins involved in signalling pathways. Due to their lipophilic nature, most can readily dissolve in cell membranes, where they can disrupt communication of the cell with its surrounding. Many antimicrobial constituents work by disrupting the cell membranes of microbial pathogens. The same thing happens in the cell membranes of our own tissues, which is why potent antimicrobial oils are also potential irritants.
Essential oil constituents typically exert their pharmacological effects by inhibiting the function of the proteins they bind to, and therefore inhibit or slow down associated biological processes. That is how lavender can induce calming and sedative effects in the brain, or how anti-inflammatory and spasmolytic effects are mediated.
In some cases, the constituents activate specific receptors and consequently enable or speed up associated processes. That is how many anxiolytics work, or why peppermint oil feels cold, or how we’re able to sense the smell of essential oils.
So far that might not sound very fascinating. But that’s how all pharmacologically active substances work, regardless of whether they’re of natural or synthetic origin. They either activate some process in the body or deactivate it. They don’t remove the cause for the condition but help the body to better cope with an out-of-balance situation, and – ideally – heal on its own.
There’s no magic panacea like the ancient Mithridatium that could protect us from every disease. I know no-one wants to hear this, but the paradox of life is that organisms inevitably sentence themselves to death by the very source they’re crucially dependent on: oxygen. All higher life forms need oxygen to be able to maintain their highly efficient metabolism. But that same efficient metabolism produces free radicals from the oxygen, which slowly, but inevitably cause our cells and bodies to ‘rust’ over the years. This is the fundamental cause for ageing, and there’s no way around it – despite the ingenious biochemical mechanisms that have evolved to slow it down, and all the antioxidants and healthy lifestyles.
Unfortunately, no medicine can provide some utterly new functionality to the body that previously wasn’t there; what they can do is help re-establish the balance to physiological processes that for whatever reason went out of ‘focus’.
How can we make sense of the different mechanisms discussed? Clearly, the mode of action will depend on the mode of application, but we can expect at least some interaction between different mechanisms – and here’s where it gets interesting.
For example, psychotherapeutic effects of essential oils can be mediated by both pharmacological and psychological mechanism. Linalool can work on the central nervous system through the pharmacological mechanism – by acting on specific ion channels and receptors – but it can also act through pleasantness, associations and expectations associated with its smell perception.
In a therapeutic context, smelling lavender can induce a calming effect through the expectancy of the calming effect itself, if such an experience has been previously associated with the smell of lavender. The calming effect can then be further consolidated by the inhibitory polypharmacological action of linalool on glutamate receptors and voltage-gated sodium channels, if sufficiently bioavailable. However, if you find the smell of lavender repulsive, or it reminds you of some bad experience in your life, it may rather act oppositely and make you frustrated.
In many cases, the mechanism behind an observed effect is not clear and further research is needed. But it’s the interactions between different modes of action that add a little bit of art to the science behind how essential oils work, making aromatherapy such a fascinating, yet often misunderstood discipline. We’ll go deeper into this subject in the next post!