Modes of Absorption

When essential oils are applied externally (the recommended method for lay people and medically unqualified aromatherapists), there are two main routes by which they may reach the bloodstream: by skin absorption and by diffusion across the thin air sacs in the lungs.

Skin Absorption

The skin is a two-way system, capable of both absorption and excretion. When we eat spicy or garlicky foods, the odour will be noticeable on our breath; the odoriferous molecules will also be secreted through the pores of the skin with the sweat. While it is true that water cannot be absorbed into the bloodstream through the skin, the upper layers of skin will temporarily hold a little water.

This is particularly noticeable after a long soak in the hath; the pads of your fingertips and toes will take on a wrinkled appearance. The skin can, however, convey certain substances to the systemic circulation, provided that their molecular structure is small enough. Essential oils are good examples of such substances.

It is thought that the tiny molecules of essential oils pass through the hair follicles, which contain sebum, an oily liquid with which essential oils have an affinity. From here the oils diffuse into the bloodstream or are taken up by the lymph and interstitial fluid (a liquid surrounding all body cells) to other parts of the body. If the skin is healthy, the aromatic molecules will be easily absorbed into the bloodstream, though different essences are absorbed at varying rates. The fastest travellers include eucalyptus and thyme, which reach the systemic circulation within half an hour of application; the slowest include coriander and peppermint, which can take up to two hours to reach the bloodstream. A classic test of the skin’s ability to absorb substances is to rub a clove of garlic into the soles of the feet; the odour will be detected on the breath an hour or two later.

If the skin is congested, or if there is much subcutaneous fat, absorption of plant essences is greatly hindered. Even so, people with congested skin often benefit from aromatic baths because warmth and moisture facilitate penetration of the aromatic molecules. The skin also becomes more receptive to essential oils when warmed and frictioned by the action of massage.

Moreover, when we take an aromatic bath or receive a full-body massage, essential oils will come into contact with sensitive areas such as the abdomen, inner thighs and upper arms: the skin in these places is softer and therefore more permeable.

Even though essential oils arc sometimes given orally, they can often be even more effective when applied to the skin. This can also be seen with other substances such as evening primrose oil; though not an essential oil, it appears to work better when applied externally for the treatment of hyperactivity in babies and children. Oral doses are not always successful because absorption from the gut is often impaired in these children. In fact, the skin of the very young child is especially permeable, which is why I no longer advocate skin applications of essential oils for babies and young children.

Critics would argue that external applications of evening primrose oil, and indeed any other fixed oil, cannot he absorbed through the skin because the oil’s molecules are too large. It would seem that this argument is born of a misunderstanding, for it is not the molecules or the vegetable oil itself which slip through the skin, rather it is the oil’s nutrients. Substances like vitamin E and essential fatty acids (found in unrefined vegetable oils) have a tiny molecular structure which can indeed diffuse into the bloodstream, just like an essential oil.

Similarly, empirical evidence would suggest that the skin can also absorb the medicinal properties of aromatic decoctions (water-based medicines) by osmosis or diffusion. Herbal baths and compresses can, it seems, be used to treat all manner of internal ailments, although experts argue about whether such treatment is truly effective. As with vegetable oil, it is not the carrier itself (i.e., the water or the vegetable oil) which is absorbed, but the medicinal substances suspended in the carrier-their passage through the skin being facilitated by warmth and moisture.

Despite advances in administering drugs by inunction (applying to the skin), many physicians underestimate the ability of the skin to convey nutrients to the blood-stream. It is often forgotten that vitamins (suspended in an oily base) were applied to the inner thighs of malnourished ex-prisoners who were too ill to take them by mouth after the Second World War, Even more remarkable, it was not only the fat-soluble nutrients which were found to enter the bloodstream by this method, but also the water-soluble molecules, vitamin C and B-complex vitamins. It is my own belief that in serious conditions, especially in life-or-death situations such as the example given, the skin becomes super-efficient at absorbing substances: becoming a kind of emergency ‘digestive system’. Therefore, it would seem that ‘feeding’ by inunction would also benefit sufferers of anorexia nervosa, (or impaired absorption from the gut is a well-known complication of this distressing illness.

There is nothing new about administering drugs by inunction. Indeed, it is well known that the ‘witches’ of antiquity anointed their bodies with hallucinogenic ‘flying ointments’ impregnated with extracts of hemlock, deadly nightshade and the poisonous toadstool known as fly agaric. Since these plants are without an essential oil, their active principles being crystalline alkaloids, this must surely counter the notion that it is only the oil-soluble, highly volatile essential oil of a plant which is capable of travelling across the skin.The use of the nicotine patch (nicotine is an alkaloid) to help smokers kick the habit is a more modern example of the effect.

Administering drugs by inunction has been made safer recently with the introduction or a measured dose. For example, oestrogen and trinitrin can be administered through a patch applied to the skin. The advantage of transdermal applications over oral dosage is that much smaller quantities of the drug may be administered because none is lost in the digestive processes, nor broken down by the liver. This method also avoids irritation of the gastrointestinal tract (a common side-effect of many drugs). Likewise, smaller doses or essential oils applied externally may be just as efficacious as larger quantities of the same essences taken by mouth.

Absorption via lungs

When inhaled, the aromatic molecules or essential oils reach the lungs from where they diffuse across tiny air sacs (alveoli) into the surrounding blood capillaries and eventually find their way into the systemic circulation from where they exert their therapeutic effect.

A Japanese experiment carried out in 1963 concluded that the effect of peppermint oil on the digestive system was stronger if inhaled than when the same oil was taken by mouth. Interestingly, aromatherapists often comment upon the appetite-stimulating ‘side-effect’ of working with essential oils in general. To the weight conscious, this could be regarded as something of an occupational hazard. But to the epicurean, the aromas of essential oils sharpen the senses, and thus enhance all of life’s pleasures, not just the enjoyment of food.

It is also interesting to note that a common side-effect reported by female hop pickers is menstrual change as a result of inhaling the aroma of hops. Some women experienced heavier menstrual flow; others whose menstrual cycles were jerky discovered the efficacy or hops in regulating the cycle. Although a certain amount of oestrogen like substances found in hops may be absorbed through the skin of the hands, it seems much more likely that inhalation is the most important route. Indeed, herbalists generally agree that it is the volatile aromatic element (the essential oil) of hops that appears to be the most active.

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