Fixed Oils and Skin Penetration
Fixed Oils And Skin Penetration
- Atopic dermatitis has been shown to respond positively to dermal application (Lovell 1981, Wright & Burton 1982, Kersher & Korting 1992) - but this is disputed by Sharpe & Farr (1990) - and other skin conditions, such as psoriasis, may also benefit (Ferrando 1986). There is some evidence that oils rich in bonded essential fatty acids do bring benefits where the skin is dry.
- The stratum corneum in normal skin is rich in saturated and polyunsaturated fatty acids and a deficiency in these essential fatty acids can result in dryness because of the increase in trans-epidermal water loss. The level of GLA in the epidermis is low because of a lack of the enzymes necessary to convert LA to GLA and in dry skin the level of LA is low.
- It has been observed that topical application of borage and evening primrose oils increases the levels of GLA in the stratum corneum up to 2% following application of the oil for fourteen days. (Hoofmann -La Roche 1989) so helping dry skin problems.
- The skin was always considered to be completely impenetrable (Fleisher 1877) up until the beginning of the 2oth century when it was generally accepted that, although relatively impermeable to water, the skin did accept lipid soluble substances, including vegetable oils and fats. Saafeld (1911) p.171) wrote that natural fats are insoluble in water: they serve as protectives to the skin against external injuries, and partially penetrate it, in virtue of which it swells, acquires a gloss, and is protected against dryness.
- He also noted that it may be difficult to introduce a medicament into a greasy scalp , unless it can be got to incorporate itself or mix with the greasy secretion. Hence an ointment containing the drug will penetrate in such circumstances more readily than anything else (Saalfeld 1911p. 178)
- Today the skin is regarded as a semi-permeable barrier to many materials. Natural sebum permeates the space between the cells, which themselves have a lipid bilayer thus enabling liquid molecules to pass the stratum corneum by finding their way around and through the cells.
- The percutaneous absorption of some vegetable oils was studied in adult albino rats: in this test sweet almond oil was found to have a relatively low percutaneous absorption: it was concluded that increased amounts of short chain and polyunsaturated fatty acids in oils enhances their percutaneous absorption ( Valette & Sobrin 1963)
- PARENTAL NUTRITION
The observation made in the previous section (namely) that carrier oil molecules are too large to be absorbed through the skin)may not always be absolutely true. Consideration given to the use of cutaneously applied vegetable oils as a way of preventing essential fatty acid deficiency in some people of their components.
- Press, Hartop and Prottey (1974) used sunflower seed oils with three patients who had developed essential fatty acid deficiency after major intestinal resections .
- The deficiency was corrected by applications of 2-3 mg of the oil per kg body weight per day for twelve weeks. Friedman et al (1976) reported the correction of essential fatty acid deficiency in two infants who were given 1400mg per kg per day of topical applied sunflower oil.
- The above work does not seem to suggest that in order for the essential fatty acids to be made available the triacylglycerol molecules have to undergo a chemical process known as hydrolysis, and this can only take place within the body, thus all the parts of the oil must necessarily have passed the skin for this to happen. However, other investigators have reported that essential fatty acid deficiency cannot be influenced by cutaneously applied vegetable oils. (Hunt et al 1978, McCarthy et al 1983, O'Neill, Caldwell & Meng 1976).
- Miller et al (1987) examined the use of safflower oil on five patients and deduced that topical applications may improve plasma fatty acid profiles but adeqacy of tissue stores remained unanswered. They noted that only the deficiency in one of the fatty acids was being addressed (linoleic acid) as safflower oil does not contain appreciable amounts of linolenic acid.
- They were cautious about extending the results of the study, and felt that because some of their subjects displayed abnormal results in liver function tests such tests should be carefully monitored in all patients given cutaneous safflower oil.
- TEMPERATURE AND VISCOSITY
- There are other influencing factors, for example viscosity of the penetrant: viscosity is the 'stickiness' or otherwise affecting the rate of flow of a substance - water has a low viscosity and treacle has a high viscosity.
- Viscosity plays a part with some carrier oils, some being relatively viscous (almond, olive) and which are absorbed only very slowly: it is known that high viscosity fats such as lard and wool fat retard or prevent percutaneous absorption(Macht 1938).
- Other oils which are less viscous (grapeseed, linseed) are relatively quickly absorbed as tests involving the abdominal skin of the rat have shown. However, avocado oil has the reputation of being easily absorbed into the skin and aiding the penetration of substances dissolved in it, despite its relatively high viscosity.
- Temperature is also a consideration as any oil becomes less viscous with a rise in temperature; therefore it can be expected that warming the oil and the hands, and applying to a warm body would aid penetration.
- FATTY ACID SATURATION
Yet another influencing factor is the degree of saturation of the vegetable oil; it appears that the greater the unsaturation of an oil, the better the penetration rate. It has been suggested increased amounts of short chain and polyunsaturated fatty acid components in vegetable oils favours their percutaneous absorption (valette& Sobrin 1963).
- Linseed oil crosses the skin more quickly than expected from its viscosity but the skin is resistant to olive oil which is mainly monounsaturated (Scheuplein & Ross 1970). On the other hand, there is some evidence that jojoba wax, consisting mainly of saturated fatty acids, can permeate the skin. Photographs have been produced showing the oil in a 'pool' at the base of the hair and moving through the follicle wall into the corneal layer (Anon 1985).
- It would therefore seem that until such time as a more clear cut answer is obtained, it is reasonable to continue to say that the molecule of carrier oils are perhaps too large to be absorbed through the skin: Zatz (1993) speaking of molecular size say that considering that the horny layer is a compact membrane and that diffusing molecules must follow a tortuous path through it, it might seem obvious that the diffusion coefficient would be inversely proportional to molecular weight and size; in other words, the bigger the molecule the less the likelihood of it passing the skin barrier.
- Essential oils are completely soluble in all proportions in vegetable oils and so it must follow that the rate of absorption of an essential oil is governed to a large extent by the characteristics of the vegetable oil employed as the carrier. The fixed oil also influences the amount of essential oil absorbed through the skin because the volatility of the essential oil is decreased and so it remains on the body longer, giving greater opportunity for absorption.
- Reference: Carrier OIls for Aromatherapy and Masage: Len Price with Ian Smith & Shirley Price
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