What is the composition of castor oil?

The castor oil is predominantly composed of ricinoleic acid (80 to 90%), a hydroxylated mono-unsaturated fatty acid found exclusively in this vegetable oil and responsible for its unique hair benefits. Indeed, this omega-9 is believed to have the ability to stimulate hair growth, a characteristic property of castor oil. According to in vitro studies, ricinoleic acid could inhibit the activity of prostaglandin D2 (PGD2), a protein that, in synergy with its metabolite 15-dPGJ2, interrupts hair growth by binding to the GPR44 molecular receptor. By doing so, ricinoleic acid and, by extension, the castor oil, would promote hair growth. While this hypothesis seems to have been validated in some in vivo studies, it is still a bit early to assert that castor oil can result in long hair.

Furthermore, ricinoleic acid possesses anti-inflammatory properties, which have been notably demonstrated. These were particularly evident in a study involving 10 mice. After inducing oedema in a paw, these mice were treated daily with a cream containing 90 µg of ricinoleic acid or the same cream without the active ingredient. After one week, the average volume of the oedema was 0.82 ± 0.10 mL in the "ricinoleic acid" group, compared to 1.39 ± 0.10 mL in the control group. It thus appears that ricinoleic acid had a positive impact on the inflammatory process, suggesting that castor oil could be a valuable ally for alleviating everyday redness and irritation.

Besides ricinoleic acid, castor oil contains other monounsaturated fatty acids, such as oleic acid (5 to 10%) and eicosanoic acid (< 1%). While the latter has been little studied, scientific literature recognises the former for its moisturising and soothing virtues. Naturally present in the hydrolipidic film, this mixture of water, sebum and sweat that covers the skin to protect it from dehydration and external aggressions, oleic acid can act to strengthen it, thus providing additional skin protection.

Furthermore, oleic acid has demonstrated in numerous studies that it can modulate inflammation by inhibiting the activation of NF-kB, an intermediary in the synthesis of several pro-inflammatory cytokines, as well as the expression of the COX-2 membrane protein, known for releasing prostaglandins. In synergy with ricinoleic acid, oleic acid contributes to making the castor oil an intriguing natural anti-inflammatory for soothing the skin and scalp.

Castor oil also contains several polyunsaturated fatty acids, notably linoleic acid (5 to 10%). This natural constituent of cell membranes plays a key role in the lipid organisation of the stratum corneum. Often referred to as intercellular cement, linoleic acid helps to maintain the cohesion between the cells of the epidermis, thereby limiting transepidermal water loss. It is also involved in the synthesis of acylceramides, a specific type of ceramides. As a reminder, the ceramides account for about 50% of the lipids in the stratum corneum and play a significant structural role in holding the corneocytes together.

In addition to its nourishing properties, linoleic acid is recognised for its healing benefits, derived from its role as a precursor in certain pathways for the synthesis of mediators of the cellular regeneration and angiogenesis. Although further work is needed to confirm this, it is possible that this fatty acid bestows healing properties on the castor oil.

Castor oil also incorporates several saturated fatty acids into its composition, including palmitic acid (2 to 5%) and stearic acid (2 to 5%). Known for their film-forming properties, these saturated fatty acids work in synergy to strengthen the hydrolipidic film. Stearic acid also has the unique characteristic of being part of the stratum corneum. A lack of stearic acid can destabilise the stratum corneum and increase water loss, which weakens the skin's water balance. An external supply, through the use of castor oil, could thus be beneficial, particularly for dehydrated skin.

Castor oil also contains various flavonoids. Among them, we can notably mention ß-sitosterol, campesterol, and quercetin. The first two are particularly renowned for their hygroscopic properties, that is, their ability to capture and retain water in the skin cells. Moreover, flavonoids have the unique feature of possessing an aromatic structure, which is the source of their antioxidant power. By neutralising free radicals, they can prevent damage caused by oxidative stress to cell membranes and preserve the integrity of the skin tissues. The flavonoids in the castor oil thus have an interesting protective role and help, for example, to delay the appearance of wrinkles and fine lines around the eye contour.

Castor oil finally contains tocopherols (400 to 500 mg/kg), which belong to the vitamin E family. Well-known for its antioxidant properties, it is responsible for the excellent oxidation stability of castor oil. Thanks to the free hydroxyl function carried by its aromatic cycle, vitamin E acts as a free radical scavenger, thus preventing these reactive species from altering the other actives in castor oil. The anti-radical action of vitamin E also helps to protect the skin and hair from oxidative stress, which accelerates skin ageing and also the appearance of white hair. It is also worth noting that mechanistic studies have shown that tocopherols have an interesting anti-inflammatory action, particularly via the inhibition of certain eicosanoids such as prostaglandin E2 (PGE2) and leukotrienes, which are mediators of inflammation.

Note : It is not certain that the vitamin E content in castor oil is sufficient to enable it to exert the various benefits mentioned above. The scientific literature only reports that this antioxidant vitamin contributes to making castor oil stable to oxidation.

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• JIALAL I. & al. Vitamin E, oxidative stress and inflammation. Annual review of nutrition (2005).

• BECKER L. Safety Evaluation of Phytosterols in Cosmetic Applications. Cosmetic Ingredient Review (2013).

• FONG P. & al. In silico prediction of prostaglandin D2 synthase inhibitors from herbal constituents for the treatment of hair loss. Journal of Ethnopharmacology (2015).

• FERNANDEZ L. & al. Chemical components of the oilseed crop Ricinus communis and their pharmacological properties: A review. Industrial Crops and Products (2016).