What is the composition of castor oil?

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.