Misconception: Oily skin does not require hydration.

A common misconception is to believe that oily skin does not require hydration. The fact that it produces excess sebum (reactive hyperseborrhea) would suggest not to add more with the application of a moisturising product, thinking that this could be counter-intuitive. Even if it seems surprising, oily skin can also suffer from dehydration with consequences on the appearance and feel of the skin: tightness, irritations, redness...

Our skin naturally and daily loses water through sweating and also through evaporation (trans-epidermal water loss). Under physiological conditions, this loss is on the order ofapproximately 5 grams per square meter per hour. These water movements contribute to the proper physiological functioning of the stratum corneum by providing it with a continuous source of hydration. This hydration rate influences apparent macroscopic parameters (elasticity, softness...) but also molecular parameters, enzyme activity and cellular signalling within the epidermis.

Indeed, although a large majority of water is stored in the form of a semi-fluid gel due to its attachment to hydrophilic macromolecules (collagen, hyaluronic acid) in the dermis, a portion of it remains mobilisable and passively diffuses towards the epidermis. The skin is also perpetually subjected to external aggressions: pollution, allergens, microbes, UV rays, lifestyle hygiene (smoking, alcohol, etc...), excessive cleansing/exfoliating, hair removal, etc...

Thus, to maintain an optimal hydration gradient in the skin, it is essential to hydrate it even if it is oily. Applying a moisturising treatment will help the skin to limit this insensible water loss (IWL), to provide a barrier against external aggressions and at the same time to rebalance the sebum level. This is an important step to respect, especially when the skin is subjected to the effects of climatic variations (wind, cold, humidity, heat, etc...), and when the hydrolipidic film can become weakened and the skin can no longer retain water. Moreover, when the epidermal barrier is damaged and the skin is assaulted by external elements, the sebaceous glands are stimulated and produce more sebum to protect it.

The aim of this action is not to hydrate per se, but rather to prevent skin dehydration by protecting the hydrolipidic film. Indeed, the term "hydrate" commonly used in cosmetics is not entirely appropriate. On the contrary, providing oily skin with a cream will allow the "oily" elements to strengthen or rebuild the lipid barrier. Furthermore, protecting and soothing oily skin will gradually regulate its overproduction of sebum. Consequently, these hydrating formulations aim to maintain an optimal level of water within the epidermis to preserve its functionality, while restoring the barrier role of the horny layer and the hydrolipidic film.

The hydration of the skin thus depends on the integrity of the skin barrier, composed of the hydrolipidic film and the horny layer. The use of a moisturising product is therefore indicated, even for the oily skin. For this skin type, it is particularly appropriate to opt for a gentle care in the form of an oil-in-water emulsion, which is perceived as light, fluid and hydrating upon application, and whose composition reconciles mattifying effectiveness with active ingredients such as zinc, copper, bamboo extract, green clay, activated charcoal... and hydration of the epidermis.

Furthermore, it is crucial that this product is non-comedogenic, meaning it will protect the skin without clogging the pores, and sebum-regulating, meaning that in addition to preserving the hydration of the epidermis, it should also purify and balance sebum production. Regarding the hydration strategy, the treatment should contain substances of the type:

• Humectant/Hygroscopic Agents: These active ingredients attach themselves to the stratum corneum and help to trap trans-epidermal water within the cells of the stratum corneum. Examples include glycerine, aloe vera, panthenol, low molecular weight hyaluronic acid , sodium PCA, hydrolysed collagen, urea, sodium hyaluronate, fructose... ;

• Hydrophilic film-forming agents: These compounds are capable of retaining numerous water molecules within their structure, thereby forming a hydrogel on the surface to slow down water loss. We can mention high molecular weight hyaluronic acid (up to 1,000 times its weight in water), thepolyglutamic acid (up to 5,000 times its weight in water), high molecular weight collagen... ;

• Emollients: These are lipids that mimic those which make up the intercellular cement of the epidermis. They integrate into the lipid matrix to fill the micro-cracks between the cells of the superficial layers of the epidermis, thus having an action referred to as internal occlusion. They are primarily vegetable oils rich in polyunsaturated fatty acids and free lipids (fatty acids, fatty alcohols, fatty acid esters, sphingolipids, ceramides, squalane, stearyl alcohol...).

If the primary goal is to provide a moisturising and mattifying treatment, it must also contain anti-inflammatory agents (ex : niacinamide, aloe vera, etc...) and antibacterial components (ex : azelaic acid, bakuchiol, salicylic acid, tea tree essential oil, etc...). Indeed, oily skin tends to provide bacteria with the ideal conditions for rapid and sustained proliferation.