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.