• Núria Estapé, science journalist

    The facts about skin hydration

    19 Jun The facts about skin hydration



    Skin Therapy Letter

    All the cells in our body need water to survive and multiply; the cells in the skin are no exception. Pharmacy, beauty centre and perfumery windows typically display hundreds of different skin moisturizing products. Who doesn’t use a moisturizer? But how does a moisturizer work if the skin is virtually impenetrable? Knowing where and how moisturizers work can help us choose the product that best suits our skin type.


    The skin tends to dry out more and more as the years go by. So moisturizers are high up in the ranking of bestselling dermocosmetic products. Each time we apply a moisturizer to our skin we are helping the outermost layer of the epidermis protect us against environmental aggressions. This protection, in turn, helps the skin cells carry out the metabolic processes that keep the skin alive and healthy. Moisturizers also reinforce homeostasis, that is, maintenance of the body’s internal balance in the face of external changes in humidity and temperature.


    Skin hydration secrets

    The outer layer of the skin is almost impenetrable. In fact, if our skin is dry and we take a bath, the skin dries again in no time. The explanation is that the skin absorbs little water from the exterior and also quickly loses water through evaporation.


    So it can be concluded that the skin receives water almost entirely from inside the body. What we call skin hydration is, in fact, the prevention of too much water evaporating from the skin’s surface layer. These are the scientific facts about skin hydration.


    Cosmetic moisturizing products may contain any of three basic types of ingredients that act differently on the skin, namely, emollients, occlusives and humectants.


    Emollients. These fill the spaces between the epidermal cells (called corneocytes). When the skin is dry, these cells shed, leaving holes through which water evaporates. Emollient substances such as ceramides, linoleic acid and other vegetable oils repair and re-waterproof the protective barrier of the surface layers of the epidermis, giving the skin a soft and flexible appearance.


    Occlusives. These prevent water evaporating from the skin by forming an extra layer that acts as an additional barrier. Petroleum jelly is the most effective occlusive, as it is capable of reducing trans-epidermal water loss by 98%. Other substances such as lanolin or silicone derivatives are also used as occlusive agents, which are most effective when applied to slightly damp skin. However, some occlusive substances leave the skin feeling greasy and may cause allergies.


    Humectants. These are water-soluble substances that help the stratum corneum capture moisture from the exterior while enhancing the skin’s lipid (fat) barrier. Glycerol, urea, lactic acid and the alpha hydroxy acids are the humectants most commonly used as ingredients in moisturizing cosmetics.


    What’s in the ideal moisturizer?

    Consumers ask: What should we look for when choosing the ideal moisturizer? A good moisturizing product should contain all three substance types: emollients, occlusives and humectants. The way these substances are combined, furthermore, should meet the needs of different skin types and the area of the body to which the product will be applied.


    Most moisturizing cosmetics are emulsions, that is, stable combinations of aqueous ingredients and oily ingredients. Changing the ratio between aqueous and oily ingredients allows specialist cosmetics to be designed for all the basic skin types: dry, normal and oily.


    Products for dry skin contain a higher proportion of oily ingredients – such as petroleum jelly, lanolin and mineral oils – and are most effective if applied in the evening. Lighter lotions or creams without many oils are more suitable for oily skin as they are less viscous; they are also often used as daytime moisturizers.


    Extra ingredients

    As well as emollient, occlusive and humectant compounds, most facial moisturizers contain additional ingredients, such as vitamins, exfoliants and sunscreens.


    Formulations may also include anti-cellulite and anti-flaccidity ingredients and even fake tanning agents. If we want an ideal moisturizer for ongoing use we also should ensure that it is hypoallergenic and does not sensitize the skin.