Friday, March 9, 2012
Question: What's the difference between moisturizing and hydrating?
Occlusives create a hydrophobic (water hating) barrier to reduce water loss from the skin, creating a barrier to reduce transepidermal water loss (or TEWL). There are three approved barrier ingredients recognized by the FDA - dimethicone, cocoa butter, and allantoin. There are other ingredients that can be occlusive, like our butters, oils, and esters, but they aren't approved barrier ingredients.
Emollients, which include oils, butters, and esters, "enhance the flexibility and smoothness of skin and provide a secondary soothing effect to the skin and mucous membranes" (Cosmetic Dermatology: Practices and Procedures).
Humectants attract and hold moisture, facilitating hydration. Hydration is defined as an adequate measure of water in our skin - anywhere between 20% to 30% in our stratum corneum, or top layer of skin. (There seems to be a debate about how much moisture should be in our stratum corneum. So far the lowest amount I've seen has been 10%, but most papers are saying around 20%, so I'll go with the majority here.)
Let's recap: Humectants draw water from the atmosphere and bind it, and the occlusives keep the water from the lotion or the atmosphere trapped in your skin by creating a barrier. The emollients make our skin feel nice and smooth. Ideally we combine all three to create an awesome product that draws water to your skin, traps it in, and makes our skin feel smooth and elastic.
The job of a moisturizer is to prevent transepidermal water loss, "which facilitates the body's own barrier repair mechanisms" (Lippincott's Primary Care Dermatology, p. 30). In other words, we use a moisturizer to take the place of our skin's barrier mechanisms while they repair themselves. We can add things that might help speed up this process - for instance, using something like evening primrose oil or borage oil, both of which are high in GLA, or sunflower, soybean, or rice bran oil, all of which are high in linoleic acid, and various additives - but the whole goal is to great a barrier between our skin and the world so we don't lose more moisture.
min-maxed toner containing 0.5% allantoin could be considered a moisturizer because it creates that barrier between my face and the very chapping wind!
If we can make a moisturizer by slathering 100% mineral oil all over our skin, what's the point of making an exotic lotion filled with fancy ingredients like evening primrose oil, green tea extract, aloe vera and lavender hydrosol, shea butter, and sodium PCA lotion? Because creating a moisturizing product isn't just about creating a barrier. We can include ingredients for their benefits, like smoothing our skin, making it feel more flexible, speeding up barrier repair, reducing inflammation, and so on. Something like evening primrose oil is filled with gamma-linoleic acid, which can behave as an anti-inflammatory and barrier repair helper, as well as phytosterols, polyphenols, vitamins, minerals, and other things good for your skin. Green tea extract contains polyphenols and anti-oxidants. Aloe vera can offer soothing to damages skin, and lavender hydrosol might help soothe wind chapped or sunburned skin. Shea butter creates a nice barrier, but it also includes fatty acids that soften our skin, and sodium PCA is a great humectant that draws water to our skin.
On a final note, what's the difference between moisturizing and hydrating? Moisturizing is about creating an occlusive barrier to keep the water we have in our skin in our skin and preventing transepidermal water loss. Hydrating is about binding water to something like a humectant and keeping it on our skin. The ideal way to do this is to create an awesome product with humectants, proteins, or other moisture binding ingredient with an occlusive ingredient and some great emollients!
As an aside, if you have really dry skin and wonder why that lotion bar isn't helping your skin get any less dry, it's because you aren't adding any moisture to the equation. The lotion bar or whipped butter or balm is creating an occlusive barrier so you don't lose any more water, but it isn't adding moisture to your skin. To resolve this problem, apply the anhydrous product over damp skin to create a layer of moisture upon which you can apply your product!
Emollients (a whole section of the blog)
Chemistry of our skin: An updated overview
Skin hydration: A review on its molecular mechanisms (great paper!)
Glycerin and the skin (another great paper!)