Friday, March 9, 2012

Question: What's the difference between moisturizing and hydrating?

What's the difference between moisturizing our skin and hydrating it? Are these fancy marketing terms or is there some difference between the two concepts? Let's review what we need for a moisturizing product...

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.

What kinds of products could be considered moisturizers? Anything that creates the barrier between our skin and the world. Slather on some mineral oil or olive oil and you've created that barrier. A fancy lotion with all kinds of extracts and exotic oils creates that barrier. A lotion bar, a whipped butter, a balm, a lotion, a cream, a body butter, a creme, and so on - anything that creates a barrier between you and the world counts as a moisturizer. You don't necessarily need to have an emollient or oil in the product, which means my 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!

Related posts:
Emollients (a whole section of the blog)
Chemistry of our skin: An updated overview

Related reading:
Skin hydration: A review on its molecular mechanisms (great paper!)
Glycerin and the skin (another great paper!)


LabMuffin said...

Is the difference regulated? Or do companies use them depending on which one markets better?

Nedeia said...

in my country, people use "hydrating" for both, which pisses me off. I hear in all kind of places that oils and butters hydrate the skin. I am glad that I have mode info now for proving they are wrong :-)

Susan Barclay-Nichols said...

Hi LabMuffin. None of these terms are regulated. I could claim that my cup of Earl Grey tea is a moisturizer and hydrator and there's nothing you can do if you don't find it moisturizing or hydrating, other than tell your friends that it wasn't moisturizing or hydrating.

It's like the idea of being "clinically proven" to do something. All I have to do is get some people into a clinic, put some of my Earl Grey tea on them and ask them what they think of it as a moisturizer and/or hydrator. Then I say things like "four out of five women saw a reduction in fine lines after using this product" or "three out of five women felt this product moisturized better than their normal brand", and I've got a marketing campaign. It's all pretty underhanded, if you ask me.

So you can call whatever you like moisturizing and/or hydrating without any proof!

Sara @Osmosis said...

And drink enough water, too!

Nedeia said...

Sara - indeed! I also advise my friends who want a hydrating "something" to first watch their diet and make sure they drink their water :-)

Mychelle said...

Great post! I always differentiate between a "moisturizer," which to me implies moisture barrier, and a "hydrator," which to me implies humectants and water. Glad to see I'm thinking along the right lines!

geri said...

It's a great article! I have one question - are all of humectants, emollients and occlusives water free?

Susan Barclay-Nichols said...

Hi Geri. Occlusives, emollients, and humectants can be water or oil soluble, so the answer to your question is no, they aren't water free.

In general, most emollients are oil soluble. Our carrier oils, exotic oils, and butters are all emollients. You can get emollients that are oil soluble - the esters like PEG-7 olivate or water soluble shea butter - but for the most part, I would consider emollients as being oil soluble or water free.

The three approved occlusive ingredients - cocoa butter, dimethicone, and allantoin - are oil soluble, oil soluble, and water soluble, in that order. There are other ingredients that are occlusive - and those tend to be oil soluble, but occlusives aren't necessarily oil soluble.

And most humectants are water soluble. In fact, the only oil soluble humectant I can think of is olive oil, and it's not a great humectant.