Welcome back! It’s been a couple days since my last post in this ongoing series. I’ve had family visiting and have been busy giving the tour of Los Angeles.
This post will be a bit long, and with good reason — moisture is extremely important to skin health.
An Introduction to Moisturizers
“Moisturizer” is a bit of a vague marketing term. Moisturizers do not necessarily add moisture to the skin as much as they reduce or slow the amount of water that skin loses. They’re important to everyone who cleanses their skin, whether with water or a cleanser, but even more critical for those with conditions that cause the skin to lose moisture more easily, such as eczema. In short, moisturizers provide a barrier against moisture loss by creating a soothing, protective film.
But let’s slow down a little. Why does the skin lose moisture in the first place, and where does it even get moisture? To answer this, I present to you a helpful diagram of the skin’s epidermis.
You may remember from Health or Biology class that skin is generally broken into three major layers: Epidermis (or the upper most layer — what we can see with our eyes), the dermis (the middle layer — where most of the nerve endings, oil glands, hair follicles, blood vessels, and collagen hang out), and the hypodermis or the subcutaneous layer (where our fat lives).
There are multiple layers of each of these three basic layers, but we will be focusing on the layers of the epidermis, particularly the stratum corneum.
The epidermis is composed of thousands of cells known as keratinocytes. They begin life deep in the stratum basale, where they slowly make their way upwards, propelled by the growth of new cells below. As these cells get closer to the surface, they flatten down, harden, die, and eventually flake away. Some skin does this well, some not so well (dry skin, acne skin, etc.).
This process is generally referred to as skin cell turnover, and as we get older, this process gets slower and slower. In children, this only takes about two weeks, and in teens it is three to four weeks. Once you’re into your adult years, this slows down to a month, and by the time you’re over 50, it’s closer to 45 to 90 days.
Nowadays, we have ingredients that can speed this process up when applied topically, but that’s another post for another time.
The water that keeps these skin cells happy and healthy comes from deep within the epidermal layers and works its way upwards, until it is eventually lost to evaporation, but just like any organ of the body, it functions best with moisture. When skin is broken or it’s natural processes disrupted by wounds, burns, exposure to surfactants (cleansers), or extreme dryness (winter), it loses water quicker. That’s why doctors tell you to keep wounds moist and covered and you feel the desire to use richer moisturizers in the winter. When you keep even the upper most layers of the skin hydrated, the cells aid in elasticity and the enzymes within the skin are able to function more efficiently.
In short, moisturizing helps to repair the skin, increases water content, reduces trans-epidermal water loss (water evaporating out of the skin), and maintains the appearance of the skin. They do this by acting as humectants, occlusives, emollients, and rejuvenators.
Occlusive is a fancy word for a sealant — a compound that is generally hydrophobic and prevents water from evaporating.
They’re not typically very appealing because they can feel heavy or greasy, but they are the most effective ingredient at reducing trans-epidermal water loss. This property makes them particularly effective when used immediately after a shower or cleansing, while the skin is damp.
The most effective occlusive agent is petrolatum — a bland little ingredient with a bad reputation. It is the most commonly used ingredient in skin care and reduces water loss by 99%. It’s able to initiate production of lipids by penetrating into the upper layers of the stratum corneum (“sealing the cracks” in the skin, so to speak), and can reduce the appearance of fine lines and dryness caused by dehydration.
The second most common ingredient is dimethicone, though it is permeable to water vapor, which makes it less ideal for compromised or damaged skin.
Occlusives, when used alone in a skin care routine, are the last step. I personally apply a touch of Vaseline on the driest parts of my face after applying moisturizers and prescription medications.
Other common types of occlusives include squalene, paraffin, lanolin, cetyl alcohol, beeswax, and cholesterol.
Humectants are kind of like the hydrators of skin care ingredients. They’re hydroscopic, meaning they are able to attract and hold onto water molecules. However, due to their water-loving way, they can pull water away from the dermis and cause excessive water loss. This is why humectants are almost always combined with occlusives, like petrolatum.
The most common humectants are glycerin or glycerol and hyaluronic acid, but there are also hydroxy acids (another post, another time), propylene glycol, and urea.
Emollients are the ingredients that serve the primary function of filling in the cracks between skin cells. Many times, occlusives can be used and function as emollients. These ingredients are what make your skin feel soft after applying a moisturizer, and are commonly natural oils like rosehip seed oil.
While simple, these ingredients can assist in the inflammatory response of the skin, and are very individual. For example, you may notice that your skin seems to act better when you use rosehip seed oil, thanks to the linoleic acid (an omega 6) within the oil.
Rejuvenators replenish the proteins in skin, and include ingredients such as collagen, keratin, and elastin. These ingredients have limited permeability due to their large molecular size, but they can fill a similar role as emollients, filling in fine lines and smoothing the skin.
A Note on Ceramides
Ceramides are a fairly new breakthrough in cosmeceuticals. They’re naturally found in the lipids of the skin, and help to maintain the skin barrier (the function of the skin that holds onto moisture and keeps out unwanted chemicals). In a 1990 study, “it was found that the level of ceramides was greatly reduced in the stratum corneum in patients with atopic dermatis. It was subsequently concluded that an insufficiency of ceramides in the stratum corneum is an important factor in atopic dry skin. Subsequently, ceramides have been added to many moisturizes used in the treatment of both atopic and normal skin.” 
You can find ceramides starring in many creams and lotions now, and even at your local drugstore in brands like CeraVe.
All Tied Together
When I first began resolving my own skin issues, the greatest barrier (and what finally solved my acne) was finding a moisturizer that worked for me. I have several sensitivities and am unable to use a handful of common ingredients as well as a large number of uncommon ingredients. Skin needs are unique and individual in this way. You may find that you need to try many products to find what works best for you, but here are some guidelines to get you started.
- Look for simple ingredient lists. Ingredient lists proven by the science are the best kind. Try to find something without fragrance, essential oils, and if you have particularly sensitive skin, without natural nut or plant oils, such as jojoba, rosehip, marula, or almond oil. This also makes it easier to rule out what causes your skin problems, so you aren’t fluttering from one product with an exhaustively long ingredient list to another, just hoping.
- Use an occlusive on your driest parts. Vaseline (brand name for petrolatum) is one of the most bland products on the market, and is comprised of one of the most boring molecules around. It is incredibly non-reactive and well-tolerated. Pat it on after a shower, after your moisturizer, on damp skin.
- Moist skin heals best. Keep cuts, scrapes, and scratches under a bandage with a glob of Vaseline. Same for acne spots with an open wound. Moist skin also scars less.
- Seek out ceramides. These crown jewels are amazing for all skin types, but especially skin that is dry or acne-prone.
- Don’t be afraid of moisture if you have acne. Many times, we fear moisture and oil as the cause of acne, but as I’ll start to explain, skin that is prone to acne tends to lack a lot of the moisture and good fats that keep it healthy.
- Hydrated skin absorbs ingredients better. Remember when I said that skin that is hydrated functions more efficiently? Skin that is hydrated also uses other ingredients more effeciently. This means that you may experience more irritation from topical prescriptions like tretinoin/retinoids, AHAs, BHAs, LHAs, etc. Simply back down on how much you use of these products and celebrate — your skin is healthy!
- Use a humidifier if it’s dry in your home. I have a simple digital humidity gauge in my bedroom so I can track the humidity of the room at any given time. Ideal humidity levels are between 40-60%. When humidity levels dip below this, such as in the winter when the heat is running frequently, I use a humidifier to boost the humidity levels. This is fantastic for your skin and prevents humectants from “pulling” too much moisture out of the deeper layers of your skin.
- Nolan, K. and Marmur, E. (2012), Moisturizers: Reality and the skin benefits. Dermatologic Therapy, 25: 229–233
- Chemistry of skin: Trans-epidermal water loss (TEWL)