Moisturizing: The Importance of the Barrier
Many of my patients ask me if they should moisturize. With some exceptions (for example, extremely oily skin,) the answer is “yes.” The reason may be a little more complex than one would think.
First, let’s look at what the skin does and the importance of barrier function. The skin has numerous functions:
1. It has a social and reproductive function; i.e. it advertises our general health and youth to others; this is important in social and work situations and in finding potential mates.
2. It mediates the sense of touch, one of the five senses.
3. It maintains our body temperature, through the mechanisms of sweating and vasodilation/vasoconstriction.
4. It pads our bones and muscles with a layer of subcutaneous fat, which functions as a shock absorber (and also contributes to temperature homeostasis).
5. It has its own immune system, which protects us from bacteria, viruses, and parasites.
6. It protects us from ultraviolet radiation.
7. Finally, (and perhaps the most important of all), it provides a barrier between us and the outside world. Obviously, without our skin, water would seep out of our bodies and we would quickly die.So, an intact skin barrier is necessary for our survival. It keeps good stuff in (i.e. water) and bad stuff out. But on a more microscopic scale, skin barrier function is important to maintain the function and appearance of the skin itself. What do I mean by that? Well, skin requires a certain level of moisture to do its job. The outer layer of skin usually contains 20-35% water. The “skin barrier”, also known as the stratum corneum, is comprised of a dead flattened layer of cells called corneocytes, forming the “bricks”, and the intervening layer of lipids, forming “the mortar.” This bricks and mortar structure prevents water loss through the skin. The living epidermis residing just below the stratum corneum is in a constant state of activity and is responsible for repairing the stratum corneum and making new lipids for the barrier. When our barrier is disrupted by trauma, changes in humidity, ultraviolet exposure, harsh cleansers, or any number of environmental insults, it sends “distress signals” to the epidermis to initiate the repair process. However, sometimes, the repair mechanisms can get overwhelmed by disease states, such as psoriasis, eczema, severe dryness, etc. Now, the skin barrier is broken and the skin cannot function well. Without adequate hydration, the processes that normally cause dead skin cells to shed is disrupted and the skin becomes flaky, rough, dull, and inelastic, leading to cracks and fissures. The “distress signals”, also known as cytokines, activate the inflammatory cascade, leading to further barrier disruption. Thus, skin disease leads to barrier dysfunction, but it also goes the other way: barrier dysfunction can lead to skin disease. This is why moisturizing is such an important part of treating skin diseases such as eczema and psoriasis. “It’s half the battle,” as I tend to say. But there’s more. We now know that inflammation is the final common pathway for multiple illnesses, from arthritis to heart disease to certain cancers to autoimmune diseases, to aging itself. With respect to the skin, we know that the inflammatory cytokines produced by distressed skin lead to upregulation of a group of proteins called matrix metalloproteinases, which chew up collagen and leave us with wrinkles. So, in order to prevent skin aging, we need to prevent skin inflammation. How do we do that? By treating our skin well, protecting it from ultraviolet radiation with sunscreens and sun avoidance, avoiding harsh cleansers, using topical antioxidants and maintaining a healthy barrier with adequate moisturization. When I was a resident, it was considered a myth that moisturizers prevented wrinkles. Sure, they made the skin look smoother, but that was it. Well, it turns out that the “myth” might actually have some truth to it! Should everyone moisturize? Probably, unless you’re naturally oily, in which case you obviously wouldn’t need to. If you suffer from psoriasis, eczema, or seborrheic dermatitis (facial dandruff), absolutely! Even acne sufferers can benefit from a lightweight moisturizer, as it has been shown that improving barrier function improves acne (Also, many acne treatments leave the skin dry). Rosacea patients tend to have dry skin and an inherent barrier dysfunction, so moisturizing is essential in this condition. Most moisturizers work by forming a seal to prevent water loss through the skin; some also contain humectants which pull moisture into the skin from the tissue below. Oil-in-water lotions are generally not as moisturizing as water-in-oil creams. Research in skin physiology has shown that there are 3 main lipids in the skin barrier: ceramides, cholesterol, and fatty acids. We are now starting to see the advent of new “physiologic” moisturizers on the market that contain one or more of these ingredients and do more than just provide an occlusive water barrier; they actually help the skin rebuild the lipid barrier.