The Skin Barrier, Explained
What your outermost skin layer does, how scientists measure it, and what research says about supporting it from the outside and the inside.
Key takeaways
- The skin barrier is your outermost skin layer: flattened cells held together by a "mortar" of lipids.
- Scientists judge it mainly by measuring how much water escapes through the skin, called TEWL.
- Dry air, age and some genetic differences can make the barrier lose water more easily.
- A few nutrients taken by mouth have been tested in small human trials of 6 to 12 weeks, with early, promising results.
Your skin barrier is the outermost layer of your skin, and it does a large share of the work of keeping you comfortable. It holds water in, and it keeps irritants out. This guide explains what the barrier is made of, how scientists measure how well it works, what tends to weaken it, and what research has, and hasn't yet, shown about supporting it.
What is the skin barrier?
The skin barrier is the stratum corneum, the thin outer layer of the epidermis. Researchers describe it as a brick wall. The bricks are flattened, tough skin cells. The mortar is a mix of fats, or lipids, packed in tight layers between them. By weight, that mortar is about 50% ceramides, 25% cholesterol and 15% free fatty acids.1
What the lipid mortar is made of
Approximate share by weight
- Ceramides: about 50%
- Cholesterol: about 25%
- Free fatty acids: about 15%
- Other lipids: about 10%
What does the skin barrier do?
Its main job is to slow water down. The lipid layers between the cells form the only continuous path through the stratum corneum, and they keep the water in your body from evaporating too quickly into the air. The skin's own lipid production and the delivery of new lipids to that layer are both needed to keep the barrier in balance.1 When the barrier works well, skin tends to feel soft and comfortable. When it doesn't, skin can feel tight and dry.
How do scientists measure it?
The most widely used measurement is transepidermal water loss, or TEWL: the amount of water that passes out through a patch of skin over a set time. A small probe rests on the skin and detects the water vapor leaving it. A lower TEWL generally means the barrier is holding on to more of its water.2
TEWL is sensitive. It changes with room humidity, temperature and airflow, with sweating, and between body sites, so good studies measure it under controlled conditions.2 Researchers often pair it with a second reading, the water content of the stratum corneum (sometimes called corneometry). When you read a skin study, these are the two numbers to look for.
What weakens the skin barrier?
Dry air. In a study of healthy volunteers, just 3 to 6 hours in a low-humidity room significantly lowered the water content of the stratum corneum on the forearm and cheek, changed TEWL, and made the skin surface measurably rougher.3 That's one reason skin can feel tight in winter or on a long flight. For the everyday signs of a stressed barrier and how long repair takes, see our guide to a damaged skin barrier.
Age. Researchers compared adults aged 20 to 30 with adults over 80. The older group's barrier was disrupted more easily: it took about 18 rounds of tape stripping to disturb it, compared with about 31 in younger adults. It also recovered more slowly. Younger skin recovered 50% of its barrier in 24 hours and 80% in 72 hours, while older skin recovered only 15% in the first 24 hours. In the same study, the total lipid content of the stratum corneum in aged mice was about 30% lower.4
Genes. Filaggrin is a protein that helps build the stratum corneum, and its breakdown products help hold water in the skin. In a study of people with atopic dermatitis (eczema), those with changes in the filaggrin gene had lower stratum corneum hydration than healthy controls. In that group, a higher TEWL went along with a higher score on a clinical severity index (r = 0.81).5 It was a small observational study, and it shows a link rather than a cause, but it helps explain why researchers see the barrier as central to eczema.
How to care for your skin barrier from the outside
The barrier's lipids work as a team. In laboratory and early human testing, applying only one or two of the three main lipids to damaged skin slowed its recovery, while a balanced mix of ceramides, cholesterol and fatty acids allowed normal recovery. Raising one of them within that mix sped recovery up further.6 That is the thinking behind barrier creams that combine all three.
The everyday basics are simple. Keep cleansing gentle and the water lukewarm rather than hot, moisturize while skin is still slightly damp, and give any change a few weeks before you judge it. In dry seasons, a humidifier can help keep indoor air from pulling water out of your skin.
Can you support the skin barrier from the inside?
This is a newer and smaller area of research. Most studies test one nutrient at a time, in a few dozen people, over 6 to 12 weeks. Here is what has been studied.
Hyaluronic acid. Your body already makes it, and it is also studied as an oral supplement. In a 6-week randomized, double-blind trial in 61 adults with dry skin, the groups taking 120 mg of hyaluronic acid a day had a greater increase in skin moisture than the placebo group.7 In a 12-week trial in 40 healthy adults aged 35 to 64, the group taking 120 mg a day had significantly better stratum corneum water content, TEWL and elasticity than the placebo group.8
A heat-killed postbiotic. Postbiotics are bacteria that have been heat-treated, so they are no longer alive. In a 12-week randomized, double-blind trial, 80 healthy adults who were conscious of dry skin took the heat-killed strain Lactiplantibacillus plantarum L-137 (HK L-137) or a placebo. The HK L-137 group tended toward larger increases in forearm water content and larger decreases in facial TEWL. Those differences were significant in participants above the median age and in those with drier skin. Scores on two skin questionnaires improved significantly more in the HK L-137 group.9
Plant ceramides. In a 6-week placebo-controlled trial in 51 healthy adults, a konjac tuber extract providing 5 mg of glycosylceramides a day was linked to less skin dryness in a dermatological assessment.10
Niacinamide (vitamin B3). So far, the clearest work on niacinamide and ceramides comes from the laboratory. In cultured human skin cells, niacinamide increased the cells' own production of ceramides 4.1- to 5.5-fold, along with other barrier lipids. In the same paper, applying niacinamide to the skin lowered TEWL in dry skin.11 Cell studies show how something might work. They don't show that taking it by mouth has the same effect in people.
The bottom line
Your skin barrier is a thin, lipid-rich layer whose main job is holding water in. Scientists track it with TEWL and skin water content, and dry air, age and genes all affect it. Gentle daily care helps from the outside. Research on support from the inside is promising but still early, so look for studies that report TEWL or water content, run at least 8 to 12 weeks, and compare against a placebo.
This article is for general education and isn't medical advice. If you have a skin condition, talk to a dermatologist or your doctor about diagnosis and treatment, and don't change a prescribed treatment without their advice.
