Your skin’s own moisturizer: what the natural moisturizing factor is

Your skin doesn’t wait for you to buy a moisturizer. It makes its own, inside the outermost layer, and it has been doing so since before you were born. Scientists call this built-in mix the natural moisturizing factor, or NMF. Once you know what it is, a lot of moisturizer ingredient lists suddenly make sense, and so does why your hands crack every January.

What is the natural moisturizing factor?

The surface of your skin, the stratum corneum, is a layer of flattened dead cells called corneocytes, stacked like bricks in a wall of fats. Inside each of those cells sits a concentrated cocktail of small molecules that love water. That cocktail is the NMF.

The name goes back to 1959, when researchers noticed that the skin’s outer layer contained “hygroscopic water-soluble substances,” hygroscopic meaning they attract and hold water from the air around them. Skin that keeps its NMF stays soft and bends without cracking. Skin that loses it dries, stiffens and flakes.

It’s not a small detail, either. The NMF makes up about 10% of the mass of each surface cell, and around 20 to 30% of the dry weight of the whole outer layer.

What’s in it?

The NMF is a mixture rather than a single ingredient. The main parts are:

• Free amino acids, the building blocks of proteins, which are the biggest share.
• PCA (pyrrolidone carboxylic acid), made from one of those amino acids and the single most plentiful component. It’s a very strong humectant, the word for any ingredient that draws water in.
• Urocanic acid, made from another amino acid, histidine, and which also absorbs some ultraviolet light.
• Lactate, the salt form of lactic acid.
• Urea, a small molecule your body also makes elsewhere.
• Sugars and mineral salts, such as sodium, potassium and chloride.

If that list looks familiar, it’s because moisturizer labels have been copying it for decades. Urea, lactic acid, sodium PCA and amino acids on a label are there to do exactly what the NMF does.

Where does it come from? Meet filaggrin

Most of the NMF starts life as a protein called filaggrin. Deep in the living part of the epidermis, filaggrin’s job is to bind the keratin fibres inside a skin cell into a tight, flat bundle. That’s part of what turns a plump living cell into a tough, flat corneocyte.

Then, as the cell moves up toward the surface, enzymes chop filaggrin into its individual amino acids. Some of those are converted further, into PCA and urocanic acid. The result is the water-holding mix inside the cell. In other words, the same protein first gives the cell its shape and then gets recycled into its moisturizer.

The clever part is what triggers the chopping. Filaggrin only breaks down when the water activity inside the cell reaches a certain range, which depends on the humidity outside. In humid air, the breakdown happens right near the surface. In dry air, it happens deeper down, so the cell arrives at the surface already stocked with NMF. Cover skin with an occlusive patch and the breakdown can stop altogether, because the cells never get dry enough to need it.

Why does it drop in dry skin?

Several things lower your NMF, and most of them happen in a Canadian winter.

Low humidity. Cold outdoor air holds very little moisture, and indoor heating dries it further. When humidity drops too far, the filaggrin breakdown itself slows, so less NMF is produced at the same time as more water is evaporating from the skin. Dry skin makes less of the thing that would fix it, which is why winter dryness tends to snowball rather than settle.

Washing. The NMF is water-soluble, which is its strength and its weakness. Soap and hot water can rinse a portion of it straight out of the surface cells. Frequent hand-washing, long hot showers and harsh cleansers all cost you some NMF each time.

Sun. Ultraviolet light impairs the filaggrin breakdown, so sun-exposed skin ends up with less NMF than covered skin.

Age. As we get older, skin makes less profilaggrin, the parent of filaggrin, and the amino acid content of the outer layer falls. That’s one reason dry skin becomes more common with age.

Why is it low in eczema?

Eczema, also called atopic dermatitis, is the itchy, dry, inflamed skin condition that’s common in children and often runs in families. One of the strongest genetic risk factors for it is a mutation in the filaggrin gene. People with one faulty copy make less filaggrin; people with two faulty copies make essentially none. Less filaggrin means a leakier outer layer and, of course, less NMF.

A 2011 study measured filaggrin breakdown products in the skin of people with atopic dermatitis. Those carrying two mutated copies of the gene had roughly a threefold drop in NMF compared with people with a normal gene. But the study also found something important: NMF was lower in everyone with moderate-to-severe eczema, mutation or not, and the worse the eczema, the lower the NMF. Inflammation itself reduces filaggrin production.

Low NMF then feeds the problem. With less of the acidic components, the skin surface becomes less acidic, which switches on enzymes that break down more barrier proteins, which lets more water out and more irritants in. It’s a loop, and it explains why moisturizing is a cornerstone of eczema care rather than an optional extra.

Ingredients that mimic the NMF

The good news is that the NMF is made of simple, well-understood molecules, and many of them can be put in a cream. Ingredients that do this are called humectants, and the ones closest to the real thing are:

• Urea, the oldest and best studied. At low concentrations (about 2 to 10%) it mainly hydrates and helps hold water in; at higher ones it also loosens thick, scaly skin. Trials in dry skin have shown it reduces water loss through the skin, and eczema guidelines treat urea-based moisturizers as a well-documented option.
• Lactic acid and lactate, which hydrate and gently loosen dead surface cells.
• Sodium PCA and amino acids, direct copies of what’s in your cells.
• Glycerin, not part of the NMF itself but a humectant that works the same way and is in almost every moisturizer.

Two practical notes. Urea and acidic ingredients like lactic acid can sting on cracked or inflamed skin, so on raw skin a bland cream or ointment may feel better until things calm down. And humectants work best when paired with something to hold the water in: an emollient (an oil or fat that fills the gaps between cells) or an occlusive (a film-former such as petrolatum). Most good moisturizers combine all three, which you’ll see if you browse moisturizers with their ingredient lists.

If your skin is dry, itchy or cracked and it isn’t better after two weeks of regular moisturizing, see your doctor or pharmacist. Persistent dryness can be eczema or something else that needs more than a cream.

The natural moisturizing factor is your skin’s own water-holding kit: amino acids, PCA, lactate, urea and salts, made by recycling the protein filaggrin. Dry air, washing, sun, age and eczema all drain it. Moisturizers with urea, lactate, glycerin, sodium PCA or amino acids top it back up, and they do it best inside a cream that also seals the water in.