Water, water: Transepidermal Water Loss

Remember back when I wrote about how skin is supposed to work? One of the main jobs that skin does is to keep water inside our bodies.

Our bodies are about 60% water, and membranes isolate every cell into its own little world. The balance between inside and outside is maintained by a complex network of osmosis, diffusion, cell pumps, eating, drinking, breathing and excreting. But inside and outside the cells is easy – outside is blood (mostly water) and extracellular fluid (mostly water). The problem is the barrier between the body (mostly water) and the air (anywhere from 3-100% water, depending on the weather). And that’s where the skin comes in.

Normally, skin is made up of a waxy layer called keratin. All the organelles and fluid in the cells either degenerate or are excreted into the spaces between the cells, and this combination of waxy keratin, oils, cholesterols and cell connectors make the skin very water resistant. It isn’t completely waterproof, but it is resistant. When you wash your hands, you don’t really absorb that much water. But when you take a bath or go for a swim, after a while your fingers wrinkle up and at some point, you’ll need to find the locker room! Water seeps into the cells and makes them swell up, resulting in wrinkled fingers. And some gets into your bloodstream by osmosis, where your kidneys then flush out the excess.

But even walking around in the air, you lose water all the time. Most of the water loss is from sweat, about 1-2 liters a day, or even 4 liters (a little more than a gallon) if you’re exercising heavily in the heat. But separate from what comes out of the sweat glands, which are actual glands that pump out water as an intentional heat control mechanism, you also lose water from simple evaporation, which is passive and completely out of your control. Water penetrates through the skin’s protection, reaches the surface, and evaporates away, taking body heat with it, at about .6 Calories per ml.

Since we are warm-blooded animals, any heat lost has to be made up by metabolism. Moving your muscles, digesting food, shivering, etc, all generate heat, which we measure in Calories. Normal water loss from evaporation is just under 10 ml per hour, per square meter of skin surface, or about 240 ml per day per square meter. (It’s also roughly 10 ml/kg hour). 240 ml is about the amount in a baby bottle or a coffee mug. A child has around a square meter of body surface.  If you take 240 ml lost through the skin and multiply it times .6 Calories/ml, you get 144 Calories lost through the skin every day.

In 2005, Dr. Mary Williams and her team at UCSF conducted a study on 10 kids with various types of ichthyosis. They took a tool that specifically measures how much water is lost through the skin, compared it with other metabolic measurements like oxygen use and food intake, and determined that kids with ichthyosis lose a LOT more water through the skin than those with normal skin.

They found that kids with ichthyosis lost as much as 15 ml/hr and in some cases as much as 40 ml/hr. Remember, normal is less than 10 ml/hr. And when they calculated it out over a day and matched it up with the weights of the specific kids, they found that those kids lost as much as 280 calories a day (about two bowls of cereal, or maybe a ham/egg/cheese sandwich) in types like ARCI-Lamellar where the skin is pretty intact and thickened, slightly more (maybe a cheeseburger or so) in types like ARCI-CIE and ichthyosis en confetti type 1, where the skin tends to be red and flaky, and as much as 1200!! Calories a day in really severe types like harlequin and Netherton’s syndrome.*

1200 calories a day is more than an entire extra meal, and a lot of these children contend with “Failure to Thrive,” which means that so much energy is being used either on skin manufacturing or on maintaining body temperature and regular old metabolism like breathing and brain power, that there’s nothing left to spend on actual growing. Evan’s mom mentioned that he’s 2.5 and still about the size of a 12 month old. Brenna’s mom posted that Brenna finally is on the growth chart. Another adult friend of ours with harlequin told us one time about putting in his own feeding tube every night so he could manage his calories. Yet another friend did everything she could to avoid a feeding tube in her daughter with Netherton’s syndrome, but she just wouldn’t gain weight, so they ended up with the tube. Even my own kids were all born around 3rd-5th percentile for size, and took 2 years to pass the 10th percentile mark.

If you went to the Denver or Orlando FIRST conferences in 2012 or 2010, you might have had your water loss measurement taken. These studies are still ongoing, but we are learning more each day.

*J Pediatr. 2004 Jul;145(1):82-92.