Winter Dehydration: Hydrating Serums for Deep Nourishment

"Deep hydration" is on a lot of serum labels. Here is where skin hydration actually happens, what "deep" can honestly mean, and how to keep water in through winter.

Where Skin Hydration Lives

Skin hydration is mostly the job of the outermost layer, the stratum corneum. Water-attracting compounds inside it hold water, while the lipid layer between the cells and a surface film slow evaporation. Dry skin is characterized by a reduction in those natural moisturizing compounds or in the lipids, and cold weather is one of the known triggers^[1]. Our earlier post on dehydrated or dry skin explains how to tell the two apart.

What Winter Does to That Layer

In a year-long study of Czech women, skin hydration and sebum were low in winter^[2]. Reviews also conclude that low humidity and low temperatures reduce barrier function^[3]. All the authors of the year-long study work at a company (Contipro), and it covered Caucasian women only. Together this explains the familiar winter pattern: the layer that holds water has less to hold, and loses what it has faster.

How "Deep" Is Deep Hydration?

Labels love the word "deep", and there is one real piece of science behind it. In a study using Raman imaging on skin sections, hyaluronic acid with a low molecular weight (20 to 300 kDa) passed through the stratum corneum, while large hyaluronic acid (1,000 to 1,400 kDa) did not^[4]. So the size of a molecule does decide whether it stays on the surface or travels further in. One author works at an ingredient company (Soliance).

What the research does not establish is that going deeper hydrates skin better. Hydration in studies is mostly measured as water content of the outer layer. Researchers can now even separate water bound tightly to skin proteins from loosely held, unbound water^[5], a reminder that "hydrated" has more than one meaning. That study involved authors from a dermatology company (Galderma) and was a laboratory model. For more on hyaluronic acid sizes, see our Journal explainer.

What "Nourishment" Means

"Nourishing" is not something a study measures. What studies measure is water content, water loss through the skin, and how skin feels. A hydrating serum adds humectants, which draw water into the outer layer. To make that last, the water has to be sealed in: basic emollients combine humectants with occlusives that slow evaporation^[1], and moisturizers are described as breaking the dry-skin cycle and keeping skin smooth^[6].

A Winter Hydration Routine

• Cleanse with lukewarm water and pat dry, leaving skin slightly damp.
• Apply a hydrating serum to the damp skin.
• Seal it with a moisturizer, and add a thin layer on the driest patches. See Moisturizer vs. Serum.
• Re-apply after washing your face, since cleansing removes some of what you applied.
• Watch the air as well: very dry rooms reduce barrier function^[3].
• Give a routine three to four weeks before judging it.