Deep hydration for mature skin: aquaporins and firmness
Mature skin that feels tight at 4 PM, a dull complexion in the evening, fine lines more visible upon waking than at bedtime: these signs rarely indicate a lack of cream. They indicate a problem with the circulation of water within the skin.
Key takeaways
- Water does not circulate freely in the epidermis: it uses channel proteins, aquaporins, driven by the osmotic gradient.
- Two isoforms are of interest in cosmetics: AQP3 (water + glycerol, deep keratinocytes) and AQP5 (superficial layers). Their presence declines with age, and with it the flexibility of the barrier.
- The most cost-effective action is free: apply the humectant to still damp skin, within 90 seconds after rinsing.
- Without a lipid phase (2 to 3 drops of prickly pear seed oil), the captured water evaporates.
- On reactive skin, a soothing base — chamomile, panthenol — makes the introduction of an active ingredient more comfortable.
- Allow 2 to 4 hours for a plumping effect, 4 to 8 weeks to assess the appearance of firmness.
Why does mature skin retain water less effectively?
It seems to absorb water with more difficulty, yet it loses it faster. The reflex is then to add oil — often without effect on a dull complexion, because that's not where the problem lies. It's due to cutaneous aquaporins, these channel proteins that allow water molecules to pass from one cell to another, and to the force that sets this water in motion. From the age of thirty, their presence in the epidermis tends to decrease; the trend accelerates after fifty. Age therefore not only changes the appearance of the skin: it changes the speed at which water circulates in it. Nothing irreversible, however; it's a less efficient water transport, on which daily actions have a real impact.
The osmotic gradient, the driving force of hydration
The cutaneous osmotic gradient is the difference in solute concentration between the dermis, which is rich in water, and the stratum corneum, which is much drier. These solutes — electrolytes, urea, lactic acid, amino acids derived from filaggrin — form the natural moisturizing factor (NMF). The difference creates a force that directs water from bottom to top. However, the pathways must be functional. Two parameters are therefore constantly at play:
- The intensity of the gradient, linked to the richness in NMF and the body's general water intake.
- The permeability of the pathways, ensured by aquaporins and the intercellular lipid cement.
AQP3, AQP5: what age changes
AQP3 is located in the keratinocytes of the basal and spinous layers and transports water and glycerol — hence its classification among aquaglyceroporins. Sougrat et al. (Journal of Investigative Dermatology, 2002) documented its functional expression in human epidermis; Ma et al. (Journal of Biological Chemistry, 2002) showed, in a murine model, that an absence of epidermal AQP3 is accompanied by impaired hydration of the stratum corneum. The glycerol in a skincare product is therefore not just a simple surface magnet: it is the natural substrate of the channel.
AQP5 is located more in the upper layers and contributes to the cohesion of the skin barrier in its most exposed part. It's what suffers when the air suddenly dries out: mid-season, airplane cabin, air-conditioned office.
With age, the presence of these two isoforms decreases, AQP3 in particular. The circuit narrows at the entrance and leaks more at the exit, with insensible water loss (TEWL) increasing. This is coupled with a depletion of NMF and ceramides in the stratum corneum, and slowed renewal that allows less reflective corneocytes to accumulate — hence the dull appearance.
To be fair: a cosmetic product does not create aquaporins and does not alter your cellular biology. What it can do, and this is significant: provide the substrate, maintain a favorable gradient, slow down evaporation, and preserve comfort.
Dry skin or dehydrated skin? The preliminary diagnosis
The two states are often confused in common language, yet they require opposite responses — and mature skin often exhibits both.
| Criterion | Dry skin (alipidic) | Dehydrated skin |
|---|---|---|
| What is missing | Lipids (sebum, ceramides) | Water in the stratum corneum |
| Nature | Skin type, constitutional | Temporary condition, reversible |
| Visual appearance | Fine texture, flaking, rough | Dull complexion, fine lines, makeup that creases |
| Pinch test | Supple but rough to the touch | Fine vertical lines, slow to fade |
| On oily skin? | No | Yes, very frequently |
| Primary response | Vegetable oils, butters, ceramides | Humectants, then light occlusion |
| Link with aquaporins | Indirect | Direct |
The pinch test is performed in thirty seconds: gently pinch the cheek. If fine vertical lines take more than a second to disappear, dehydration dominates; if the skin remains smooth but feels rough to the touch, lipids are lacking.
Humectant comparison: what each one actually does
They all attract water, but not to the same depth, nor at the same speed, nor in the same dosage. This table compiles common usage values in formulation; it helps to decipher an INCI list rather than to believe a packaging promise.
| Active (INCI name) | Molecule size | Usual dosage | Where it acts | Perceived comfort from | Limit to know |
|---|---|---|---|---|---|
| Glycerin (Glycerin) | 92 g/mol — very small | 3 to 15 % | Stratum corneum and living keratinocytes, via AQP3 | 2 hours | Sticky beyond 20%; possible discomfort beyond 30% |
| Hyaluronic acid high molecular weight | More than 1,000 kDa | 0.1 to 0.5 % | Surface film | Immediate (tightening effect) | In dry air and without occlusion, it draws from the skin rather than the air |
| Hyaluronic acid low molecular weight | 10 to 50 kDa | 0.05 to 0.2 % | Superficial horny layer | 24 to 48 hours | Little visible smoothing effect at any given moment |
| Urea (Urea) | 60 g/mol — very small | 2 to 10 % | Horny layer; native component of NMF | 24 to 72 hours | Stings on cracked skin; keratolytic beyond 10% |
| Panthenol (pro-vitamin B5) | 205 g/mol | 1 to 5 % | Horny layer and epidermis; renowned for soothing | 3 to 7 days | Not very spectacular in the short term |
| Aloe vera gel (Aloe barbadensis) | Polysaccharides (acemannan) | 5 to 40 % juice | Hydrogel network on the surface and in the horny layer | First application | Possible sensitivity to Asphodelaceae: 24-hour patch test |
| Betaine (Betaine) | 117 g/mol | 0.5 to 3 % | Horny layer, osmolytic role | 1 to 2 weeks | Supportive, never alone |
A relevant formula combines at least two depths: a surface active ingredient that smoothes immediately, and an active ingredient that penetrates deeper. Reading reminder: ingredients are listed by decreasing concentration up to 1%.
Fats that seal
Capturing water is one thing, keeping it is another. The role of the oil phase is not to "nourish" in a vague sense: it is to slow down evaporation. Unsaturated fatty acids and plant sterols do most of the work.
| Fat | Linoleic acid | Oleic acid | Feel | Place in the protocol | Quantity |
|---|---|---|---|---|---|
| Prickly pear (Opuntia ficus-indica seeds) | approx. 60 % | approx. 20 % | Dry, rapid penetration, satin finish | Final sealing; unsaturated fatty acids, γ-tocopherol and β-sitosterol | 2 to 3 drops |
| Vegetable squalane | — | — (saturated hydrocarbon) | Very dry, almost invisible | Neutral occlusion, practical under makeup | 2 drops |
| Jojoba (liquid wax) | approx. 5 % | approx. 10 % | Dry-satin, very stable | To mix into cream to thicken it | 2 drops |
| Hemp | approx. 55 % | approx. 12 % | Dry, distinct herbaceous scent | Seasonal alternative, keep refrigerated | 2 to 3 drops |
| Sweet almond | approx. 20 % | approx. 65 % | Oily, persistent film | Only in the evening, on very dry skin | 3 drops |
These profiles vary with origin and extraction: orders of magnitude, not guaranteed batch. For prickly pear seeds, Ramadan and Mörsel (Food Chemistry, 2003) situate linoleic acid around 60% and describe a notable unsaponifiable fraction of γ-tocopherol and β-sitosterol.
Nopal: A Dry Oil and Betalains
Botanically speaking, the prickly pear holds a unique place: a single plant provides two unrelated raw materials, an oil from its seeds and pigments from its pulp.
Prickly pear oil is extracted from the fruit's seeds—it takes about a ton to produce one liter. It absorbs quickly and leaves a satiny finish; its occlusion remains acceptable even in the morning, under sun protection. Without this lipid phase, water trapped by glycerin would evaporate in one to two hours in a dry room.
Nopal also contains rare pigments: betalains. Unlike the anthocyanins found in most colored fruits, betalains are only synthesized by Caryophyllales—the botanical order that includes cacti, beets, and amaranths; the two families are mutually exclusive (Gandía-Herrero and García-Carmona, Trends in Plant Science, 2013). Their cosmetic characterization is still recent; they are primarily described as pigments with antioxidant profiles, meaning they are useful for formula stability and perceived skin comfort. Nothing more at this stage: a purple color in cosmetics is not an active ingredient on its own.
Glycerin, Aloe, and Chamomile: The Synergy of Water
Three pillars, two of which are botanical, structure the aqueous phase of a skincare product for mature skin.