Water and Skin Hydration — Does Drinking 8 Glasses a Day Improve Your Complexion?

“Drink 8 glasses of water a day and your skin will be radiantly hydrated” — it's one of the most frequently repeated pieces of skincare advice. You'll find it in magazines, on blogs and on the lips of influencers. The problem is that science doesn't back it up. Drinking water is, of course, essential for your health, but the mechanism behind skin hydration works completely differently from what most people assume.

Where did the 8-glass myth come from?

The “8 × 8” recommendation (eight 8-ounce glasses, roughly 2 litres a day) most likely traces back to a 1945 report by the US Food and Nutrition Board, which recommended 2.5 litres of fluid a day — but with a crucial caveat: “most of this quantity comes from food.” That second sentence was forgotten, and the number took on a life of its own.

In 2002, Dr Heinz Valtin of Dartmouth Medical School published an extensive review in the American Journal of Physiology, systematically analysing the scientific literature and finding no evidence to support the “8 × 8” rule for healthy adults living in a temperate climate. Fluid requirements are individual and depend on physical activity, climate, diet and health.

Why does drinking water NOT hydrate the skin directly?

The study by Palma et al. (2015), published in Clinical, Cosmetic and Investigational Dermatology, is one of the few to directly examine the effect of increased water intake on skin hydration. The results? In people who drank less than 2 litres a day and increased their intake, a slight improvement in the hydration of deeper skin layers was observed — but the effect was clinically negligible and did not translate into a visible improvement in appearance.

Why? Because skin hydration is primarily a matter of the epidermal barrier, not the amount of water in the body. The water you drink reaches the digestive tract, is absorbed in the intestines, circulates in the blood and is filtered by the kidneys — and only a small fraction reaches the dermis, with even less reaching the epidermis.

TEWL — the real key to hydrated skin

The key concept in skin hydration is TEWL — Transepidermal Water Loss. This is the process by which water from the deeper layers of the skin evaporates through the epidermis into the surrounding environment.

A healthy epidermal barrier — built from corneocytes (dead skin cells) and intercellular lipids (ceramides, cholesterol, free fatty acids) — effectively limits TEWL. When the barrier is damaged (by harsh washing, retinol, frost or skin conditions), TEWL increases and the skin becomes dry and irritated.

This is an external process — no amount of water you drink will fix it. Repairing the epidermal barrier requires:

• Emollients — they rebuild the lipid layer (ceramides, shea butter).
• Occlusives — they create a physical barrier that prevents evaporation (petrolatum, silicones).
• Humectants — they draw water into the epidermis (hyaluronic acid, glycerin, urea).

Humectants work independently of how much water you drink

Humectants — such as hyaluronic acid, glycerin, urea and panthenol — draw water from two sources: the deeper layers of the skin (dermis) and humidity in the air. Their effectiveness does not depend on how much water you drink.

Hyaluronic acid can bind up to 1,000 times its own weight in water — but it does so locally, at the level of the epidermis. Glycerin (glycerol) is one of the best-studied humectants — a systematic review by Fluhr et al. (2008) confirmed its effectiveness in improving skin hydration and barrier function.

Check which of your cosmetics contain effective humectants — use the PurScore search engine.

Dehydration vs. dry skin — they are not the same thing

There is a fundamental difference between bodily dehydration and dry skin, even though the two terms are often confused:

• Dehydration — a systemic condition in which the body loses more fluid than it takes in. It manifests as thirst, dark urine, fatigue and a drop in blood pressure. The skin may lose its turgor (the skin-pinch test). It's a condition that requires fluid replenishment.
• Dry skin (xerosis) — a dermatological condition in which the epidermis has too few lipids and/or too little water in the stratum corneum. It can occur in perfectly well-hydrated people. The treatment is moisturising cosmetics, not drinking water.

Someone who drinks 3 litres of water a day can still have dry skin if their epidermal barrier is damaged. And conversely — someone who drinks moderate amounts of fluid can have perfectly hydrated skin if they follow the right skincare routine.

Is it worth drinking water for your skin at all?

Of course — severe dehydration worsens the appearance of the skin. But for most people in developed countries who drink normally and have no hydration disorders, an extra glass of water won't change the condition of their skin.

Instead of counting glasses, focus on:

1. Drinking when you're thirsty — the thirst mechanism in healthy adults works properly.
2. Proper skincare — a cream with ceramides, humectants and occlusives.
3. Protecting the barrier — gentle washing, avoiding excess SLS, protection against frost.

Discover the moisturising ingredients in the PurScore database — check whether your cream really contains what it promises.