What helps hydrated skin from the inside and the outside

Key takeaways

• Your skin’s outer layer holds water with a built-in mix of water-binding compounds called natural moisturizing factor, sealed in by fats such as ceramides¹³.
• In a 30 day study, women who drank about 2 extra liters of water a day scored higher on skin hydration, especially those who usually drank less⁵.
• Dermatologists suggest keeping indoor humidity between 45 and 60 percent and putting on moisturizer within minutes of a short, warm shower⁸¹⁶.
• In placebo-controlled trials, people taking oral hyaluronic acid, collagen peptides or seed oils such as evening primrose scored better on skin moisture measures³⁴¹².
• The longer supplement trials ran about 12 weeks, so give a new routine 8 to 12 weeks and choose a quality-tested product³⁴¹³.

Step out of a long, hot shower on a January morning, and your shins may feel tight within minutes. That snug, papery feeling is a sign the skin’s outer layer is running low on water. Changes in temperature, air flow and humidity can pull water away from the skin and weaken its barrier¹.

Your skin is built to hold on. More than half of all the hyaluronic acid in your body, a sugar-based molecule known for holding moisture, sits in your skin². The outer layer adds its own water-binding compounds and a seal of fats that slows evaporation¹.

Researchers have studied ways to help from both directions. They have measured what happens when people drink more water or change daily habits. They have also tested oral supplements such as hyaluronic acid, collagen peptides and seed oils. Here is what they found, and how to put it to work.

Your outer skin layer is built to hold water

Scientists often describe the skin’s outer layer (stratum corneum) as a brick and mortar wall¹. The bricks are flat, protein-rich cells called corneocytes. The mortar is a blend of fats, and ceramides make up 40 to 50 percent of it³.

Within this layer sits a mix of water-binding compounds called natural moisturizing factor, or NMF¹. You can picture NMF as a built-in sponge. It helps the outer layer hold on to water even when the air around you is dry¹.

The wall is not meant to be watertight. A small, steady stream of water moves up from deeper layers and escapes into the air. Scientists call this transepidermal water loss, or TEWL¹. Water content drops sharply as skin cells move toward the surface, so the very top of your skin is its driest part¹.

A healthy barrier keeps that stream slow. When the barrier is disrupted, water escapes faster and skin starts to feel dry. Because water loss is so closely tied to barrier health, researchers use TEWL readings as a sign of how well skin is doing¹.

Below the outer layer sits the dermis, the thick support layer that gives skin its strength. Collagen makes up nearly three quarters of its weight¹. The dermis also holds far more hyaluronic acid than the outer layers do². This molecule has a high capacity to hold water⁴.

Food helps build the wall. Linoleic acid, an omega-6 fat, gets built into special ceramides in the outer layer³. The more of it those ceramides carry, the stronger the barrier tends to be³. When people do not get enough essential fats, their skin becomes dry and scaly and loses more water³.

The Linus Pauling Institute at Oregon State University puts it simply. Caring for the outer layer’s fats and NMF is a key strategy for comfortable skin, through nutrition or through skin care¹.

How researchers measure skin hydration

You can feel dryness, but scientists want numbers they can compare. The most common tool is a small probe called a Corneometer. It uses a capacitance method, an electrical reading tied to the water content of the outer layer⁴. Hyaluronic acid researchers have used it on the skin just below the eye, for example².

Results come in arbitrary units rather than percentages. In one recent trial, facial scores sat around 49 at the start⁴. That makes the Corneometer most useful for tracking the same spot on the same people over time.

A second tool, the Tewameter, measures TEWL, the water vapor rising off the skin. It reports grams of water per square meter per hour⁴. A smaller number means the barrier is holding water in more tightly¹⁴.

Conditions matter. In a 2021 trial, volunteers rested in a room held at about 68°F (20°C) and 50 percent humidity before testing⁴. Steady air keeps readings fair, since temperature and humidity can pull water from the skin¹.

Some studies also look deeper. One research team used probes at different frequencies to read surface hydration and deeper skin hydration separately⁵. The same team also tracked how well skin stretches and springs back⁵.

These tools explain the language you see in skin studies. When a group “scored higher on skin moisture,” a Corneometer reading usually went up. When TEWL went down, the skin was losing less water to the air.

What drinking more water does for your skin

How much water do adults need each day? The National Academies set an adequate intake of 3.7 liters of total water a day for men and 2.7 liters for women⁶. That works out to roughly 15 and a half cups and 11 and a half cups. “Total water” counts everything: plain water, other drinks and the water in food⁶.

Does extra water reach the skin? A 2015 study in Clinical, Cosmetic and Investigational Dermatology followed 49 healthy women ages 22 to 34⁵. The researchers first estimated each woman’s usual water intake with a food questionnaire. Then everyone added about 2 liters of water a day for 30 days⁵.

By the end, both surface and deeper skin layers scored higher on hydration⁵. The change was larger in the 38 women who had been drinking less than 3.2 liters a day, and it stood out from day 15 on⁵. Women who already drank more saw a smaller change⁵. The authors concluded that more water in the regular diet may have a positive effect on normal skin, especially for people who drink less⁵.

A 2018 systematic review in Skin Research and Technology pulled together six studies on fluid intake and skin⁷. Extra water may raise the water content of the outer layer, the authors found. The effect showed up mainly in people who had been drinking less to begin with⁷. Signs of dryness and roughness went down, and skin stretched and bounced back a little better⁷. Scientists are still learning how this works and how it plays out in older adults⁷.

Water loss through the skin did not shift in a clear way in these studies⁵⁷. One way to picture it: drinking water helps fill the tank, and the outer barrier controls how fast it drains. That is why the most hydrated skin usually comes from good habits on both sides.

Food counts toward your total, too⁶. Soups, fruits and vegetables add water along with fiber and nutrients. Cucumbers, lettuce, melon, oranges and tomatoes are easy ways to add more without reaching for another glass.

A few simple habits make the target easier to reach. Keep a refillable bottle on your desk, drink a glass of water with each meal, and sip more on hot days or after a workout. If you tend to forget, take heart: the women in the 2015 study who started low had the most to gain⁵.

Indoor air and daily habits that help skin hold water

The air around you is always trading water with your skin. Changes in temperature, air flow and humidity can draw water out of the outer layer¹. Dry climates and cold, windy weather are both linked with dry skin¹⁵. Indoors, heating and air conditioning pull moisture out of the air you live in¹⁶.

The American Academy of Dermatology suggests keeping indoor humidity between 45 and 60 percent¹⁶. A humidifier helps, especially in the bedroom while you sleep, and a simple humidity gauge shows where you stand⁸¹⁶.

The academy offers a short list of habits that help skin keep its water⁸¹⁶. Here is a version you can use every day:

• Keep baths and showers to 5 to 10 minutes, in warm water⁸.

• Pat your skin dry with a towel instead of rubbing⁸.

• Apply moisturizer within about 3 minutes of bathing, while your skin is still damp¹⁶.

• Run a humidifier in your bedroom, and clean it regularly⁸.

• Wear gloves for dishwashing, gardening and other wet chores, and outdoors in cold, dry weather⁸.

• Drink water through the day and add water-rich foods to meals⁵⁶.

Why apply moisturizer to damp skin? A thicker cream or ointment spread over damp skin traps that water so it cannot evaporate as quickly⁹. Dermatologists generally favor creams and ointments over lotions for dry skin, since they add more moisture⁸.

Ingredients matter as well. Some, such as glycerin, attract moisture to the skin¹⁵. Skin scientists call these water-attracting ingredients humectants. Hyaluronic acid is another, and the academy describes it as known for its intense moisturizing qualities⁹. It also lists ceramides, waxy fats that help hydrate skin⁹. For rough, cracked heels, it points to creams with 10 to 25 percent urea¹⁰.

Moisturizer types and dry skin each have their own articles in this library if you want to go deeper.

Oral supplements studied for hydrated skin

Creams work from the outside. Researchers have also asked what happens when people take nutrients by mouth that support the skin’s own water-holding systems. Three groups stand out in human research: hyaluronic acid, collagen peptides and oils rich in essential fats.

Hyaluronic acid

Hyaluronic acid has been studied as an oral skin nutrient since at least 2001². A 2014 review in Nutrition Journal described six human studies with 271 participants, most of them in Japan². Doses were often 120 milligrams a day, and most studies lasted 4 to 12 weeks². Most of these trials reported higher skin moisture scores².

How does a swallowed molecule reach the skin? In lab studies in rats, about 90 percent of swallowed hyaluronic acid was absorbed². In people, researchers describe gut microbes breaking it into small sugar chains that are absorbed and carried to the skin⁴.

A 12 week trial published in Nutrients in 2021 put oral hyaluronic acid to a careful test⁴. Forty healthy adults in Taiwan, ages 35 to 64, took either 120 milligrams of hyaluronic acid or a placebo capsule each day⁴. By week 12, facial moisture scores in the hyaluronic acid group had climbed from about 49 to 55, while the placebo group stayed near 49⁴.

The hyaluronic acid group also lost less water through the facial skin than the placebo group by week 12, and their wrinkle scores looked better too⁴. The researchers saw no adverse events tied to the hyaluronic acid⁴.

In 2025, a team from the University of California, Irvine, pooled seven randomized controlled trials of oral hyaluronic acid¹¹. People taking it scored better than placebo groups on skin hydration, elasticity and wrinkle depth¹¹. The authors described oral hyaluronic acid as a promising part of a skin care routine and encouraged larger trials to build on the results¹¹.

Collagen peptides

Collagen is the main protein in the dermis¹. Hydrolyzed collagen, often sold as collagen peptides, is collagen broken down into small pieces. Researchers study it as a way to support the body’s own collagen production and skin moisture.

A 2021 meta-analysis in the International Journal of Dermatology pooled 19 randomized, placebo-controlled trials with 1,125 participants ages about 20 to 70¹². About 95 percent were women¹². People taking hydrolyzed collagen scored better than the placebo groups on skin hydration and elasticity, and the authors pointed to about 90 days of use¹². Results vary from study to study, and researchers are still working out the best forms and doses.

Seed oils rich in essential fats

Because essential fats help build the skin’s mortar, researchers have tested oils pressed from seeds³. In a 12 week trial of 45 women ages 18 to 65, some took 2.2 grams a day of flaxseed oil, which is rich in the omega-3 fat ALA. Others took borage oil, which is rich in the omega-6 fat GLA³. Both groups saw less water loss through the skin and less roughness and scaling than the placebo group³.

In another 12 week trial, 40 healthy adults ages 32 to 56 took 1.5 grams of evening primrose oil a day or a placebo³. The evening primrose group scored better on skin moisture, water loss, elasticity, firmness and roughness³.

Everyday eating patterns line up with these trials. One study of middle-aged U.S. women paired national diet survey data with skin exams. Women who ate more linoleic acid were less likely to have dry skin³.

The table below gathers the main findings in one place.

Supplement	Amount studied	Study length	What was observed
Hyaluronic acid	120 mg a day	12 weeks	40 adults: higher facial moisture, less water loss than placebo
Hyaluronic acid (six studies)	Often 120 mg a day	Mostly 4 to 12 weeks	271 people: higher skin moisture scores in most trials
Hydrolyzed collagen	Varied by trial	About 90 days	19 trials, 1,125 people: better hydration and elasticity than placebo
Flaxseed oil or borage oil	2.2 g a day	12 weeks	45 women: less water loss, roughness and scaling than placebo
Evening primrose oil	1.5 g a day	12 weeks	40 adults: better moisture, elasticity and firmness than placebo

Sources:²³⁴¹²

Smart use: getting the most from a hydration supplement

A few practical steps help you line up your routine with the research.

Start with the amounts studied. Hyaluronic acid trials often used 120 milligrams a day²⁴. The seed oil trials used 1.5 to 2.2 grams a day³. Collagen amounts varied across the trials, so follow the directions on a product made for daily use¹².

Give it time. In the 2021 trial, the hyaluronic acid group pulled ahead of placebo on moisture by week 8, and the gap grew by week 12⁴. Collagen research points to about 90 days¹². Plan on 8 to 12 weeks of steady use before you judge how your skin feels.

Look for a quality seal. Independent groups such as U.S. Pharmacopeia, NSF International and ConsumerLab.com test supplements, and products that pass can display their seal¹³. The seal shows the product passed an outside quality check¹³.

Keep your care team in the loop. Tell your doctors, pharmacists and dietitian about every supplement you take, and keep a written list alongside your medicines¹³. That matters most before any surgery¹⁴. Your team can help you fit a new supplement into your full routine.

Read the label for the source, too. Collagen peptides can come from different animals, so check the label if you avoid certain foods. Store opened seed oils as the label directs.

Then pair the supplement with the basics. Water through the day, a humidifier in dry months and moisturizer on damp skin all work toward the same goal from different angles.

The bottom line

Hydrated skin depends on teamwork. Inside the outer layer, natural moisturizing factor holds water and a mortar of ceramides keeps it from slipping away¹³. Deeper down, hyaluronic acid in the dermis acts like a reservoir²⁴. You can support every part of that system.

Start with what is simple. Drink enough water, especially if you tend to fall short, and count watery foods toward your total⁵⁶. Keep showers short and warm, moisturize while skin is damp, and keep indoor air from getting too dry⁸¹⁶.

If you want support from within, oral hyaluronic acid, collagen peptides and evening primrose oil all have encouraging trial results³⁴¹¹¹². Choose a quality-tested product, give it 8 to 12 weeks, and let your care team know what you take. Small, steady habits add up, one glass of water and one damp-skin moisturizer at a time.

Sources

1. Michels AJ, Linus Pauling Institute. Skin Health. Micronutrient Information Center, Oregon State University. 2011 (updated 2026). lpi.oregonstate.edu

2. Kawada C, Yoshida T, Yoshida H, et al. Ingested hyaluronan moisturizes dry skin. Nutrition Journal. 2014. pmc.ncbi.nlm.nih.gov

3. Angelo G, Linus Pauling Institute. Essential Fatty Acids and Skin Health. Micronutrient Information Center, Oregon State University. 2012 (updated 2026). lpi.oregonstate.edu

4. Hsu TF, Su ZR, Hsieh YH, et al. Oral Hyaluronan Relieves Wrinkles and Improves Dry Skin: A 12-Week Double-Blinded, Placebo-Controlled Study. Nutrients. 2021. mdpi.com

5. Palma L, Marques LT, Bujan J, Rodrigues LM. Dietary water affects human skin hydration and biomechanics. Clinical, Cosmetic and Investigational Dermatology. 2015. dovepress.com

6. Institute of Medicine (National Academies). Dietary Reference Intakes: Recommended Dietary Allowances and Adequate Intakes, Total Water and Macronutrients. NCBI Bookshelf, National Library of Medicine. 2011. ncbi.nlm.nih.gov

7. Akdeniz M, et al. Does dietary fluid intake affect skin hydration in healthy humans? A systematic literature review. Skin Research and Technology. 2018. pubmed.ncbi.nlm.nih.gov

8. American Academy of Dermatology. Dermatologists’ top tips for relieving dry skin. AAD.org. 2026. aad.org

9. American Academy of Dermatology. How to pick the right moisturizer for your skin. AAD.org. 2022. aad.org

10. American Academy of Dermatology. How to care for dry, cracked heels. AAD.org. 2022. aad.org

11. Amin P, Sarabi A, Choe S, Scott S, Suh S, Mesinkovska NA. Oral Hyaluronic Acid Supplement: Efficacy in Skin Hydration, Elasticity, and Wrinkle Depth Reduction. Journal of Drugs in Dermatology. 2025. jddonline.com

12. de Miranda RB, Weimer P, Rossi RC. Effects of hydrolyzed collagen supplementation on skin aging: a systematic review and meta-analysis. International Journal of Dermatology. 2021. pubmed.ncbi.nlm.nih.gov

13. NIH Office of Dietary Supplements. Dietary Supplements: What You Need to Know. National Institutes of Health. 2023. ods.od.nih.gov

14. National Center for Complementary and Integrative Health. Using Dietary Supplements Wisely. National Institutes of Health. 2019. nccih.nih.gov

15. Cleveland Clinic. Dry Skin (Xeroderma): Symptoms, Causes, Treatment and Prevention. Cleveland Clinic. 2022. my.clevelandclinic.org

16. American Academy of Dermatology. How to care for your skin in your 60s and 70s. AAD.org. 2021. aad.org

This article is for general education and is not medical advice. Talk with your healthcare professional about your own health, especially before starting a new supplement if you are pregnant, nursing, taking medicine, or managing a health condition. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Reviewed October 2026.