The evidence table

After 10 minutes in water at about 41 °C, the skin's water loss rose from 25.75 to 58.58 g/h/m². In water at 11 °C it rose to 34.96.

Limitation: Measured on the hands, straight after leaving the water. Wet skin can push the reading up slightly.

Herrero-Fernandez M 2022Clinical, n=50

Plain water did not change water loss at either 20 °C or 40 °C. A harsh foaming agent (SLS) significantly increased water loss at 40 °C compared with 20 °C.

Limitation: Experimental irritation model; longer contact than in a shower.

Ohlenschlaeger J 1996Randomised experimental, n=20·Berardesca E 1995Experimental, n=10

Skin washed in hard water held more foaming-agent residue; the residue increased water loss and caused irritation. Softening the water reduced the effect.

Limitation: The figures are in the full text; only the abstract was used.

Danby SG 2018Experimental, n=80

Harsh foaming agents damage the skin's proteins and lipids; the result is tightness, dryness and itching. The first thing washing strips away is the skin's fatty acids.

Limitation: Reviews; the authors work for cosmetics companies.

Ananthapadmanabhan KP 2004Review·Ananthapadmanabhan KP 2013Review·Walters RM 2012Review + laboratory

In skin that dries out in winter, a body wash with moisturising ingredients reduced the signs of dryness over 4 weeks; with bar soap they increased.

Limitation: Sample size not given in the abstract; industry authors.

Hoffman L 2008Randomised, 5 weeks

Water loss rose after every wash, and the effect built up with repeated washing.

Limitation: 15 people.

Voegeli D 2008Experimental, n=15

Dermatologists found dry skin in 29.4% of 48,630 workers they examined. It becomes more common with age.

Limitation: Working population in Germany.

Augustin M 2019Cross-sectional, n=48,630

Low humidity and low temperatures generally weaken the skin's barrier function.

Limitation: Systematic review.

Engebretsen KA 2016Systematic review

Applying moisturiser straight after washing or 30 minutes later gave the same result; both were better than not applying any.

Limitation: Ages 18–25, healthy skin.

Nguyen KH 2022Crossover clinical, n=60

500 ml of underarm sweat collected under sterile conditions had no odour. Left to stand with bacteria, it developed sweat odour; the main odour molecule is 3M3SH.

Limitation: Laboratory.

Troccaz M 2004Laboratory·Shelley WB, Hurley HJ, Nichols AC 1953Experimental

Most of the odour is produced by a few species of bacteria. The enzyme that releases the sharp smell is found in only one group of staphylococci.

Limitation: Laboratory.

Bawdon D 2015Laboratory·Rudden M 2020Laboratory·Natsch A 2003Laboratory

When antiperspirant was stopped, the number of bacteria in the underarm rose; when it was applied again, it dropped sharply.

Limitation: Small samples (17 and 9 people).

Urban J 2016Experimental, n=18·Callewaert C 2014Intervention, n=9

A prebiotic is a substance that is selectively used by host microorganisms and confers a health benefit. The definition covers the skin too.

Limitation: Consensus statement.

Gibson GR 2017Consensus statement

A face serum containing GOS did better than placebo on water-holding capacity and water loss over 8 weeks.

Limitation: A leave-on serum; shower gel is rinsed off.

Hong KB 2020Randomised, placebo-controlled, n=60

With a prebiotic shower gel and lotion, the skin's bacterial profile changed over 6 weeks. Hydration and water loss did not differ from the control group.

Limitation: Manufacturer-supported; no difference between groups on the main outcome.

Li M 2023Randomised clinical, n=53

In the lab, some prebiotics held back an unwanted bacterium and left alone a beneficial species found naturally on the skin.

Limitation: Laboratory; not shown in humans.

Di Lodovico S 2020Laboratory·Le Bourgot C 2022Laboratory

Very little is known about the effects of prebiotics applied to the skin; most of the evidence comes from the lab.

Limitation: Reviews.

Al-Ghazzewi FH, Tester RF 2014Review·Zeng M 2025Review