Sebum, but Not Hydration, Shows Steeper Age-Related Differences After Age 40 in Women: A Cross-Sectional Study of 7743 Participants in 12 Countries

Skin changes in women around age 40 are often attributed to impaired barrier function or reduced hydration, but large-scale data comparing sebum, hydration, and transepidermal water loss (TEWL) across age groups are scarce. In this cross-sectional study, we analyzed standardized cheek measurements from 7743 women aged 20–59 years in 12 countries (5274 Asian and 2469 Western). Piecewise regression tested changes in age-related slopes at age 40, including differences by region. The negative age–sebum slope was significantly steeper from age 40 onward in both Western (slope change −0.762 µg/cm2/year, p = 0.010) and Asian women (−1.256 µg/cm2/year, p < 0.001), a result that held with robust standard errors, log transformation, and country adjustment and was replicated at two other facial sites. Hydration showed no change in slope at age 40, and resting TEWL decreased rather than increased. Sebum was 34.9% (Asian) and 25.4% (Western) lower at 50–59 than at 20–29 years, whereas hydration differed by less than 3.5%. Men showed a different pattern, with no net sebum difference between these age groups. The regional difference in slope change was not significant (p = 0.168). After age 40, the oil–moisture profile of women is characterized by lower sebum with stable hydration; longitudinal studies with hormonal assessment should test whether this reflects menopause.