As shown in Fig. 1, 418 infants were included in this study, with 219 (52.4%) boys. In total, 114 (27.3%) infants were diagnosed with AD within the first 1 year; the other 304 infants comprised the non-AD group.
Infants were born at 39.3±1.0 weeks with half by caesarean delivery; 83.3% were breastfed, and 54.6% started solid food before age 6 months. Compared with the non-AD group, the AD group showed a generally larger body weight and height across the first 6 months; maternal and paternal allergy history was also more often reported among infants with AD (30.7% vs. 17.4%; 22.8% vs. 15.1%) (Table 1).
Dynamic trends of skin barrier functional parameters in infants from birth to age 6 months based on non-AD group
The values of TEWL, SCH, skin pH, and sebum content varied across the time points at birth (36±22 hours), 42 days (44±5 days), and 6 months (6.1±0.2 months) for all five body sites (Table 2). From birth to age 6 months, dynamic changes appeared an overall increase trends for TEWL and SCH but a decline trends for skin pH and sebum content with age.
We found some differences in dynamic trends of the four parameter values between exposed and less-exposed body sites. The postnatal rise in TEWL on exposed body sites (cheek and forehead) was greater than that on less-exposed sites (abdomen, volar forearm, and lower leg) from birth to 42 days and declined in a faster rate to levels similar to those at birth by age 6 months (Table 2).
SCH values showed an overall increasing trend from birth throughout early infancy, with a similar trend for exposed and less-exposed body sites. Skin pH on all tested body sites and sebum content of the forehead showed an overall declining trend after birth until age 6 months, indicating overall acidification during early infancy (Table 2).
Sex differences were only found for sebum content on the forehead at birth or at 42 days, values in boys significantly higher than those in girls (ps<0.001), but the difference became non-significant at age 6 months (p=0.57) (Table S1).
Trends of skin barrier parameters on the cheek and forearm among infants with and without AD
Mixed models for repeated measures showed significant differences in the dynamic trends of TEWL and sebum content between the two groups (p<0.001), but not for pH or SCH (Fig. 2). TEWL did not differ at birth, but TEWL reached higher levels in the AD group than the non-AD group at 42 days (21.02 vs. 17.01 on the cheek; 12 vs. 10 on the forearm; p<0.001). Only significant differences for the forearm TEWL remained at 6 months (12.78 vs. 9.95, p<0.001). Infants with AD had significantly higher sebum content values at birth and 42 days, but the difference became smaller and did not significantly differ at 6 months (Fig.2).
Associations of cheek and forearm skin barrier parameters among infants with AD
In univariate analysis, we found a weak association of AD risk within 1 year with cheek pH at birth, cheek TEWL values at 42 days, and forehead sebum content at 42 days. Additionally, the association of forearm TEWL at 42 days values with AD was significant (Table S2).
After adjusting for covariates such as parental allergy history, infant sex, birth weight, and feeding history, significant associations remained for cheek TEWL (OR=1.26, 95% CI 1.00–1.57, p=0.045 at birth, and OR=1.52, 95% CI 1.17–1.97, p=0.002 at 42 days). The results indicated that infants with higher cheek TEWL values by 1 SD at birth (4 g/m2/h) or at 42 days (7.8 g/m2/h) have increased risk of AD, by 26% and 52%, respectively; a similar association for forearm TEWL at 42 days with AD was also found (OR=1.31, 95% CI 1.02–1.68, p=0.032) (Table 3). No associations were not found between SCH, skin pH, and sebum content at birth or 42 days with AD risk age 1 year.