Statistical analysis
Participant characteristics are described separately for mothers who
received 1000 IU cholecalciferol and those who received placebo using
frequency and percentage distribution for categorical variables, mean
(SD) for normally distributed continuous variables and median
(interquartile range) for non-normally distributed continuous variables.
We used logistic regression to examine associations between being
randomised to the active group and developing eczema at ages 12, 24 and
48 months, expressing results as odds ratios (ORs) and 95% confidence
intervals (CIs). Models were adjusted for duration of breastfeeding as
descriptive analyses showed differences in breastfeeding duration
between the groups. Sensitivity analyses were undertaken to examine
whether the effect of the intervention differed in mothers breastfeeding
for <1 and ≥1 month because of its reported association with
eczema and the influence of gestational supplementation on breast milk
vitamin D content, and to take account of season of birth. We
additionally used logistic regression to examine the associations
between the offspring’s risk of atopic eczema at ages 12, 24 and 48
months and maternal late pregnancy serum 25(OH)D concentration and
single-nucleotide polymorphisms (SNPs) in or near key vitamin D
metabolism genes, specifically rs12785878 (encoding 7-dehydrocholesterol
reductase (DHCR7) in the epidermal vitamin D biosynthesis
pathway), rs10741657 and rs6013897 (encoding 25-hydroxylase
(CYP2R1) and 24-hydroxylase (CYP24A1) ,
respectively).10 Analyses were performed using Stata
(Stata version 15.1, Statacorp LP, TX).