Vitamin D status during pregnancy modulates the effect of pre-pregnancy obesity on gestational diabetes mellitus risk: a birth cohort study

Abstract Objective: To determine whether gestational vitamin D status modulates the effect of pre-pregnancy obesity on gestational diabetes mellitus (GDM) risk while stratifying by maternal age. Design: Birth cohort. Setting: A major maternity hospital in Kuwait. Participants: Pregnant women in their second/third trimester of gestation were enrolled. Pre-pregnancy BMI (kg/m 2 ) was categorised as under/normal weight (< 25·0), overweight (25·0 to < 30·0) and obesity (≥ 30·0). Gestational 25-hydroxyvitamin D concentrations were categorised as deficiency (< 50 nmol/l) or insufficiency/sufficiency (≥ 50 nmol/l). GDM status was ascertained according to international guidelines. Adjusted OR (aOR) and 95 % CI were estimated using logistic regression. Results: Data from 957 pregnant women were analysed, with GDM affecting 166 (17·4 %) pregnancies. Pre-pregnancy obesity and gestational vitamin D deficiency were ascertained in 275 (28·7 %) and 533 (55·7 %) pregnant women, respectively. The association between pre-pregnancy obesity and GDM risk differed according to maternal age and gestational vitamin D status ( P interaction[BMI × age × vitamin D] = 0·041). Among women aged < 35 years ( n 710), pre-pregnancy obesity compared to under/normal weight was associated with increased GDM risk among women with gestational vitamin D deficiency (aOR: 2·72, 95 % CI: 1·18, 6·23) and vitamin D insufficiency/sufficiency (2·55, 1·15, 5·62). In contrast, among women aged ≥ 35 years ( n 247), pre-pregnancy obesity compared to under/normal weight was associated with increased GDM risk among women with gestational vitamin D deficiency (6·92, 1·45, 33·04), but not among women with vitamin D insufficiency/sufficiency (1·13, 0·36, 3·56). Conclusions: Gestational vitamin D status modulates the effect of pre-pregnancy obesity on GDM risk in an age-specific manner.