An investigation of the association between maternal body weight and vitamin D status during pregnancy

Abstract

The combined effects of maternal obesity and low vitamin D status have adverse implications for maternal and fetal health. The overall aim of this PhD thesis was to investigate the association between maternal body weight and vitamin D status throughout pregnancy. From the systematic review, this thesis reports that few observational studies have assessed vitamin D status across BMI categories. Obese or obese class 2 pregnant women had lower vitamin D status compared to non-obese pregnant women. Maternal vitamin D status was negatively associated with maternal BMI and positively associated with infants vitamin D status. Vitamin D status was lower in infants born to obese mothers than infants born to normal weight mothers. When we examined data from a previous cohort of pregnant women in Northern Ireland (from 2006) who were not taking vitamin D supplements in pregnancy, we were able to examine the true association between BMI and vitamin D status in this unique cohort. Our results demonstrate that pregnant women with obesity had significantly lower vitamin D status than those who were normal weight, this was evident particularly during the winter months. There was a high prevalence of vitamin D insufficiency (<50 nmol/L) among pregnant women living in Northern Ireland in 2006. When we assessed the association between BMI and vitamin D status in early pregnancy in a cohort that were recruited for the purposes of this PhD work, we found that pregnant women with overweight and obesity had significantly lower vitamin D status compared to pregnant women of normal weight in early pregnancy despite 62% of women reporting to take a vitamin D supplement. Maternal BMI was found to be a significant negative predictor of vitamin D status and a high prevalence of vitamin D insufficiency still exists among pregnant women living in Northern Ireland in 2017. This insufficiency was highest among non-supplement users during winter months. We assessed the effect of supplementation with 10μg vs 20μg vitamin D3/d throughout pregnancy on vitamin D status of normal weight, overweight and obese pregnant women and on the cord blood of their infants. We report that vitamin D status increased throughout the pregnancy in both treatment groups, with a significantly higher increase observed in the 20µg group. In the 20µg group, maternal and cord vitamin D status reached and maintained sufficiency (≥50 nmol/L) throughout pregnancy, even in those who started pregnancy with an insufficient status. In obese women who started pregnancy with an insufficient vitamin D status, the related cord blood vitamin D status was deficient (<25 nmol/L) in both treatment groups (4). Overall, maternal obesity was negatively associated with maternal and infant vitamin D status and the current recommendation of 10µg/d is inadequate to reach and maintain sufficiency in those who start pregnancy with an insufficient status.

Date of Award	Jul 2018
Original language	English
Sponsors	Royal Embassy of Saudi Arabia
Supervisor	Maria Mulhern (Supervisor) & Mary Mc Cann (Supervisor)