Pregnancy, lactation and early childhood are life stages when the risk of low vitamin D status is high and the knowledge basis for determining nutritional requirements for vitamin D is weak. The current dietary reference intervals (DRI) for vitamin D in pregnant and lactating women are the same as those in non-pregnant adult females below 70 years (600 IU/15 μg/d)(1). The aim of the current study was to investigate vitamin D requirements during lactation. We conducted a double-blind randomised placebo-controlled trial across three intervention groups using 20 μg/d of vitamin D3 (to achieve a total vitamin D intake of ~25 μg/d), with or without 500 mg Ca, or placebo, over 12 weeks of lactation. The study protocol was implemented across a calendar year to account for seasonal effects.
Concentrations of serum 25-hydroxyvitamin D (s25(OH)D) were measured at baseline (BL) and endpoint (EP) in mothers and in umbilical cord blood using ELISA. Vitamin D metabolites (D3, D2 and 25(OH)D) were quantified in expressed breast milk at four time points during the intervention study using HPLC. Dietary intakes of vitamin D and Ca, anthropometric data, socio-demographic and lifestyle data were collected, as well as antenatal supplement use and habitual sunshine exposure. The s25(OH)D data are described here.
Median; interquartile range in parentheses; *One-way ANOVA followed by Tukey's test.
† ANCOVA adjusting for BL s25(OH)D, dietary intake of vitamin D and season of EP blood sampling.
At BL, 21 and 63% had a s25(OH)D level below the thresholds for vitamin D deficiency and sufficiency of 30 and 50 nmol/l, respectively. Season of blood sampling was the main determinant of BL s25(OH)D (adj. R 2=0.338; β=0.571; P<0.001), as expected. Other determinants were parity and total vitamin D intake (β=0.157 and 0.150, respectively; P<0.05). Mean (sd) cord s25(OH)D levels were 33.8 (14.8) nmol/l (n 92) and were 78% of maternal levels on average (R 2=0.7; P<0.001). Season of birth (adj. R 2=0.274; β=0.137) and antenatal vitamin D supplement use (adj. R 2=0.004; β=0.140) were independent predictors of cord s25(OH)D levels (both P<0.05).
A final sample of 100 women completed the intervention protocol, of which ninety were more than 80% compliant. The intervention considerably increased s25(OH)D levels in the treatment groups by ~30 nmol/l (see Table) with no difference in the EP concentrations between women who received vitamin D only and those who received vitamin D+Ca. Given that the average habitual vitamin D intake in the group was 4.3 μg/d, supplementation with 20 μg/d vitamin D3 to achieve a total intake of ~25 μg/d, maintained s25(OH)D levels >30 nmol/l in all lactating women and brought 96% above the desirable threshold of 50 nmol/l.
In conclusion, the current DRI of 15 μg/d is inadequate to achieve a target s25(OH)D of 50 nmol/l in 97.5% of lactating women at a latitude of 51°N.
Funded by the Irish Department of Agriculture, Fisheries and Food through the Food Institutional Research Measure.