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        Response of serum 25-hydroxyvitamin D concentrations to vitamin D supplementation during lactation
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        Response of serum 25-hydroxyvitamin D concentrations to vitamin D supplementation during lactation
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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.

1.IOM (Institute of Medicine) (2011) Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The National Academies Press.