Vitamin D requirements during the first year are unknown and an adequate intake (AI) value of 10 μg/d( 1 ) has been recommended for infants in the absence of sufficient data to establish an estimated average requirement. Data on the vitamin D content of human milk are variable and maternal factors that influence milk vitamin D levels are poorly described. The objectives of this analysis were to measure total vitamin D content in human milk, to investigate the impact of adjusting for fat content on composition estimates and to evaluate the association between maternal serum 25-hydroxyvitamin D [s25(OH)D] concentration and milk vitamin D content.
In a sample of 108 women, 98% of whom were of European origin, expressed breast milk samples were collected at 2–3 weeks postpartum using a standardised procedure, i.e. in the afternoon once baby had completed a feed, and were frozen immediately. Vitamin D2, D3 and 25(OH)D were quantified using HPLC. Fat creamatocrit (v/v, %) of human milk was measured using a micro-haematocrit centrifuge. Fat content (g/L) was calculated using the formula: [(creamatocrit-0.59)/0.146]( 2 ). Maternal serum 25(OH)D2, 25(OH)D3 and 3-epi-25(OH)D3 were measured by LC-MS/MS.
* Values are median (interquartile range); ∞ Total vitamin D=(D2+D3)+25(OH)D*5; †Total s25(OH)D=25(OH)D2+25(OH)D3.
The total vitamin D content of milk was associated with milk fat (R 2=0.036; β=0.19; P=0.05), but not with maternal s25(OH)D, days post-partum or maternal BMI. Vitamin D3 was significantly associated with fat content (β=0.254; P=0.008) and maternal s25(OH)D (β=0.215; P=0.044) and negatively associated with number of days post-partum (β=−0.174; P=0.05) [total adjusted R 2=0.122, P=].
In summary, the total vitamin D content in human milk is largely determined by the vitamin D3 content, which is associated with milk fat composition and maternal s25(OH)D concentrations, and negatively associated with the duration of lactation. Assuming an average consumption of milk of 600–800 ml/day (up to ~6 month)( 3 ), an exclusively breastfed infant in this sample would receive 1.3–1.8 μg vitamin D/day, which is substantially lower than the adequate intake (AI) of 10 μg( 1 ). First stage infant formula would provide ~7 μg of vitamin D/day in 700 ml formula. Further research to define the vitamin D requirement in infants is required.
Funded by the Irish Department of Agriculture, Fisheries & Food through the Food Institutional Research Measure.