A dietary/nutritional supplement containing 400 IU of vitamin D3 taken before conception and during pregnancy raises vitamin D levels in human milk but does not impact the concentrations of B-vitamins.

This double-blind randomized controlled trial aimed to scrutinize the impact of maternal supplementation from preconception through pregnancy until birth on the concentrations of vitamin D3 and B-vitamins in human milk. Additionally, the research sought to characterize the longitudinal changes in vitamin concentrations in human milk.

The supplements provided to both control and intervention groups included calcium, iodine, iron, β-carotene, and folic acid. The intervention group's supplement also included zinc, vitamins B2, B6, B12, and D3, along with probiotics and myo-inositol. Human milk samples were collected at four time points from 1 week to 3 months post-delivery from 158 mothers in Singapore and at seven time points from 1 week to 12 months from 180 mothers in New Zealand.

The quantification of human milk vitamin D was executed using supercritical fluid chromatography. On the other hand, B-vitamins were assessed with the aid of mass spectrometry. Linear mixed models with a repeated measures design were employed to evaluate potential intervention effects on human milk concentrations of vitamins D3, B2, B6, and B9, as well as other B-vitamins (B1 and B3).

Throughout the initial 3 months of lactation, concentrations of human milk 25-hydroxyvitamin D3 were 20% higher in the intervention group (95% CI 8%, 33%), with more pronounced effects observed in New Zealand. No discernible intervention effects were witnessed on human milk concentrations of vitamins B1, B2, B3, B6, and B9. In New Zealand mothers, vitamin D3 concentrations steadily escalated from early lactation up to 12 months, while vitamins B1 and B2 peaked at 6 weeks, B3 at 3 weeks, and B6 and B9 at 3 months.

Maternal supplementation during preconception and pregnancy led to elevated human milk vitamin D concentrations but did not impact B-vitamin concentrations during lactation. Further investigations are necessary to assess the specific benefits of vitamin D use initiated either preconception or during pregnancy and to comprehensively understand the effects of supplementation on subsequent health outcomes in offspring.