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Context
Routine vitamin D supplementation (400 IU/day) of breastfed infants has been recommended in North America for >50 years.1 Historically, the practice was advocated to prevent rickets; recently, there has been greater emphasis on its role in maintaining serum 25-hydroxyvitamin D concentrations (25(OH)D) above conventional thresholds of sufficiency (eg, 50 nmol/L).2 Yet, some breastfeeding advocates have argued that this policy conflicts with the message that mother's milk is a complete source of nutrient requirements in the first 6 months of life. The recognition that breast milk vitamin D inadequacy reflects maternal vitamin D insufficiency has prompted efforts to define maternal postpartum vitamin D supplementation regimens to be used in place of infant supplementation.
Methods
Healthy exclusively breastfeeding mother–infant pairs (≥35 weeks gestation) were enrolled at 4–6 weeks postpartum in a randomised controlled trial comparing three masked vitamin D3 regimens: (1) maternal 400 IU/day and infant 400 IU/day; (2) maternal 2400 IU/day and infant 0 IU/day; (3) maternal 6400 IU/day and infant 0 IU/day. The primary analyses were between-group comparisons of the mean maternal or infant serum 25(OH)D at 4 or 7 months postnatal age, and the proportion of mothers or infants in each group who attained 25(OH)D≥50 nmol/L. Group 2 was stopped prematurely due to ‘safety concerns for the infants’ (no further details provided). Analyses only included mother–infant pairs in groups 1 and 3 who maintained exclusive/full breastfeeding throughout follow-up (ie, breast milk alone until 6 months of age; breast milk as the sole milk after introduction of complementary foods).
Findings
Of 334 pairs randomised to groups 1 or 3, only 148 (44%) and 95 (28%) mother–infant pairs were included in 4-month and 7-month analyses, respectively. About half of the participants discontinued exclusive/full breastfeeding before 7 months. Mean maternal 25(OH)D did not differ between groups at baseline (82.1 vs 90.7, p=0.06), but women assigned to 6400 IU/day had significantly higher mean 25(OH)D at 4 and 7 months postpartum (p<.0001). Infant 25(OH)D did not significantly differ between groups at baseline, 4 or 7 months (p range 0.1–0.9). Nearly all mothers and infants had 25(OH)D≥50 nmol/L at 4 and 7 months. There were no supplement-related safety concerns reported for groups 1 or 3.
Commentary
In the minority of enrolled participants included in analyses, high-dose postpartum vitamin D supplementation (6400 IU/day) markedly increased maternal serum 25(OH)D compared to 400 IU/day. Importantly, the maternal dose of 6400 IU/day achieved similar vitamin D status in exclusively/fully breastfeeding infants versus the infant supplement (400 IU/day). These findings were consistent with vitamin D pharmacokinetics and previous observations of dose-dependent mother-to-infant delivery of vitamin D via breast milk.3 ,4 Postpartum vitamin D supplementation appears safe and efficacious with respect to biochemical end points, but this trial does not greatly advance knowledge required to introduce new maternal-infant vitamin D supplementation guidelines. A key limitation was the exclusion of participants who changed breastfeeding status. A preferred trial design would have involved a standardised algorithm to address this common scenario, so that an intention-to-treat analysis could have been performed. Based on the pharmacological data from the researchers’ pilot study,3 the present trial might have been designed to compare real-world supplementation protocols. Eligibility criteria were unnecessarily restrictive; for example, initiation of infant vitamin D supplementation is recommended ‘in the first few days of life’,1 yet infants receiving vitamin D before the study were excluded. Overall, the reporting of the trial appeared incomplete—there was insufficient information about group 2, adherence rates or clinical outcomes. The high dose itself presents a dilemma for implementation: 6400 IU/day is above the tolerable upper intake level of 4000 IU/day.2 Given the very high attained serum 25(OH)D and cholecalciferol concentrations in mothers assigned to 6400 IU/day, it remains unclear if such a high dose is necessary.
Implications for practice
Methodological flaws limit the translation of these findings into practice. Yet, pharmacological data available thus far suggest that maternal postpartum vitamin D supplementation will be a viable alternative to infant supplementation in the future.
Footnotes
Competing interests None declared.
Provenance and peer review Commissioned; internally peer reviewed.