Vitamin-B12 status among the mother–infant dyad: an explorative overview in a single centre in Lombardy

Introduction: Maternal vitamin B12 deficiency is an increasingly recognized cause of secondary neonatal cobalamin deficiency and can be incidentally detected through expanded newborn screening. Methods: We retrospectively evaluated 240 infants and 238 mothers referred to a Regional Clinical Centre in Lombardy, Italy, between 2016 and 2026 following a positive newborn screening result suggestive of methylmalonic acidemia or vitamin B12 deficiency. Clinical, dietary, and biochemical data were collected at diagnosis and during follow-up after vitamin B12 supplementation and complementary feeding introduction. Results: At baseline, 56.2% of infants had vitamin B12 concentrations below the laboratory reference range, while elevated homocysteine and plasma methylmalonic acid were observed in 52.8 and 44.8% of cases, respectively. Maternal and neonatal vitamin B12 concentrations were positively correlated, and infants born to vitamin B12-deficient mothers showed significantly lower vitamin B12 levels than those born to mothers with adequate status. Vegetarian mothers had lower vitamin B12 concentrations than omnivorous mothers, whereas exclusively breastfed infants showed lower vitamin B12 concentrations than formula-fed infants. Compared with conventional laboratory reference ranges, the National Institute for Health and Care Excellence criteria identified significantly more infants with biochemical evidence of functional vitamin B12 deficiency. Following supplementation, vitamin B12 concentrations increased markedly and functional biomarkers normalized in almost all infants, with improvements maintained after supplementation discontinuation. Discussion: These findings confirm the strong influence of maternal vitamin B12 status on neonatal biochemical status, support the role of newborn screening in identifying secondary neonatal vitamin B12 deficiency, and suggest that currently adopted laboratory reference intervals may underestimate the burden of clinically relevant vitamin B12 deficiency in mother-infant dyads.