Abstract
BACKGROUND: The first 1000 days of life represent a critical window for developmental programming, during which specific nutritional exposures, such as vitamin D levels, may influence long-term health trajectories. Vitamin D plays a central role in skeletal development, but increasing evidence also supports its possible involvement in immune, metabolic, and neurodevelopmental processes during early life. In this narrative review, we summarize current evidence on the biological functions of vitamin D across the first 1000 days, focusing on its roles in skeletal, immune, metabolic, and neurodevelopmental processes, and its potential role as a programming factor. METHODS: We conducted our research using the PubMed, Scopus, and Cochrane databases. We included systematic reviews, randomized controlled trials, and high-quality observational studies published from 2015 onward, focusing on pregnancy, neonatal life, and early childhood. RESULTS: Vitamin D acts through placental, epigenetic, skeletal, immune, metabolic, and neurodevelopmental pathways that are particularly active during early development. Low maternal or early-life vitamin D status has been associated with adverse birth outcomes and impaired bone health. It has also been linked to increased susceptibility to infections and allergic diseases, altered metabolic trajectories, and mild neurodevelopmental differences. Evidence from supplementation trials remains heterogeneous, with benefits appearing more consistent in populations with baseline deficiency. CONCLUSIONS: Vitamin D fulfills several biological plausibility criteria for a potential early-life programming factor, although current human evidence remains heterogeneous.