Vitamin D Status and Multisystem Health Outcomes at 5–6 Years in Very-Low-Birth-Weight Preterm and Term-Born Children: A Cross-Sectional Secondary Analysis of Two Prospective Cohorts

Background/Objectives: Vitamin D deficiency is common in children, but its associations with growth and organ function remain unclear. This cross-sectional secondary analysis of two prospective cohort studies evaluated vitamin D status and multisystem health outcomes in very-low-birth-weight preterm and term children aged 5–6 years. Methods: Serum 25-hydroxyvitamin D [25(OH)D] was categorized as deficiency (<20 ng/mL), insufficiency (20–30 ng/mL), or sufficiency (>30 ng/mL). Assessments included anthropometry, blood pressure, body composition and bone mineral density, pulmonary function, cardiac indices, kidney and metabolic biomarkers, and cognitive function (preterm children only). Results: Overall, 213 children (144 preterm, 69 term) were included; only 17.8% achieved sufficient vitamin D levels. No statistically significant associations were detected between serum 25(OH)D and any of these outcomes. In contrast, 25(OH)D levels were positively correlated with physical activity, sun exposure, and vitamin D supplementation (all p < 0.05; second cohort only). Conclusions: Vitamin D deficiency was common, but no statistically significant associations with multisystem health measures were detected; several features of this analysis limit the strength of these null findings. Sun exposure, physical activity, and vitamin D supplementation were associated with higher 25(OH)D concentrations. These exploratory cross-sectional findings require confirmation in prospectively designed studies.