Sodium and Milk Fortification Evaluation of Body Composition Among Very Preterm Infants

Very preterm infants (born \<32 weeks gestational age) are at high risk for postnatal growth faltering, which is associated with adverse neurodevelopmental outcomes. Despite routine use of enriched human milk fortification in the NICU, many infants fail to achieve adequate somatic and lean mass growth. Sodium depletion has been identified as a potentially modifiable contributor to this problem: renal tubular immaturity in preterm infants results in obligate urinary sodium wasting. Total body sodium depletion has been associated with poor growth in infants and poor muscle mass development in animal studies. Current NICU practice relies primarily on serum sodium to guide supplementation; however, serum sodium is a late and insensitive marker of total body sodium depletion. Urine sodium is a more sensitive indicator of sodium balance and may identify depletion before serum levels fall. Gestational-age-specific urine sodium thresholds to guide supplementation have recently been proposed (Stalter et al., 2025), but have evaluated body composition in a prospective randomized trial. This trial tests whether a structured, urine sodium-guided supplementation protocol based on every other week urine and serum sodium monitoring improves anthropometric growth and lean mass accrual compared to standard enriched fortification alone. The intervention runs from day of life 14 through 36 weeks postmenstrual age, with tapering through 38 weeks PMA. Body composition assessment incorporates two complementary modalities: serial point-of-care ultrasound (POCUS) of the biceps, rectus femoris, and subcutaneous fat for longitudinal lean and fat mass tracking, and Air Displacement Plethysmography (PeaPod) at 36 weeks postmenstrual age or discharge for criterion-standard whole-body composition. Fat mass, fat-free mass, and body fat percentage will be referenced against the Norris et al. preterm normative curves. The trial uses an open-label design with block randomization stratified by gestational age. A Data Safety and Monitoring Board will conduct oversight and interim safety reviews.