Background <p>Very preterm infants are at high risk of vitamin D deficiency (VDD), which contributes to metabolic bone disease (MBD) and other morbidities. Despite guidelines, optimal dosing remains uncertain. We determined VDD incidence and evaluated 25-hydroxyvitamin D [25(OH)D] responses to varying vitamin D intakes during the first 8 weeks of life.</p> Methods <p>This prospective cohort enrolled infants born at &lt;32 weeks’ gestation or birth weight &lt;1500 g. Vitamin D supplementation followed institutional policy. Infants were categorized by total intake (parenteral plus enteral) during weeks 0–4: &lt;400, 400–700, or &gt;700 IU/kg/day. Serum 25(OH)D was measured at birth, 4, and 8 weeks. Biochemical markers and MBD screening were performed. VDD was defined as 25(OH)D &lt; 20 ng/mL and excess (VDE) as &gt;100 ng/mL.</p> Results <p>Among 126 infants (gestational age 30 [27, 31] weeks; birth weight 1230 [950, 1570] g), 94.3% had VDD at birth. At 4 weeks, VDD persisted in 17.8% receiving &lt;400 IU/kg/day and 6.5% receiving 400–700 IU/kg/day; vitamin D excess (VDE) occurred in 3.3% and 3.2%, respectively. At 8 weeks, normal 25(OH)D was achieved in 90.1% receiving &lt;400 IU/kg/day and 77.4% receiving 400–700 IU/kg/day, while VDE increased to 8.6% and 22.6%, respectively. Biochemical markers remained normal; only one infant developed MBD.</p> Conclusions <p>VDD is highly prevalent at birth in very preterm infants. Daily intake &lt;400 IU/kg generally normalizes vitamin D status by 8 weeks while minimizing risk of excessive vitamin D. Higher doses may provide no additional benefit and increase risk of exceeding 25(OH)D levels.</p> Clinical trial registration <p>Thai Clinical Trials Registry. Registration number TCTR20230725007. Web link: <a href="http://www.thaiclinicaltrials.org/show/TCTR20230725007">http://www.thaiclinicaltrials.org/show/TCTR20230725007</a>.</p>