Vitamin D status and the adequacy of its supplementation during the first year of life in preterm infants in northern Japan.

Background: Vitamin D (VD) deficiency (VDD) is a major concern in preterm infants. The prevalence of VDD in mothers and infants varies between countries and is affected by a range of factors, such as geography and lifestyle. Thus, strategies aimed at preventing VDD must consider the status of each region. However, few reports have explored VDD in preterm infants in Japan and the safety and efficacy of VD supplementation in addressing VDD remain unclear.

Methods: This study was conducted between September 2019 and October 2022. The participants were 108 preterm infants who were divided into three groups based on their gestational age: <28 weeks (Group 1), 28-33 weeks (Group 2), and 34-36 weeks (Group 3). VD status at birth was assessed, and 25-hydroxyvitamin D (25OHD) levels and biochemical markers were monitored during supplementation with 400 IU/day of VD over the first year of life.

Results: Levels of 25OHD at birth were 10.0 (10.0-16.1), 10.5 (10.0-18.0), and 13.0 (10.0-19.0) nmol/L in Groups 1, 2, and 3, respectively. Infants in all three groups exhibited marked VDD. Their 25OHD levels gradually increased with VD supplementation before plateauing at 6 months. Nevertheless, VDD persisted in the majority of infants at 1 month of age. Serum intact parathyroid hormone levels peaked at 1 month of age and declined thereafter, negatively correlating with 25OHD levels. None of the infants exhibited symptoms of VD toxicity.

Conclusion: Preterm infants in northern Japan exhibited substantial VDD, regardless of gestational age. In our cohort, VD supplementation at 400 IU/day safely increased 25OHD levels. However, VD levels improved gradually over the months, and several of these infants developed secondary hyperparathyroidism. Further studies are warranted to determine the optimal VD supplementation dose for preterm infants in this region.