NRDS is a common acute disease in pediatrics [11]. It has been reported that the onset of NRDS is closely related to lung hypoplasia, progressive alveolar collapse and high permeability lesions of pulmonary alveoli [12]. NRDS is a self-limiting disease [13]. However, there is a greater risk of concurrent infection during the onset of the NRDS, and NRDS can seriously threaten the lives of the infants. Therefore, exploring biochemical indicators that may be able to detect and judge the risk of NRDS is necessary.

Accumulating evidence has explained the association of vitamin D levels at birth and pulmonary disease morbidity in very preterm infants [14]. The use of vitamin D as adjuvant therapy in RDS cases resulted in significantly lower severity, complication rates, and length of hospital stay in preterm infants with RDS [15]. Barchita et al. reported that spontaneous and idiopathic preterm labor is associated with vitamin D deficiency. Preterm birth is an important factor leading to NRDS, and it was initially revealed that vitamin D deficiency caused by preterm birth may be related to NRDS [16, 17]. Decreased vitamin D is related to preterm birth, and vitamin D supplementation during pregnancy can reduce the rate of preterm birth [18, 19]. Aylor et al. reported that the levels of alveolar surfactant protein B and vascular endothelial growth factor increased through inhalation of vitamin D, suggesting that vitamin D might exhibit special functions in the management and treatment of NRDS [20]. Decreased vitamin D is a common and changeable risk factor for bronchopulmonary dysplasia and RDS [21]. A recent large prospective study which covered 402 newborn infants and their mothers suggested that cord blood vitamin D deficiency was associated with increased risk of preterm birth, NRDS and hospitalization during the first year of life [22]. Interestingly, some studies found no correlation between vitamin D and neonatal lung disease or lung co-development. For example, in a study that included 44 extremely preterm infants (gestational age <29 weeks), neither cord blood nor the 36 weeks' corrected age 25(OH)D3 levels were associated with development of bronchopulmonary dysplasia [23]. Furthermore, no significant association was found between vitamin D status and selected clinical outcomes when using a cut-off of 25 nmol/l (severe vitamin D deficiency) in preterm infants [24]. Considering a large number of animal and clinical studies that have revealed the importance of vitamin D for the development of the neonatal respiratory system, we believe that the differences in these results are because the cutoff values used in the above studies were not strong enough to distinguish differences in respiratory disease in preterm infants with vitamin D deficiency from other preterm infants.

There is still few clinical research on the effect of serum 25(OH)D3 concentration on the extra-skeletal effects of premature infants. It is still unclear how much serum 25(OH)D3 concentration can satisfy the needs of preterm infants. This study indicates that the 25(OH)D3 level in the serum of newborns with NRDS was statistically decreased than the control group, suggesting that the low 25(OH)D3 of newborns may be related to NRDS. The level of serum 25(OH)D3 of newborns could be used to predict NRDS taking 57.69 nmol/L (24ng/ml) as the cut-off value. In addition, we found that the serum 25(OH)D3 level in the cord blood serum of children was significantly positively correlated with Apgar score, and significantly negatively correlated with OI, duration of oxygen support and continuous positive airway pressure ventilation. The possible mechanism is that, lung surface active substances are produced by alveolar type II epithelial cells, which have vitamin D receptors and renal tubules 1α-hydroxylase protein expression. Therefore, alveolar type 2 epithelial cells may be the target cells for vitamin D bioregulation, which will have a certain impact. For the developing fetus, the lack of vitamin D leads to insufficient alveolar surface-active substances, thus affecting the development of lung structure and function. Therefore, 25(OH)D3 is related to NRDS in participants of our research.

In addition, in order to further analyze the relationship between neonatal 25(OH)D3 and NRDS, the risk factors of NRDS were analyzed. Our results suggested that cord blood serum 25(OH)D3 <57.69 nmol/L (24 ng/ml), gestational age<31 weeks, birth weight<1.86kg, Apgar score (1 min) <7 and Apgar score (5 min) <8 are independent risk factors for NRDS (p<0.05). Our data is consistent with the data of previous related studies. Decreased vitamin D in preterm infants is an independent risk factor for RDS [10]. In addition, Fettha et al. confirmed that higher serum 25(OH)D3 level in preterm infants could prevent RDS in preterm infants [25]. However, the difference is that in the latest prospective cohort study, when using 10 ng/ml as the cut-off value, vitamin D was not related to RDS, and the serum 25(OH)D3 level of infants did not affect the incidence of RDS [26]. This may be due to the use of 10 ng/mL as the cutoff value in the above-mentioned study was not sufficient to distinguish the difference between the two groups. At present, there is no consistent international standard for the optimal level of 25(OH)D3 in preterm infants [27]. Many scholars believe that 25(OH)D3 concentration <20 ng/ml indicates insufficient, and serum concentration<10 ng/ml indicates severe deficiency [26, 28, 29]. The cut-off value in this study is 57.69 nmol/L (24 ng/ml), which suggests that we may need a higher serum 25(OH)D3 level to prevent NRDS. However, patients who take vitamin D excessively for a long time may suffer from vitamin D poisoning, when the content of 25(OH)D3 in their blood exceeds 100ng/ml. Although vitamin D toxicity is rare in clinical practice, we should actively monitor the 25(OH)D3 level in patients' blood during treatment.

In fact, there are still many deficiencies in our research. First, due to the limitations of hospital size and department beds, we only recruited 82 qualified infants with NRDS. Insufficient number of infants may affect the accuracy of statistical results. We plan to conduct future multicenter joint studies to expand the number of patients with NRDS. Second, we mainly analyzed the relationship between vitamin D in umbilical cord blood and the incidence and treatment of postnatal NRDS. However, there is no reliable study to explain why vitamin D deficiency increases the NRDS incidence rate. The key role of vitamin D in the formation of fetal alveolar cells and the establishment of lung tissue structure remains unclear. We would like to further reveal the important role of vitamin D in fetal lung development in future mechanism research. In addition, the risk factors of NRDS revealed in this study are very limited. We only analyzed the relationship and correlation between 25(OH)D3 level, pregnancy time, birth weight, Apgar score (1 min) and NRDS in this study. However, previous studies have shown that the risk of NRDS may be due to maternal diabetes, fetal gender, delivery time, and difficult labor experience. Due to the limitation of manpower and information integrity, we did not analyze the relationship between these factors and NRDS incidence rate in this study. We intend to fully cover these factors in the future. Moreover, our study also lacked data on the association of cord blood 25(OH)D3 levels with subsequent incidence of neonatal bronchopulmonary dysplasia, and the association of cord blood 25(OH)D3 levels with maternal vitamin D status. We will continue to refine these studies in future research.