The result of the present study showed that the prevalence of vitamin D deficiency and insufficiency were 37% and 58% in our population study. It delineates that the majority of NICU admitted neonates (95%) had abnormal levels of vitamin D. Consistent with our finding, Tanbakuchi et al. indicated such a high prevalence of vitamin D deficiency/insufficiency (vitamin D < 30) in 90% of Iranian high-risk preterm neonates (20). Another study from Iran also showed that 93.3% of 522 newborns had mild to severe vitamin D deficiency at birth (21). Investigations from other countries indicated different prevalence rates ofvitamin D deficiency/insufficiency among neonates. For instance, Panda et al. from Australia have demonstrated that 35.7% of NICU admitted preterm neonates had vitamin D insufficiency or deficiency (22). A study from Albany has shown vitamin D deficiency/insufficiency in 80% of NICU admitted preterm infants with birth weight < 1500 gr (23). Other studies from Saudi Arabia and Jordan demonstrated vitamin D deficiency in 86% and 94.1% of newborns (24, 15). These discrepancies in the results may arise from diversities in some influential factors like the mother's type of clothing, geographic region, and sunshine exposure, maternal and neonatal multiple disease states, gestational age, implementing of different laboratory cut-offs, ethnicity, and prenatal intake of vitamin D supplements (11, 25, 26)
The present results showeda significant association between neonates' vitamin D status and neonatal age at hospital admission; younger neonates had lower serum vitamin D levels. This finding may indicate the importance of evaluation of vitamin D status in the neonatal period particularly for NICU hospitalized subjects (27). Panda et al. has also indicated a significant increase in vitamin D levels from the first days of life (57 nmol/L) to 3–4 weeks post-natal period (63.5 nmol/L) among NICU admitted preterm neonates. However, participated subjects in this reported study were prescribed postnatal vitamin D supplements (22).
Our findings showed that neonatal vitamin D status was significantly correlated with the season of birth. Of all vitamin D deficient neonates, the majority of them were born in winter. This association between vitamin D status and season of birth was reported by Maden et al. They showed that neonates who admitted during summer had higher levels of vitamin D compared to those admitted in other seasons. The least levels of vitamin D pertained to subjects who admitted through fall or winter (9). The correlation between vitamin D levels and season of birth was also reported by Korj-Bulos et al. They showed low vitamin D levels in neonates who were born during winter months (15).
Results of the present investigation showed a significant correlation between serum vitamin Dand calcium statuses. Consistent with our findings, Mcnallyet al. have demonstrated a positive significant association between vitamin D and calcium levels (28). This finding may confirm the important role of vitamin D in calcium homeostasis. Moreover, it may reveal that in hospitalized newborns, functions of different organs and systems including cardiovascular and hemostasis, immune system, enzymes, and cell receptors may be disturbed by the imbalance of calcium levels resulting in severe medical complications (28).
According to our results, the mean value of vitamin D in term neonates was significantly lower than preterm neonates. Another study from Iran has also indicated a lower level of vitamin D in term neonates compared to the preterm group, however, the difference was not significant (13.39 vs. 13.91 ng/ml; p = 0.850) (27).On the other hand, we could not find any statistically significant difference between term and preterm neonates regarding the prevalence of vitamin D deficiency, insufficiency or sufficiency. It has been reported that there is no significant correlation between gestational age and vitamin D level in infant-mother pairs (27). However, several studies have indicated a greater risk of vitamin D deficiency among preterm infants compared to term infants (5, 29, 30).
Findings of the present study demonstrated no significant differences in neonates' vitamin D concentrations regarding medical or surgical causes of hospitalization. Consistent with our results, Arnson et al. did not also find any relationship between vitamin D levels and the causes of hospitalization (6).
We did not find any association between serum vitamin Dstatuses with neonatal mortality. Accordance with our findings, Azim et al. reported no association between vitamin D status and mortality rate among hospitalized critically ill patients (4). Mcnally et al. have shown that the survival of an infant was more associated with the type and severity of underlying complications rather than vitamin D status (28).
Based on the results, no significant association was observed between duration of NICU hospitalization and serum vitamin D status. However, this finding was not confirmed by other studies; another study from Iran has demonstrated a significant relationship between neonatal vitamin D levels with the duration of NICU hospitalization (31). Kim et al. have also reported that the average duration of NICU hospitalization in vitamin D deficient group was significantly longer than vitamin Dinsufficient or sufficientgroups (32).
Our study had several limitations. For instance, the sample size, particularly in vitamin D sufficient group, was so small. The mother's vitamin D levels and its association with neonate's vitamin D status were not assessed because of some shortages in our financial recourse. The associations between neonate's serum vitamin D/calcium levels did not evaluate with some other possible involving factors like parathyroid response and prenatal vitamin D supplementation. Future studies with larger sample sizes and considering more affecting variables are suggested.