Abstract: Objective To investigate the levels of serum 25-hydroxyvitamin D [25-（OH）D] in children aged 0-6 years in Haikou and the relationship between vitamin D receptor （VDR） gene polymorphism and bone mineral density （BMD）. Methods Healthy children aged 0 - 6 years who underwent physical examination in Hainan Women and Children's Medical Center from January to December 2020 were selected as the subjects to detect the serum 25-（OH）D levels, VDR gene polymorphism and BMD. The serum 25-（OH）D levels of children with different gender, age, body mass index （BMI） and BMD, the VDR gene polymorphism of children with normal and abnormal BMD were compared, and the relationship between VDR gene polymorphism and BMD was analyzed. Results A total of 1 580 healthy children aged 0-6 years were investigated, including 838 boys and 742 girls, with an average age of （2.49±1.20） years. The serum 25-（OH）D level was （34.66±5.87） ng/mL, and the proportion of children with vitamin D deficiency, insufficiency, and sufficiency was 4.49%, 21.01%, and 74.49%, respectively. There was no significant difference in serum 25-（OH）D levels and vitamin D nutritional status between children of different genders （both P>0.05）. There were significant differences in serum 25-（OH）D levels and vitamin D nutritional status between children with different age groups, BMI and BMD （P<0.05 or P<0.01）. The results of genotyping showed that there were polymorphisms in APa Ⅰ locus of VDR gene in healthy children aged 0-6 years, and the genotypes were AA, Aa, and aa. There were significant differences in genotype and allele distribution of APaⅠ locus of VDR gene between normal and abnormal BMD children （all P < 0.01）. Multivariate Logistic regression analysis showed that children with Apa Ⅰ genotype aa of VDR gene （OR = 3.729） and a allele （OR = 2.656） had a higher risk of abnormal BMD. Conclusion The level of serum 25-（OH）D in children aged 0-6 years in Haikou is related to children's age, BMI and BMD, and the VDR gene polymorphism in children is closely related to the occurrence of abnormal BMD.

Key words: Children, 25-hydroxyvitamin D, Vitamin D receptor, Gene polymorphism, Bone mineral density