2.2. Infant vitamin D examination

Detailed method was reported in previous study (Zhao, ²⁰¹⁵). Briefly, 200μL finger‐sticking blood samples were collected from each participant and placed directly into a 0.5 milliliter micro‐tube. Within 10 min, the blood samples were centrifuged at 3,500 rpm for 15 min. Serum samples were stored at −80℃ until enzyme‐linked immunosorbent assay, the purpose of which was measuring for serum 25(OH)D levels (IDS Ltd.). The interassay and intra‐assay coefficients of variation were <10%, respectively.

Although the optimal vitamin D level is surrounded by debate, vitamin D deficiency has been historically defined and recommended by Endocrine Society's clinical practice guidelines as <50 nmol/L (Holick, ²⁰⁰⁶, ²⁰⁰⁷; Holick et al., ²⁰¹¹). Meanwhile, to maximize the effect of vitamin D on bone and extra‐skeletal health, it was suggested that the level of 25(OH)D should be above 75 nmol/L. (Holick et al., ²⁰¹¹). Additionally, 25(OH)D < 50, 50–75, and ≥75 nmol/L as deficiency, insufficiency, and sufficiency were often used in Chinese population (Guo, ²⁰¹⁸; Zhang et al., ²⁰¹³; Zhu, ²⁰¹²). Thus, vitamin D nutritional status was assessed by 25(OH)D concentration as “deficiency” (<50 nmol/L), “inadequacy” (50 to <75 nmol/L), and “sufficiency” (≥75 nmol/L) in the current study, respectively.

The seasons of specimens collection were divided into spring (from March to May), summer (from June to August), autumn (from September to November), and winter (from December to February).