The study was conducted from May to September 2018 with 1134 children aged 4–14 years from different latitudes and UV index (UVI): Antofagasta (north of Chile: 23.65°; 6–7 UVI), Santiago (central zone: Santiago 33.70°; 4–6 UVI), and Concepción (southern region: Concepción 36.73°; 3–5 UVI). Study participants were recruited independently of nutritional status according to gender, age group, and socioeconomic status. Children were grouped according to their serum 25(OH)D levels [6].
Anthropometric measurements were made with children in light clothing, weight, and height were measured with a portable stadiometer (SECA 813/213). The nutritional status classification was carried out according to the WHO standards for the age and gender of the children. Children with BMI/Age < +1 DS were classified as healthy weight, those with BMI/Age between +1 SD and +2 SD were classified as overweight, and those with BMI/Age ≥ +2 SD as obese. Children with chronic diseases, such as diabetes, cancer, or chronic kidney disease, were excluded. Informed consent was obtained from all subjects. The study protocol was reviewed and approved by the Ethics Committee of the University of Chile.
Biochemical measurements
Serum 25(OH)D was measured in triplicate using an Enzyme-Linked Immunosorbent Assay kit (ELISA, DiaSource, Belgium). Sensitivity of 2 ng/ml with an intra-assay variation coefficient of 5.1% and an inter-assay variation coefficient of 5.9%.
Statistical analysis
Results are presented as percentages (%) and means ± SD. All calculations were performed using the software package SPSS version 15.0.1 (SPSS Inc, Chicago, IL). Shapiro–Wilk normality test was performed to determine the distribution of the variables. Pearson correlation test, factorial ANOVA, one-way ANOVA, and Tukey posthoc analysis were used. In all cases, a value of p < 0.05 was considered significant.