prevalence of vitamin d deficiency according to climate conditions among a nationally representative sample of iranian adolescents: the caspian-iii study

background: sunlight is the main source of vitamin d; therefore, environmental factors might have an important role in the high prevalence of hypovitaminosis d in children. this study aimed to assess the vitamin d status according to the climate of the living area in a nationally representative sample of iranian adolescents. material and methods: this nationwide cross-sectional survey was performed among a representative sample of 1,095adolescents aged 10-18 years, selected by multistage cluster sampling method from 27 provinces of iran. serum 25-hydroxyvitamin d [25(oh) d] concentrations was compared in inhabitants of humid-rainy, cold-mountainous, and sunny regions. results: vitamin d deficiency was documented in 40% of participants including 40.70% of boys and 39.30% of girls. we found significant difference in 25(oh) d concentrations among participants living in the three different climates of the living area (p<0.05). the median inter-quartile range (iqr) level for 25(oh)d was lower in humid-rainy climate: 11.40 (18.64). hypovitaminosis d was more frequent in humid-rainy climate (42.30%), compared to other climates; this difference was more prominent in urban areas (p<0.05). boys living in various climates had significantly different levels of 25(oh) d(p<0.05), however this figure was not significantly different for girls (p>0.05). the highest frequency of hypovitaminosis d (45.2%) was documented among boys living in humid-rainy regions. conclusion: the high prevalence of hypovitaminosis d, notably among inhabitants of humid-rainy region underscores the necessity of implementing national preventive strategies. this is of great importance especially in regions with lower exposure to sunlight.