**Abstract**

In addition to its contribution to bone metabolism, vitamin D seems to fulfill a broad spectrum of biological functions which justifies the interest in monitoring its body content. The aim of this study is to analyze the prevalence of hypovitaminosis D and associated factors in schoolchildren and adolescents living in a region of northern Spain. A cross-sectional clinical and analytical study (calcium, phosphorus, calcidiol, and parathyroid hormone) was accomplished in a group of 602 Caucasian individuals (aged 3.1–15.4 years). Gender, age, body mass index, residence, and season of the year were recorded, and their association with vitamin D deficiency was analyzed by multiple regression. Vitamin D status was defined according to the US Endocrine Society criteria. The prevalence of hypovitaminosis D was 60.4% (insufficiency: 44.6%; deficiency: 15.8%). The female sex, adolescence, season of blood sample collection (autumn, winter, and spring), an urban residence, and severe obesity showed an association with an increased risk of hypovitaminosis D.

**Keywords:** associated factors, calcidiol, children, deficiency, insufficiency, prevalence, sufficiency, vitamin D

#### **1. Introduction**

Vitamin D (cholecalciferol) is a hormone that is basically synthesized in the skin after exposure to sunlight. This substance undergoes initial hepatic (25-OH-D or calcidiol) and subsequent renal [1, 25-(OH)2-D or calcitriol] hydroxylation before the definitive functional activation. The stimulation of ultraviolet radiation (type B) promotes the endogenous synthesis of vitamin D from epidermal 7-dehydrocholesterol, which is the main source of vitamin D, whereas dietary sources account for less than 10% of the total. The function in bone metabolism and calcium homeostasis has been long considered and known. Vitamin D deficiency causes a reduction in the absorption of dietary calcium and an increase in parathyroid hormone (PTH) secretion with the aim to keep steady levels of serum calcium. A deficiency in vitamin D produces osteoclastic activity and causes loss of bone mineral density [1–6].

Recent studies have shown the presence of vitamin D receptors in the majority of organs (blood vessels, B and T lymphocytes, heart, muscles, skin, brain, mammary gland, colon, prostate, gonads, etc.) and the activity of specific enzymes that are not regulated by PTH and induce calcitriol synthesis. All these findings reveal the biological significance of vitamin D. In fact, several observational studies have suggested that vitamin D deficiency could be related to a higher cardiovascular risk, as well as a risk to present autoimmune, endocrine, infectious, psychiatric, and/or neurological diseases and several types of cancer [1, 4, 7–13]. Additional studies are needed to evaluate the underlying mechanisms.

In other words, in addition to its contribution to bone metabolism, vitamin D seems to fulfill a broad spectrum of biological functions related to cell proliferation, differentiation, and metabolism, which justifies the interest in monitoring its body content. Gender, age, race, season of the year in which serum is collected, sun exposure, and nutritional status have been associated with lower levels of serum calcidiol, but there are disparities among the different authors in the interpretation of these findings [7, 9, 14–20].

The aim of this study is to determine the prevalence of hypovitaminosis D and associated factors among children in northern Spain.
