5.2. Vitamin D Status
There is currently no definitive consensus regarding the optimal concentration of vitamin D in children and adults. Most authors define vitamin D in normal range from 30 to 100 ng/mL, which might be the optimal range to ensure its immunoregulatory effects; insufficient between 20 and 29 ng/mL, and deficient if serum levels are <20 ng/mL (50 nmol/L), since this level is necessary to maintain optimal bone mineralization and calcium homeostasis in 97.5% of the population [109,110]. Severe vitamin D deficiency is defined as <10 ng/mL; below this cut-off, the risk of developing rickets is very high. Concentrations >100 ng/mL may instead be harmful, although toxicity is more commonly seen over 200 ng/mL.
A 2016 study [111] combined data from 14 European population studies, including children, adolescents, and adults, and found an estimated prevalence of insufficient vitamin D levels of 13% in the general population. The prevalence according to age in pediatric populations varied from 4%–7% (1–6 years), 1%–8% (7–14 years) and 12%–40% (15–18 years). Italian data usually regards smaller populations and reports a high prevalence of vitamin D deficiency and insufficiency. A 2014 study from Stagi and colleagues found a 30% prevalence of vitamin D insufficiency in Italian children and adolescents and a 58.7% prevalence of vitamin D deficiency [112]. In the same year, Vierucci and colleagues reported a prevalence of 32.3% insufficiency and 49.9% deficiency [113]. Cadario et al. [114] described in Italian newborns a high frequency of vitamin D deficiency (40.1%) and severe deficiency (38%).