[Vitamin D supplementation: not too much, not too little!]

Vitamin D deficiency is common in the general population and even more frequent in patients with chronic diseases. The prevention of rickets with native vitamin D supplementation is one of the oldest and most effective prophylactic measures ever reported in medicine, leading to an almost complete eradication of vitamin D-deficient rickets in developed countries. We report on two children with vitamin D abnormalities: the first, 10-year-old child developed rickets without any vitamin D supplementation despite different risk factors (autism, ethnicity, nutritional problems, chronic antiepileptic therapies). In contrast, the second, 8-month-old child received double doses of native vitamin D from birth for several months and was referred for acute and symptomatic hypercalcemia. As such, vitamin D supplementation must follow specific rules: neither too much nor too little! We also discuss the emergence of "new" genetic diseases such as mutations in the 24-hydroxylase (CYP24A1) gene inducing neonatal hypercalcemia and nephrocalcinosis: we believe that before prescribing conventional vitamin D supplementation as recommended by the national guidelines, pediatricians should quickly rule out a potential genetic abnormality in phosphate/calcium metabolism (namely a history of lithiasis or hypercalcemia) that would lead to further biological investigations.