A long-lasting and severe deficiency of vitamin D, as defined by a serum level of 25(OH)D lower than 6 ng/ml, is associated with defective mineralization resulting in rickets in children and osteomalacia in adults. Vitamin D insufficiency, the preclinical phase of vitamin D deficiency, as defined by a serum level of 25(OH)D comprised between 6 and 30 ng/ml, causes a reduced
calcium supply and a secondary hyperparathyroidism. If this state remains chronic, osteopenia results.