[Osteomalacia due to vitamin D deficiency. Part two: Aetiology (author's transl)].

The mechanisms of vitamin D deficiency already described are triggered off by a variety of causes. Confinement indoors leads to defective photosynthesis and dietary restrictions to insufficient intake. Malabsorption results from digestive tract diseases: mainly adult coeliac disease, but also sequelae of gastrectomy, exocrine pancreatic insufficiency, chronic biliary obstruction and all other causes of steatorrhoea. Practically, osteomalacia of digestive origin usually results from multifactorial hypovitaminosis D. The same applies to primary or nutritional biliary cirrhosis, which frequently entails low vitamin D blood levels despite subnormal 25-hydroxylation. Osteomalacia is also found in renal osteodystrophy, where it is partly due to inhibition of 1,25-hydroxylase and subsequent deficiency of 1,25-dihydrocholecalciferol, though other, non vitaminic substances may also be involved. Two misleading forms of the disease must be borne in mind: one with renal tubular lsions, the other associated with functional pseudo-hypoparathyroidism. The aetiology of most cases of osteomalacia due to vitamin D deficiency can be elucidated by a few simple tests.