Advances in knowledge of vitamin D metabolism in the past decade have provided an expanded base for understanding the pathogenesis of metabolic bone disease in digestive disorders and have suggested new approaches to treatment.1 The purpose of this article is to reemphasize the various digestive diseases that are complicated by bone disease and to discuss the pathogenesis, diagnosis, and therapy of vitamin D deficiency in these conditions.
VITAMIN D ABSORPTION AND METABOLISM
Vitamin D is derived from dietary sources and is produced endogenously in the skin from the action of ultraviolet light on precursor 7-dehydrocholesterol. Vitamin D is absorbed predominantly in the proximal jejunum. Bile salts are required for absorption of this fat-soluble vitamin, and delivery into the circulation is mainly through intestinal lymphatics.2,3 The first step in bioactivation occurs in the liver where vitamin D is converted to 25-hydroxyvitamin D (25OHD3) by hepatic 25-hydroxylase. The major