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Evolution of the Calcium Paradigm: The Relation between Vitamin D, Serum Calcium and Calcium Absorption
Endocrine & Metabolic Unit, Royal Adelaide Hospital, Adelaide 5000, South Australia, Australia
Discipline of Medicine, University of Adelaide, Adelaide 5000, South Australia, Australia
Received: 3 August 2010; in revised form: 18 August 2010 / Accepted: 24 August 2010 / Published: 16 September 2010
Abstract: Osteoporosis is the index disease for calcium deficiency, just as rickets/osteomalacia is the index disease for vitamin D deficiency, but there is considerable overlap between them. The common explanation for this overlap is that hypovitaminosis D causes malabsorption of calcium which then causes secondary hyperparathyroidism and is effectively the same thing as calcium deficiency. This paradigm is incorrect. Hypovitaminosis D causes secondary hyperparathyroidism at serum calcidiol levels lower than 60 nmol/L long before it causes malabsorption of calcium because serum calcitriol (which controls calcium absorption) is maintained until serum calcidiol falls below 20 nmol/L. This secondary hyperparathyroidism, probably due to loss of a “calcaemic” action of vitamin D on bone first described in 1957, destroys bone and explains why vitamin D insufficiency is a risk factor for osteoporosis. Vitamin D thus plays a central role in the maintenance of the serum (ionised) calcium, which is more important to the organism than the preservation of the skeleton. Bone is sacrificed when absorbed dietary calcium does not match excretion through the skin, kidneys and bowel which is why calcium deficiency causes osteoporosis in experimental animals and, by implication, in humans.
Keywords: calcium; vitamin D; parathyroid hormone; osteoporosis; osteomalacia
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Nordin, B.E.C. Evolution of the Calcium Paradigm: The Relation between Vitamin D, Serum Calcium and Calcium Absorption. Nutrients 2010, 2, 997-1004.
Nordin BEC. Evolution of the Calcium Paradigm: The Relation between Vitamin D, Serum Calcium and Calcium Absorption. Nutrients. 2010; 2(9):997-1004.
Nordin, Borje E. Christopher. 2010. "Evolution of the Calcium Paradigm: The Relation between Vitamin D, Serum Calcium and Calcium Absorption." Nutrients 2, no. 9: 997-1004.