Calciferols vitamin hypervitaminosis

The effects of the steroid hormone calcitriol (see p. 330) in bone are complex. On the one hand, it promotes bone formation by stimulating osteoblast differentiation (top). This is particularly important in small children, in whom calcitriol deficiency can lead to mineralization disturbances (rickets see p.364). On the other hand, calcitriol increases blood Ca "" levels through increased Ca "" mobilization from bone. An overdose of vitamin D (chole-calciferol), the precursor of calcitriol, can therefore have unfavorable effects on the skeleton similar to those of vitamin deficiency (hypervitaminosis see p.364). 

There are few dietary sources of cholecalciferol. The richest sources are oUy fish (especially fish liver oUs), although eggs also contain a relatively large amount, and there is a modest amount in mUk fat and animal liver. In many countries, margarine is fortified with vitamin D. No common plant foods contain vitamin D, although some tropical plants contain calciferol glucuronides that are hydrolyzed in the intestinal lumen and are a source of the vitamin. Indeed, this can be a cause of hypervitaminosis and calcinosis in grazing animals. 

Toxicity. Most cases of hypervitaminosis D found in the literature are the consequences of overdosing with either chole-calciferol or ergocalciferol. Toxic concentrations of vitamin D... 

Another substance, chemically related to vitamin D2 (calciferol), has an effect similar to that of parathormone. This is dihydrotochysterol (AT 10). The effect is, however, in some aspects more comparable to D-hypervitaminosis. Dihydrotachysterol is used frequently in tetany caused by a deficiency of the parathyroid hormone, because the peptide hormone is not readily available. 

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