| Popis: |
Biologic assay of the antiricketic activity in serum and tissues has been used, with concomitant measurements of intestinal net absorption of calcium, to study aspects of vitamin D metabolism in man. Patients with chronic renal failure appear to require a greater intake of the vitamin and a higher serum antiricketic activity, to promote calcium absorption, than does the healthy subject. This increased requirement is not due to intestinal malabsorption of the vitamin nor, apparently, to its metabolic inactivation nor loss in the urine. It could be caused by relative unresponsiveness of the target organ; or, if due to an acquired abnormality of vitamin D metabolism, this is likely to involve some metabolic transformation subsequent to the initial hydroxylation. When the intake of vitamin D is high, the antiricketic activity of serum appears to be determined principally by the amount absorbed from the gut, and its concentration is a power function of the daily oral dose. Under such conditions of high dosage, the vitamin is stored in various tissues, of which voluntary muscle may be quantitatively the most important. The implications of these relationships are discussed. In healthy British adults, the antiricketic activity of serum is low compared with the concentrations measured in adults in the United States. This is probably due to a habitually low dietary intake of the vitamin in Britain and limited solar exposure. Although adequate to prevent development of vitamin D deficiency, this nutritional state may provide insufficient reserve to meet an increased requirement caused by disease. It is postulated that when chronic renal failure increases the requirement for the vitamin, the British patient is likely to have an effective vitamin D deficiency. The American, with a habitual luxus consumption of vitamin D, may have acquired sufficient body stores to meet the needs imposed by renal impairment. Differences in the nutritional state of the different populations may thus explain the frequency of azotemic rickets and osteomalacia in Europe and Asia, as compared with their reputed infrequency in the United States. |
