To examine the role of vitamin D in human phosphate absorption, we studied patients with chronic renal disease on hemodialysis, before and after correction of vitamin D deficiency. Thirty-centimeter segments of jejunum were perfused with test solutions containing varying concentrations of phosphate; phosphate absorption rate and electrical potential difference were measured. The data reveal that dialysis patients have depressed phosphate absorption, but the degree of this depression is modest, compared to the extent of their depressed calcium absorption. Therapy with 1,25-(OH)2D3 for 1 wk restored phosphate absorption rate to near normal. With or without 1,25-(OH)2D3 therapy, phosphate absorption was not influenced by calcium in the perfused test solutions. Examination of kinetic data suggests that the vitamin D deficiency of chronic renal failure causes a reduction by half in the rate of active phosphate absorption. By contrast, our data suggest that vitamin D deficiency does not alter passive phosphate absorption. By aspirating jejunal contents after ingestion of different foods, with and without aluminum hydroxide, the physiologic luminal phosphate concentration was found to vary between 0.7 and 12.2 mM. At the lower end of this range, phosphate absorption would be mediated entirely by active transport; at the higher phosphate concentrations, phosphate absorption would be mainly mediated by passive transport.
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