1. Peak 47Ca absorption and 7 day 47Ca retention were measured by a whole-body radioactivity counting technique in 10 haemodialysis patients before and after treatment with 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] and 24,25-dihydroxycholecalciferol [24,25-(OH)2D3].

2. Before treatment all patients had low peak 47Ca absorption and 7 day 47Ca retention.

3. After treatment with 1,25-(OH)2D3 (0.25-1 μg/day for 4–12 months) peak 47Ca absorption and 7 day 47Ca retention returned to normal.

4. After treatment with 24,25-(OH)2D3 (2 μg/day for 4–12 months) peak 47Ca absorption and 7 day 47Ca retention remained at pre-treatment levels.

5. It is concluded that physiological doses of 24,25-(OH)2D3 have no effect on calcium absorption or retention in uraemic man.

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