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Low Dietary Calcium Reduces 25-Hydroxycholecalciferol in Plasma of Rats¹

Reinhold Vieth^*,2, Donald Fraser and Sang Whay Kooh

^* Department of Clinical Biochemistry Department of Paediatrics and Physiology, University of Toronto Department of Research Institute, Hospital for Sick Children ^* Department of Research Institute of the Queen Elizabeth Hospital, Toronto, Ontario, Canada M5G 2A2

We investigated whether dietary factors that are known to increase 1,25-(OH)₂-cholecalciferol production can deplete plasma 25-OH-cholecalciferol. Plasma concentration of 25-OH-cholecalciferol, its metabolism in vivo and activities of renal mitochondrial 25-OH-cholecalciferol 1-hydroxylase (1-OHase) and 25-OH-cholecalciferol 24-hydroxylase (24-OHase) were measured in rats fed various amounts of calcium (Ca) and phosphorus (P). All diets contained 5 µg (200 KI) cholecalciferol per 100 g. For rats fed the "normal" diet (0.7% Ca and 1.2% P) the mean plasma 25-OH-cholecalciferol level was 11.0 ± 0.8 nmol/L, and the mean 1-OHase activity was 30 ± 5 fmol/(mg·min). All rats fed the low Ca (0.014%) diet had 1-OHase activities above 200 fmol/(mg·min) and undetectable plasma 25-OH-cholecalciferol levels (<2.5 nmol/L). The chi-square test interrelating plasma 25-OH-cholecalciferol and dietary Ca showed statistical significance (P < 0.001). The high activity of 1-OHase that resulted from dietary Ca restriction increased utilization of 25-OH-cholecalciferol to the point of causing depletion of this metabolite in the circulation.

KEY WORDS: • dietary calcium • vitamin D requirement • 25-OH-cholecalciferol 1-hydroxylase

¹ This work was supported by the Medical Research Council of Canada.

² To whom correspondence should be addressed.

Manuscript received 6 November 1985. Revision accepted 5 January 1987.

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