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Effects of Sodium Bicarbonate and 1,25-Dihydroxy-cholecalciferol on Calcium and Phosphorus Balances in the Rat¹

Department of Medicine, University of Otago Medical School, Dunedin, New Zealand

Metabolic balance studies were undertaken to determine whether sodium bicarbonate (NaHCO₃) supplements (4.5 mmol/day) altered 7-day cumulative calcium (Ca) phosphorus (P) balances in growing rats consuming either a basal diet providing 0.6% Ca and 0.3% P, or this diet plus 1,25-dihydroxycholecalciferol [40 ng 1,25(OH)₂D₃/day]. Feeding bicarbonate lowered urinary Ca but raised fecal Ca so that Ca balance became less positive. However, 1,25(OH)₂D₃ increased net absorption of Ca and P to the same degree when given to control rats and rats consuming bicarbonate. Nevertheless, bicarbonate-fed rats had lower net Ca absorption than controls, even when treated with high doses of 1,25(OH)₂D₃. Changes in net Ca absorption induced by bicarbonate may occur at a point in the gut distal to the duodenum since duodenal ⁴⁵Ca absorption was not decreased by bicarbonate feeding. The present results show that bicarbonate consumption depressed net Ca absorption in the rat. The effect appears to be independent of changes in 1,25(OH)₂D₃ metabolism because it is manifest in animals receiving high doses of 1,25(OH)₂D₃, which stimulate alimentary Ca absorption maximally, and because bicarbonate-fed rats are able to respond normally to exogenous 1,25(OH)₂D₃ by increasing their net absorption of Ca and P. In view of this demonstration that NaHCO₃ supplements elevate fecal Ca loss in the rat, it is suggested that studies should be undertaken to determine whether bicarbonate exerts similar adverse effects on Ca balance in humans.

KEY WORDS: • sodium bicarbonate • calcium balance • hypervitaminosis D • antacids

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

Manuscript received 23 September 1983.