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Robert P. Heaney, Karen Rafferty, K. Michael Davies, Letter re: Long-Term Persistence of the Urine Calcium-Lowering Effect of Potassium Bicarbonate in Postmenopausal Women, The Journal of Clinical Endocrinology & Metabolism, Volume 90, Issue 7, 1 July 2005, Page 4417, https://doi.org/10.1210/jc.2005-0662
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To the editor:
Frassetto et al. (1) provide evidence that the acute reduction of calciuria produced by potassium bicarbonate (which they had earlier described) endures for as long as the potassium bicarbonate intake continues. My colleagues and I are able to confirm that this is so. Using a different study design, i.e. 190 peri- and postmenopausal women studied on diets matching their usual nutrient intakes, we have recently reported that urine calcium is inversely related to potassium intake (2). The slope of the relationship in our women (0.011 mmol urine calcium/mmol diet potassium) was slightly greater than, although in the same range as, the relationship presented in Fig. 1 of the report by Frassetto et al. (1).
Frassetto et al. (1) recognize that the potassium-associated reduction in urine calcium spillage could produce an important calcium advantage for women with high potassium intakes, potentially resulting in total skeletal bone gain of as much as 1.7% per year. However, as they also note, such gain assumes no offsetting adjustments of other calcium fluxes. They acknowledge that a reduction in intestinal calcium absorption would counteract to some extent the beneficial effect of potassium intake on urinary calcium loss, but they are able to provide no data on absorption. Our study, however, did specifically evaluate calcium absorption, using the gold standard, double-tracer method. In brief, we found that women with high potassium intakes not only had the reduced calciuria reported by Frassetto et al. (1) but also had reduced calcium absorption, and that the two effects were of approximately equal magnitude. Effectively, what was gained at the kidney was lost at the gut. This disappointing finding could perhaps have been anticipated in that Sakhaee et al. (3) had previously shown that a thiazide-produced reduction in calciuria was also offset by a corresponding reduction in calcium absorption efficiency.
