Influence of Dietary Calcium:Phosphorus Ratio on Mineral Excretion and Nephrocalcinosis in Female Rats

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Abstract
The effects of dietary calcium (Ca) concentration and calcium:phosphorus (Ca:P) ratio on mineral balance and nephrocalcinosis were studied in female rats. In the first experiment there were two dietary Ca concentrations (0.25 and 0.50%, wt/wt) at two different Ca:P ratios (0.6 and 1.3). In the second experiment the diets were formulated to contain 0.40% P and either 0.13, 0.25, 0.50 or 0.75% Ca. The diets contained 0.03% magnesium (Mg). The fecal outputs of Ca, P and Mg were lower (P < 0.01) after feeding low Ca diets than after feeding high Ca diets. Urinary excretion of P decreased with increasing dietary Ca and increased with increasing P intake. In rats fed the 0.25% Ca diets whole-body retentions of Ca and P were lower than in the rats fed 0.50% Ca. Both increases in dietary Ca from 0.13 to 0.50% and P from 0.20 to 0.40% elevated Ca and P content of kidneys as well as the degree of nephrocalcinosis. However, after feeding the highest Ca concentration (0.75%) nephrocalcinosis was essentially absent while kidney concentrations of Ca and P were relatively low. When compared with 0.50% Ca in the diet, 0.75% Ca increased group mean whole-body retention of Ca but lowered that of P. In individual rats the degree of nephrocalcinosis and the concentrations of minerals in kidney were positively correlated.