The Role of Sulfate in the Calciuria of High Protein Diets in Adult Rats

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Abstract
Diets containing different sources of protein were evaluated for their effect on urinary calcium (Ca) excretion in adult male rats. High protein diets were prepared by adding 24 g N/kg as lactalbumin (Hi-Lact), egg white (Hi-EW), casein (Hi-Cas) or gelatin (Hi-Gel) to a control diet containing 24 g N/kg as casein (Cas). There were significant differences in the degree of hypercalciuria produced by different dietary protein mixtures. A peak in Ca excretion occurred at about 2 days, when the relative values were as follows (Cas = 100): Hi-Lact, 489; Hi-EW, 429; Hi-Cas, 340, and Hi-Gel, 263. Subsequently urinary Ca declined, but a moderate hypercalciuria persisted to the end of the 8-week experiment, when the corresponding values (% Cas) were 200, 183, 160 and 136, respectively. The degree of hypercalciuria was proportional to the sulfur content of the diets, i.e., Hi-Lact > Hi-EW > Hi-Cas > Hi-Gel > Cas. Supplementing the Cas diet with sulfur amino acids to the level present in the Hi-Lact diet resulted in a comparable increase in Ca excretion. Supplementing the Cas diet with 1.42% sulfate produced a 570% increase in urinary Ca on day 2 and a 306% increase at 5 weeks. There was a linear relationship between Ca excretion and sulfate excretion. It is proposed that a major factor in the hypercalciuria of high protein feeding is the production and excretion of sulfate. Variability in the calciuric effect of different proteins appears to be related mainly to differences in their sulfur amino acid content.