Sulfur-containing amino acids are a major determinant of urinary calcium

Epidemiological evidence suggests a relation between dietary protein intake and nephrolithiasis. In addition, generation of acid equivalents from dietary protein increases urinary calcium excretion. To further evaluate the usefulness of epidemiological or clinical tools to study the relation between dietary protein and urinary calcium, the correlation between urinary urea and sulfate excretion on the one hand and calcium excretion on the other hand was examined in 42 healthy individuals. Only a modest correlation was found between urea and calcium excretion (r = 0.33); in contrast, the correlation between sulfate and calcium excretion was marked (r = 0.73) and highly significant (p less than 0.01). It remained significant when the influences of urea, sodium, hydroxyproline, and oxalate excretion were taken into account using partial regression analysis. In 9 healthy male probands, addition of 6 g L-methionine (40 mmol) to their usual diet caused an increase of urinary sulfate (+ 3,795 mg/24 h) and urinary calcium (+ 86.4 mg/24 h). The variable methionine/cysteine-cystine content of dietary proteins may explain why urinary calcium is correlated better with urinary sulfate than urinary urea.