SALT INTAKE, URINARY SODIUM, AND HYPERCALCIURIA

Several studies have reported that high sodium (Na) intake increases n ot only urinary Na but also urinary calcium (Ca), suggesting that high Na intake could be involved in the pathogenesis of hypercalciuria. No research data are available on the relationship of Na intake to the p revalence of hypercalciuria within the general population. Moreover, i t is not clear if Na intake relates only to urinary Ca or also to othe r indices of Ca homeostasis, including intestinal Ca absorption. In th e present paper, two distinct studies addressed these points using 24- hour urinary Na as an index of salt intake in individuals on their hab itual unrestricted free diet. Study 1 analyzed the relationship betwee n 24-hour urinary Na and hypercalciuria (24-hour urinary Ca greater th an or equal to 7.5 mmol in men, greater than or equal to 6.25 mmol in women) in a population sample of 203 men and women, aged 20-59 years. Study 2 analyzed the relationship between 24-hour urinary Na and intes tinal strontium (Sr) absorption, used as an index of intestinal Ca abs orption, urinary (24-hour and fasting) and plasma Ca, and plasma parat hyroid hormone in 36 healthy men and women, aged 18-65 years. Within t he population sample (study 1), 24-hour urinary Na was directly and si gnificantly correlated with prevalence of hypercalciuria when controll ing for gender, age, weight, and urinary creatinine: the relationship was continuous and linear for urinary Na ranging between 40 and 200 mm ol/24 h. In the 36 volunteers (study 2), 24-hour urinary Na was relate d to 24-hour and fasting urinary Ca (p < 0.001) but not to intestinal Sr absorption: the relationship between 24-hour urinary Na and urinary Ca (both 24 h and fasting) was also significant, controlling for othe r variables. The results indicate that in adults on their habitual die t, urinary Na, which reflects dietary salt intake, correlates with the prevalence of hypercalciuria independently of intestinal Ca absorptio n and mainly via renal mechanisms.