Effect of Sodium Intake on Calcium Retention in Black and White Adolescent Girls.

Optimal calcium retention is a prerequisite for building maximal peak bone mass within the genetic potential, a key to reducing risk of osteoporosis later in life. The investigators have determined that maximal calcium retention averages 423 mg/day during the period of rapid skeletal accretion in white girls at a mean dietary calcium intake of 1300 mg/d. Urinary calcium explains more than 50% of the variance in calcium retention. However, urinary sodium (i.e. sodium intake)is a major determinant of urinary calcium excretion and the effect of sodium intake on maximal calcium retention is not known. Nor is its effect known in black adolescents who have higher bone density and lower calcium excretion than white adolescents.

The primary aim was to test the hypothesis that high dietary sodium increases the calcium intakes required for optimal calcium retention in both black and white adolescent girls. Calcium retention was measured at two levels of dietary sodium in a randomized crossover design on one of two levels of dietary calcium intake in black and white adolescent girls during three week metabolic periods. The investigators hypothesized that the mechanisms which regulate sodium reabsorption in the renal tubules also regulate calcium retention. Increased incidence of hypertension in blacks compared to whites has been attributed to increased sodium retention. Sodium intake induced changes in calcium and sodium retention in both races were related to changes in sodium handling (plasma renin activity, serum aldosterone, and salt sensitivity) and calcium regulating hormones, biomarkers of bone turnover and bone mass.

The subjects were resident in a Purdue fraternity house, which was transformed during the summer into a metabolic unit. Subjects were supervised at all times by trained staff. The balance study was divided into 2 sessions of 3 weeks each during the summer of 1999 and 2000, with 2 levels of dietary Na+ during each summer. During the summer of 1999 subjects consumed a low calcium diet while in the summer of 2000 subjects consumed a high calcium diet. The Na+ intake periods were separated by a 2-week period, in which subjects were free to consume self-selected diets. Subjects collected fecal and urine daily for 20 days. Other measurements included daily body weight, blood pressure every other day, blood sample at the end of each session. Baseline measures included bone mass, self-assessment of pubertal development, a physical examination and diet history.