s Description): 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 adolescent girls at a mean dietary calcium intake of 1300 mg/day. 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 of this proposal is 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 will be 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 hypothesize 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 will be 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. These studies are expected to define optimal calcium and sodium intakes for optimizing peak bone mass in both white and black adolescents. Furthermore, they should provide an explanation for the paradox that blacks can build higher bone density, have a lower rate of bone turnover, and excrete less urinary calcium on the same sodium loads compared to whites.
