An important strategy to reducing the risk for osteoporosis and fracture in later life is to optimize the development of peak bone mass during adolescence. Because calcium is the main mineral component of bone, maximizing calcium absorption and retention during adolescence is important to maximize peak bone mass.~Our two research arms address two different mechanisms by which calcium absorption and retention may be increased:~We hypothesize that a smaller particle size calcium carbonate supplement (than that which is now commercially available) may be better absorbed and lead to higher calcium retention in adolescents.~We hypothesize that vitamin D supplementation will increased calcium absorption and retention in adolescents.~Subjects will be assigned to one of the two research arms.~1)Small and large particle size calcium supplements or placebo or 2) calcium supplements with or without vitamin D supplements will be given to subjects during two 3-week study periods in which subjects live on-site.~Calcium absorption will be measured by parathyroid hormone suppression after a calcium load and by stable calcium isotope in some subjects.~Calcium retention will be measured by calcium intake minus calcium excretion.
This study has two research arms:~The purpose of the 1st is to determine if a smaller particle size calcium carbonate supplement (than that which is now commercially available) improves calcium absorption and retention in adolescents girls.~The purpose of the 2nd is to determine if vitamin D supplementation improves calcium absorption and retention in adolescents girls.