The previous infusion studies were done only in Caucasian and Asian volunteers as there are extensively documented physiologic differences in bone metabolism between African-Americans and Caucasians. Much of the racial differences noted in bone metabolism come from the osteoporosis literature. African-Americans are known to have higher bone mineral densities (BMD) and to be at lower risk of developing fragility fractures. There are many factors which may explain these racial differences in bone metabolism, including altered calcium economy, vitamin D differences, peak attained bone mass, muscle mass and obesity, mechanism of falls, remodeling rates, bone micro-architecture, hip axis geometry, and other unknown hereditary differences. It is also well established that African-Americans on average, have lower 25-OH vitamin D concentrations and thus higher PTH levels. Despite elevations in PTH, there is paradoxically no increase in bone loss indicating that a relative skeletal resistance to PTH may exist. We hope that by performing this seven-day infusion protocol in healthy African-American volunteers we can learn more about racial differences in bone turnover, renal calcium, PTH concentrations, vitamin D metabolism, and skeletal responses to lactation in this under-studied population.