The Impact of Phosphate Metabolism on Healthy Aging

Inclusion Criteria: * Children above the age of 13 years * Younger and older adults with XLH and HHRH with confirmed NPT2c mutations affecting both copies of the NPT2c gene (HHRH) or one copy of the PHEX gene (XLH) * Be willing to provide access to prior medical records to determine eligibility including imaging, biochemical, medical, and surgical history data * Be willing and able to complete all aspects of the study * Be willing to adhere to the study visit schedule and comply with the assessments (in the opinion of the investigator). Exclusion Criteria: * Subjects will be excluded, if they are children younger than age 13 years * Subjects that have other diseases likely to impact bone and mineral metabolism (e.g. renal, hepatic, gastrointestinal disorders, and malignancy), * Subjects that are currently pregnant, * Subjects that received medical therapy or developed any condition, which in the opinion of the investigator, could present a concern for either subject safety or difficulty with data interpretation. * Subjects will be excluded from Aim 2, if they are unable to tolerate supplemental phosphate.