The foundation of this diet is a phe-free medical product/formula made from free amino acids. Based on longitudinal studies, it has been reported that the most benefit is attained by individuals who maintain a phe-restricted diet throughout life. Despite the obvious benefits of the diet, it has been suggested that the dietary restrictions may be associated with poor bone health in these patients. However, data supporting this has been reported in studies with small sample sizes and/or inadequate sample populations that include children. There is a paucity of data on bone health in adults with HPA.