Calcium and Phosphorus Balance and Calcium Kinetics in Patients With Stage 3/4 Chronic Kidney Disease

The purpose of this study is to gain a better understanding of calcium absorption and metabolism in patients with Chronic Kidney Disease (CKD). It is important that the body get enough calcium to support many important body functions including bone health. CKD changes the calcium balance or how calcium is absorbed and excreted. Because of this, the knowledge of calcium absorption and excretion in patients with normal kidney function cannot be used to assess patients with CKD. In patients with CKD bone heath is often negatively affected due to a combination of poor calcium absorption, increased bone turnover (process where old bone is removed and new bone is formed), increased level of parathyroid hormone (PTH \[ a hormone that acts to increase calcium in the blood\]) and decrease in vitamin D levels. This negative effect is referred to as Chronic Kidney Disease Mineral Bone Disorder (CKD-MBD). Treatment to correct CKD-MBD should begin early in the course of CKD. In the normal population calcium supplements are frequently used to help prevent age related bone loss. Calcium supplements can also be used in CKD patients to help bind phosphate. Maintaining correct levels of phosphate in the body is crucial in CKD. However, calcium supplements may have adverse effects by promoting calcium phosphate deposits in soft tissues like the vascular system which could increase the risk of cardiovascular disease. Therefore this formal balance study is needed to determine if positive calcium balance occurs in patients with advanced CKD who are given calcium with meals as a phosphate binder. This study will also evaluate how the body handles phosphate.