The purpose of this study is to gain a better understanding of calcium absorption and metabolism in patients with Chronic Kidney Disease (CKD) using calcium balance and kinetic methods.

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.

Inclusion Criteria:

1. Patients with a GFR of < 45 ml/min;
2. Intact serum PTH > 37 pg/ml;
3. Age > 35 years (both genders and all races);
4. Able to perform two three-week balance studies;
5. Not on oral calcium or vitamin D other than multi vitamin, or willing to stop calcium or vitamin D for one month prior to entry in the study (day 1 of first calcium balance period);
6. Female patients must be post-menopausal (defined as last menstrual period at least 12 months prior to screening visit) or surgically sterile by hysterectomy;
7. On stable doses of diuretics, bisphosphonates, anti-epileptics (except dilantin) for at least 2 months.

Exclusion Criteria:

1. Serious underlying systemic disease (including uncontrolled diabetes, lupus, hypertension, amyloid, etc);
2. Taking drugs that alter calcium and phosphate balance or homeostasis including high dose cholecalciferol or ergocalciferol (1000 U/day or 50,000U/ wk, respectively), active vitamin D metabolites, calcimimetics, PTH analogues in the last 30 days;
3. Taking drugs that the investigator feels will alter calcium balance;
4. Plan to initiate dialysis in the next six months;
5. Hypercalcemia defined as serum calcium > 10.5 mg/dl;
6. Hyperphosphatemia defined as serum phosphate >5.5mg/ml;
7. Intestinal disease that alters absorption or normal intestinal function including celiac disease, small bowel resection, bariatric surgery;
8. Smoking

Due to the characteristic short treatment periods of balance studies, the results may not reflect the effects of chronic treatment with calcium carbonate on mineral balance.