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What is chronic kidney disease-mineral and bone disorder (CKD-MBD)?
CKD-MBD occurs when the kidneys fail to maintain the proper levels of calcium and phosphorus in the blood, leading to abnormal bone hormone levels. CKD-MBD is a common problem in people with kidney disease and affects almost all patients receiving dialysis.
CKD-MBD is most serious in children because their bones are still growing. The condition slows bone growth and causes deformities. One such deformity occurs when the legs bend inward toward each other or outward away from each other; this deformity is referred to as “renal rickets.” Another serious complication is short stature. Symptoms can be seen in growing children with renal disease even before they start dialysis.
The bone changes from CKD-MBD can begin many years before symptoms appear in adults with kidney disease. For this reason, the disease is known as a “silent crippler.” If CKD-MBD in adults is left untreated, the bones gradually become thin and weak, and a person with CKD-MBD may begin to feel bone and joint pain. CKD-MBD also increases the risk of bone fractures.
Doctors used to use the term renal osteodystrophy to describe the mineral and hormone disturbances caused by kidney disease. Now renal osteodystrophy is used only to describe the bone problems that result from CKD-MBD.
Why are hormones and minerals important?
In healthy adults, bone tissue is continually being remodeled and rebuilt. The kidneys play an important role in maintaining healthy bone mass and structure because one of their jobs is to balance calcium and phosphorus levels in the blood and ensure the vitamin D a person receives from sunlight and food becomes activated.
Calcium is a mineral that builds and strengthens bones. Calcium is found in many foods, particularly milk and other dairy products. If calcium levels in the blood become too low, four small glands in the neck called the parathyroid glands release a hormone called parathyroid hormone (PTH). This hormone draws calcium from the bones to raise blood calcium levels. Too much PTH in the blood will remove too much calcium from the bones; over time, the constant removal of calcium weakens the bones.
Phosphorus, an element found in most foods, also helps regulate calcium levels in the bones. Healthy kidneys remove excess phosphorus from the blood. When the kidneys stop working normally, phosphorus levels in the blood can become too high, leading to lower levels of calcium in the blood and resulting in higher PTH levels and the loss of calcium from the bones. Even before blood levels of phosphorus become elevated, the kidneys are forced to work harder to clear phosphorus from the body.
Healthy kidneys produce calcitriol from vitamin D that is received from sunlight and food. Calcitriol helps the body absorb dietary calcium and phosphorus into the blood and bones. Calcitriol and PTH work together to keep calcium balance normal and bones healthy. If calcitriol levels drop too low, PTH levels increase and calcium is removed from the bones. In a person with kidney failure, the kidneys stop making calcitriol. The body then cannot absorb calcium from food, leading to increased PTH levels. The combination of decreased calcium absorption from food and PTH drawing calcium from bones makes the bones weak and brittle.
How is CKD-MBD diagnosed?
To diagnose CKD-MBD, a doctor may take a blood sample to measure levels of calcium, phosphorus, PTH, and sometimes vitamin D. The doctor may perform a bone biopsy to see if the bone cells are building normal bone. A bone biopsy is done under local anesthesia and involves removing a small sample of bone from the hip and analyzing it with a microscope. Determining the cause of CKD-MBD helps the doctor decide on a course of treatment.
How is CKD-MBD treated?
Controlling PTH levels prevents damage to bones. Usually, overactive parathyroid glands are controllable with a change in diet, dialysis treatment, or medication.
CKD-MBD can be treated with changes in diet. Reducing dietary intake of phosphorus is one of the most important steps in preventing bone disease. Almost all foods contain phosphorus, but it is especially high in milk, cheese, dried beans, peas, nuts, and peanut butter. Drinks such as cocoa, dark sodas, and beer are also high in phosphorus. Often, medications called phosphate binders—such as calcium carbonate (Tums), calcium acetate (PhosLo), sevelamer hydrochloride (Renagel), or lanthanum carbonate (Fosrenol)—are prescribed with meals and snacks to bind phosphorus in the bowel. These medications decrease the absorption of phosphorus into the blood. A renal dietitian can help develop a dietary plan to control phosphorus levels in the blood.
Increasing dialysis dose by increasing a patient’s flow rate or time in treatment can also help control phosphorus.
If the kidneys are not making adequate amounts of calcitriol, a person can take synthetic calcitriol as a pill (Rocaltrol) or in an injectable form (Calijex). Other types of vitamin D that may be prescribed are ergocalciferol (Calciferol, Drisdol), cholecalciferol (Delta D3), doxercalciferol (Hectoral), and paricalcitol (Zemplar). A doctor may prescribe a calcium supplement in addition to calcitriol. The drug cinacalcet hydrochloride (Sensipar), approved by the U.S. Food and Drug Administration in 2004, lowers PTH levels by imitating calcium’s effects on the parathyroid gland. If PTH levels cannot be controlled, the parathyroid glands may need to be removed surgically.
A good treatment program, including proper attention to diet, dialysis, and medications, can improve the body’s ability to repair bones damaged by CKD-MBD. Overall bone health can also be improved by exercising and not smoking. People on dialysis should consult a health care professional before beginning any exercise program.
Points to Remember
- Chronic kidney disease-mineral and bone disorder (CKD-MBD) occurs when the kidneys fail to maintain the proper levels of calcium and phosphorus in the blood.
- CKD-MBD is a common problem in people with kidney disease and affects almost all patients receiving dialysis.
- If calcium levels in the blood become too low, or phosphorus levels too high, four small glands in the neck called the parathyroid glands release a hormone called parathyroid hormone (PTH). This hormone draws calcium and phosphorus from the bones to raise blood calcium levels.
- Healthy kidneys convert vitamin D into calcitriol to help the body absorb dietary calcium and phosphorus into the blood and bones. If calcitriol levels drop too low, PTH levels increase and the bones can become weak and brittle.
- Reducing dietary intake of phosphorus is one of the most important steps in preventing bone disease.
- Medications called phosphate binders are prescribed with meals and snacks to bind phosphorus in the bowel.
- Increasing dialysis dose by increasing a patient’s flow rate or time in treatment can also help control phosphorus.
- If the kidneys are not making adequate amounts of calcitriol, a person can take synthetic calcitriol or other forms of vitamin D as a pill or in an injectable form.
- If PTH levels cannot be controlled, the parathyroid glands may need to be removed surgically.
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About the Kidney Failure Series
You and your doctor will work together to choose a treatment that's best for you. The publications of the NIDDK Kidney Failure Series can help you learn about the specific issues you will face.
Learning as much as you can about your treatment will help make you an important member of your health care team.
This content is provided as a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the National Institutes of Health. The NIDDK translates and disseminates research findings through its clearinghouses and education programs to increase knowledge and understanding about health and disease among patients, health professionals, and the public. Content produced by the NIDDK is carefully reviewed by NIDDK scientists and other experts.
The NIDDK would like to thank:
Mary Leonard, M.D., University of Pennsylvania Health System; Sharon Moe, M.D., Indiana University School of Medicine
This information is not copyrighted. The NIDDK encourages people to share this content freely.