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Having healthy bones is essential to maintaining body structure and mobility. The human skeleton supports body weight and protects the brain and other organs. The skeleton also stores two important minerals—calcium and phosphorus.
In people with bone disease related to kidney failure, bone cells called osteoclasts and osteoblasts are often not in balance. This condition is called renal osteodystrophy. The way these bone cells get out of balance is when calcium, parathyroid hormone (PTH), phosphorus and activated vitamin D are out of balance.
Over time, renal osteodystrophy can cause bones to break easily, harden the soft tissues of the body including the heart and may even lead to a higher death rate in people with end stage renal disease (ESRD).
Calcium is essential for building new bone cells and keeping existing bones strong. Milk, yogurt, cheese and canned fish with bones are rich with calcium, but also provide too much phosphorus for a kidney diet. Sometimes calcium pills are prescribed to provide extra calcium instead of these high-phosphorus foods for people with chronic kidney disease (CKD). If calcium levels in your blood becomes too low due to kidney failure, your parathyroid glands (four small glands in your neck) release a hormone called PTH. If blood calcium levels are too low, the PTH will begin removing calcium from your bones to get calcium blood levels back to normal. Over months and years, as calcium is stripped from the bones, this can make the bones weak, so their texture becomes more like a piece of chalk than of a sturdy bone.
After calcium, phosphorus is the second most common mineral in the body. About 85 percent of phosphorus in your body is stored in your bones and teeth. Phosphorus is found in milk and milk products, whole grains, dried beans and peas, nuts and seeds, organ meats, meat and fish, colas, chocolate and some types of baking powder. In addition, many processed foods contain phosphate additives.
A high blood phosphorus level causes your body to pull calcium from your bones in an attempt to balance all the body’s minerals. When calcium is being pulled from your bones, your bones begin to break down and lose the ability to provide structural support.
Healthy kidneys activate vitamin D. The activated form of vitamin D is called calcitriol. Calcitriol helps the body absorb calcium. Working together, calcitriol helps maintain normal PTH levels, and they carefully balance calcium in your system. When kidneys fail, they stop converting inactive vitamin D to calcitriol. Your body is unable to absorb calcium from food, so it “borrows” the calcium it needs from the greatest calcium storage depot—your bones.
Renal osteodystrophy is often called the “silent crippler” because symptoms do not occur until a patient has been on dialysis for a few years. Typical symptoms can be:
Early indicators of renal osteodystrophy include high phosphorus and/or high PTH levels, red eyes, itching and sores from calcium-phosphorus deposits.
Children with kidney disease can be especially affected by renal osteodystrophy because their bones are still growing.
Testing for renal osteodystrophy involves taking a blood sample to measure your levels of calcium, phosphorus and PTH. If you are on dialysis, calcium and phosphorus tests are done on a monthly basis (or sometimes more frequently). PTH is measured quarterly for most patients—although testing may be done more frequently for people just starting vitamin D therapy or those with severe bone disease while the healthcare team is determining the correct dosage of vitamin D. Your renal dietitian will review your lab results and recommend changes in your diet or change your phosphorus binder prescription.
The goal of treating renal osteodystrophy is to restore balance between calcium, PTH, phosphorus and vitamin D in the body. Renal osteodystrophy can be managed with phosphorus binders, activated vitamin D and a low-phosphorus diet. If you have a high level of PTH in your blood, it’s important to bring it back to a normal level to prevent calcium loss from your bones. Medicines to treat renal osteodystrophy can be given in pill form, but are usually given intravenously during dialysis treatments for people on in-center hemodialysis. These medicines are only used in people with kidney disease. Another drug that may be used acts directly on the parathyroid glands to block PTH release. In severe cases the parathyroid glands may be surgically removed.
In addition to a low-phosphorus diet and taking prescribed medicines, exercise can also help increase your bone strength. Check with your doctor before beginning an exercise program.
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