By Daniel W. Coyne, MD

As we age, the risk of broken bones and fractures increases dramatically. Regardless of age, people on dialysis or who have received a kidney transplant have a higher chance for fractures. Several factors affect your likelihood of avoiding fractures: the density of bones, structurally strong bones, strong muscles and your balance.

The health of bones is complex. Healthy bones need to be dense, which can be measured by a test called a DEXA scan. The DEXA scan assesses bone mineral density (BMD). The density of bones is made up of calcium and phosphorus deposited as crystals onto a protein matrix. But density alone is not enough to avoid fractures. Fluoride has been tied to increase bone strength. It dramatically increased bone density, but led to brittle weaker bones.

The best dense bones have natural calcium and phosphorus deposited into a protein matrix, or scaffold. For this to occur, you need a hormone called PTH (parathyroid hormone) to be neither too low nor too high in your blood. You also need adequate vitamin D, and its active product calcitriol or a similar active form of vitamin D. With chronic kidney disease (CKD), the PTH can get too high, partly due to low vitamin D and calcitriol levels. High phosphorus in your blood can also make the PTH too high.

While good architecture and strong bones are important for avoiding fractures, a third component may be less obvious – the strength of your muscles and good balance. Regular exercise can build and maintain muscle strength and help reduce your chance of falls and fractures. The stress and strain of exercise actually stimulates bones to get stronger.

If you have a tendency to fall due to an injury, weakness or other malady, use of a cane or walker can lower your chances of a serious fall. A physical therapist can recommend simple exercises to improve strength, and decide if you need a cane or walker.

Phosphorus and Calcium
Phosphorus is an element crucial to building healthy bones. We get phosphorus in many foods we eat, though some foods have very high phosphorus content. Our kidneys keep the phosphorus in a normal range by dumping extra phosphorus in the urine. When kidneys are damaged, the phosphorus can get too high in the blood, which makes the PTH too high, and can weaken your bones. You can help keep your phosphorus normal by following a diet low in phosphorus, and taking binders with meals. Binders literally bind some of the phosphorus in your meals so you absorb less. They should be taken with your meals.

There are several types of binders. The least expensive are calcium carbonate. Also relatively inexpensive is calcium acetate, a prescription medication. Both of these binders also provide some calcium. For many people with kidney disease, the calcium is fine. For others, it may increase the calcium in your blood too high. If your blood calcium gets too high, your doctor may need to decrease or stop calcium binders. Non-calcium binders are mainly sevelamer and lanthanum carbonate. These prescription medications are more expensive. They work the same way as the other binders, but don’t give you calcium. This may be better for patients, but much more work needs to be done to prove if this is true. The most important thing for your health is not the type of binder you take, but controlling your blood phosphorus by adjusting your diet, taking your binders and getting regular blood tests. By keeping phosphorus in the normal range, you can help keep PTH normal and your bones stronger.

Vitamin D
Vitamin D is made when ultraviolet light strikes the skin. For many reasons – like wearing clothes! – at least half of all people have low vitamin D levels. Inexpensive vitamin D supplements bought over the counter or prescription supplements can be taken as pills to keep your vitamin D stores normal. There appear to be many benefits and no clear harm from taking small doses of vitamin D, such as vitamin D3 (cholecalciferol) 1000 or 2000 IU per day, or vitamin D2 (ergocalciferol) 50,000 IU capsules once or twice per month.

This may not only help your bones, but also improve your muscle strength and balance. Studies in the frail elderly have shown treatment with vitamin D increases strength and balance, and can decrease the risk of falls. Low vitamin D levels have also been associated with the risk of developing certain cancers, developing hypertension and having poorer heart function. We don’t know if the low vitamin D levels caused these other problems, but having a normal vitamin D level appears safe.

If you have kidney disease, ask your doctor to check your vitamin D levels and recommend a supplement if they are low. He or she can recommend a safe dose for you if you have low vitamin D levels.

Calcitriol, the Active Vitamin D
Your kidneys should make calcitriol from vitamin D, but due to damage, they make less and less. Calcitriol, also called active vitamin D, helps keep your PTH from getting too high and helps you absorb calcium and phosphorus. Your doctor can give you small doses of calcitriol to help push down PTH. Too much calcitriol can cause problems, such as too much calcium or phosphorus in your blood. To avoid this, your doctor will need to monitor your blood tests regularly. He or she can also use a drug similar to calcitriol which is less likely to cause problems with high calcium and phosphorus, such as paricalcitol or doxercalciferol. These drugs seem to suppress PTH better than calcitriol and cause less of these side effects.

Controlling PTH
Your doctor may prescribe calcitriol, paricalcitol or doxercalciferol to lower your PTH toward normal. These drugs may even make you live longer. Studies suggest people with kidney disease who get one of these drugs live longer than similar patients not given these medications. Further study is needed, but if your doctor determines you should get these medications, make sure to take them regularly and have routine blood work testing. In patients on dialysis, cinacalcet is sometimes given to lower the PTH. This pill should be taken every day, and works in a different fashion than the active vitamin D medications. Side effects of cinacalcet include nausea and vomiting, and low blood calcium. It is not unusual to use cinacalcet with calcitriol or one of the other active forms of vitamin D.

Summary
Maintaining healthy bones in people with kidney disease can be a challenge. Even without kidney disease, we are more prone to weak bones as we age. The best treatments to date appear to be keeping your vitamin D level, blood calcium and phosphorus in the normal range, and keep your PTH under control with various prescription medications when required. Regular exercise can help keep your muscles stronger and actually stimulate your bones to stay healthy.

Daniel W Coyne, MD, is Professor of Medicine in the Department of Internal Medicine, Renal Division at the Washington University School of Medicine, St Louis, Missouri. He also serves as the Director of Hemodialysis at the Chromalloy American Kidney Center.

This article originally appeared in the May 2008 issue of Kidney Beginnings: The Magazine.

Back