By Micki Misiaszek, RD, LDN Calcium is a mineral your body needs to build strong bones and teeth. Calcium also plays an important role in blood clotting, muscle contractions, transmission of nerve impulses, and maintaining a normal heart rhythm. Dairy products such as milk, cheese and yogurt, are rich in calcium. Calcium is a good thing, but as we all know, too much of a good thing can be harmful under certain conditions. Calcium and the Kidney Your kidneys play a key role in balancing the amount of calcium and phosphorus in your blood. As kidney function declines, the body is unable to get rid of calcium it absorbs that is not used by the bones. The failing kidney also has difficulty eliminating excess phosphorus from the blood. When calcium and phosphorus are not in balance, serious health problems can result: High phosphorus levels in the blood can cause bones to lose calcium. This makes the bones weak and brittle. The extra phosphorus and calcium join to form hard deposits in the soft tissues of the body, known as calcifications. Calcifications can occur in your blood vessels and block the flow of blood. Calcifications can lead to heart disease, lung disease and painful joints.
Sources of Calcium A person who has chronic kidney disease (CKD) can obtain calcium from several different sources: The average recommended dietary elemental calcium intake for a person following a renal diet, which is limited in sodium, potassium and phosphorus, is approximately 500-800 mg per day. This value will be even higher if calcium-fortified foods or calcium supplements are used. The recommended daily allowance (RDA) for calcium for adults with normal kidney function ranges between 1000-1200 mg per day. When kidney function decreases to a certain level, phosphate binders must be taken with all meals and snacks to control the amount of phosphorus in your blood. Phosphate binders are medications that combine with the food in your intestine, soaking up the phosphorus before it can be absorbed into your blood. The “soaked-up” phosphorus passes out of your body through your bowel movements. There are different types of phosphate binders: Calcium based, such as PhosLo® (calcium acetate) and Tums® (calcium carbonate). Metal based, such as Alternagel®, MagneBind® and Fosrenol®. Calcium free, metal free, such as Renagel®.
Calcium-based binders contain between 169 mg and 500 mg of elemental calcium per tablet. The average dose of calcium-based binders is three to four tablets per meal. If you eat three meals per day and are taking three to four calcium-based binders with each meal, you are getting at least 1500 mg and as much as 6000 mg of calcium each day from your phosphate binder. This is well above the RDA of 1000-1200 mg per day! If you are prescribed a calcium-free phosphate binder, you are not receiving any extra calcium from this medication. Calcium: How Much Is Too Much? The National Kidney Foundation has released clinical practice guidelines to assist doctors in managing calcium and phosphorus metabolism and bone disease in their patients who have CKD. These guidelines were developed by a team of experts in kidney disease and are based on a review of all scientific evidence available. Improved patient survival, decreased morbidity (illness) and a better quality of life for patients with kidney disease are the main objectives of these guidelines. The guidelines recommend the upper limit of elemental calcium intake for the person with CKD be less than 2000 mg per day from all sources (diet, phosphate binders and dialysis). High calcium intakes, in addition to high phosphorus levels in the blood, have been associated with the development of calcifications in the heart, blood vessels, lungs and around the joints. This can lead to impaired circulation, heart failure, shortness of breath and painful joints. Calcifications resemble bone forming in your blood vessels or organs. High calcium intakes in dialysis patients have also been linked to high levels of calcium in the blood and “low turnover” bone disease. Low turnover bone disease means your bones are unable to take in calcium and phosphorus to rebuild new bone. Hypercalcemia (high levels of calcium in the blood) has been associated with increased risk of death for patients on hemodialysis. Calcium-Fortified Foods Many nondairy foods and over-the- counter medications are now being “fortified,” “enriched” or “enhanced” with calcium. This means foods or medicines that normally do not contain calcium are having calcium added to them. For example, fruit juice normally is not a good source of calcium, but now you can buy calcium-enriched juice that provide as much calcium as a glass of milk. Some brands of aspirin and toothpaste are even being fortified with calcium! Cereals, cereal or granola bars, cookies, pancake and muffin mixes, butter spreads, hot chocolate, juices, sodas, cheese, ice cream and bread are just some of the food products that have been fortified or enriched with calcium. Be careful when you go grocery shopping – read the labels! Avoid buying calcium-fortified, calcium-enhanced or calcium-enriched foods. Beware when a food label states a product is a good source of vitamins and minerals. You do not want to increase your daily calcium intake to an unsafe level by eating calcium-fortified foods or taking calcium-fortified medicines. Safe Calcium Intake To keep your daily calcium intake within a safe range: Talk to your dietitian about the amount of calcium you receive from your diet and phosphate binders. Ask if there is a calcium-free phosphate binder appropriate for you. Avoid calcium supplements unless prescribed by your nephrologist (kidney doctor). Check your daily vitamin and mineral supplement or “bone health” supplement. Both may be a source of calcium. Consult with your doctor before taking calcium-based antacids. Do not take Vitamin A or Vitamin D supplements unless prescribed by your nephrologist. Avoid calcium-fortified foods and over-the-counter medications enriched with calcium.
References NKF K/DOQI Clinical Practice Guidelines for Bone Metabolism and Disease in CKD.Am J Kidney Dis. 2003; 42(suppl 3) Goodman, WG, et al. N Engl J Med. 2000:342: 1478-1483 Guerin AP, et al. Nephrol Dial Transplant. 2000;15:1014-1021 Chertow GM, et al, Kidney Int .2002; 62:245-252 Block G, J Am Soc Neph. 2004; 15: 2208-2218
Micki Misiaszek is a clinical consultant for Genzyme Corporation. Previous to her employment with Genzyme, Micki worked as a renal dietitian in the Baltimore area for more than 12 years. The information contained herein is intended for educational purposes only. It is not intended and should not be construed as the delivery of medical care. Persons requiring diagnosis or treatment or with questions specific to a single individual are urged to contact their local health care provider for appropriate care. This article originally appeared in the December 2005 issue of Kidney Beginnings: The Magazine, Vol. 4, No. 5.
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