By Matt Morrison, PharmD, & Ajit Kamdar, PharmD

What’s the deal with phosphate in kidney disease anyway?

Phosphate and calcium are both minerals that are essential in maintaining a healthy body. Together, they help build strong, healthy bones and teeth, and also play a role in regular cell and nerve function. Through a delicate process called homeostasis, the body keeps both phosphate and calcium at healthy levels. Both the kidneys and the parathyroid glands play vital roles. First, the kidneys, which activate vitamin D, control the amount of phosphate that is absorbed and excreted from the foods you eat. Second, a hormone called parathyroid hormone (PTH) regulates phosphate by increasing or decreasing the blood’s phosphate concentration. A normal blood phosphate level in adults is 3-4.5 mg/dL and a normal PTH is 10-55 pg/mL.

However, in kidney disease and kidney failure, the body is not able to keep the levels of phosphate at healthy levels. When the kidneys start to fail, they cannot excrete excess phosphate from your body. In addition, excess PTH production, which occurs in kidney failure, prevents the body from maintaining healthy levels of phosphate.

Why is it important to keep phosphate levels under control?

Excess phosphate in your body can lead to blood vessel disease and bone damage. When the phosphate level in the blood is high, phosphate binds with calcium and clumps in blood vessels. The calcium-phosphate can stick to the lining of your blood vessels and deposit into your joints. Because calcium is bound to phosphate in clumps, the blood level of calcium goes down. This stimulates the parathyroid gland to produce more PTH, which releases calcium and phosphate from bones into the blood. Your blood vessels can become completely blocked due to the clumping of phosphate and calcium in the bloodstream. Moreover, your bones can become weak as PTH continuously releases calcium and phosphate from your bones. The medical term for this kind of bone weakness is “renal osteodystrophy.”

How do I control my phosphate levels?

There are three ways of decreasing phosphate levels: diet, drugs and dialysis.

Diet:

Avoiding foods high in phosphate can help lower phosphate levels. This can be challenging but is very important. Many of the common foods we eat are high in phosphate and it is very difficult to avoid some of these foods. Since these foods cannot easily be avoided, portion control is important. Speaking to a renal dietitian and learning how to read nutrition labels can help you control your phosphate levels.

Drugs (Phosphate binders):

There are various phosphate binders available in the United States: calcium carbonate (OScal®), calcium acetate (PhosLo®), sevelamer (Renvela®), lanthanum (Fosrenol®) and aluminum hydroxide (Amphojel®). They work by binding to phosphate in the gastrointestinal tract and forming a compound that cannot be absorbed into the blood and instead is then excreted in the feces. These medicines are most often used in patients who have end-stage renal disease (ESRD) or are on dialysis. However, they are sometimes used in patients with stages 3 and 4 chronic kidney disease (CKD).

Timing is important when taking phosphate binders. Phosphate binders must be taken with meals, otherwise they do not work. This is because they only bind phosphate in food that you consume. If a dose is not taken within 30-minutes from when you last ate, it should be skipped and the following dose taken as scheduled with the next meal. Also, do not double up on the medicine if a dose is missed because this will increase the risk of side effects.

Dialysis:

Patients who progress to end-stage renal disease (ESRD) usually require dialysis. Dialysis does not remove phosphate from the blood as effectively as it removes other substances. Regular dialysis treatments helps remove just a little of the excess phosphate from your body. Thus, dialysis alone is not enough to control phosphate levels in your body. Diet and medications are also necessary.

What else can I do to get the most out of my treatment?

Here are several tips that will help you achieve the best outcome with phosphate binders. First, always take your medicine on time and everyday as prescribed. Second, you should receive regular lab testing to monitor your calcium, phosphorus and PTH levels. Finally, if you have problems with your medicines or are unsure if you are taking the medicine correctly, talk to your pharmacist for tips and advice.

Ajit Kamdar, PharmD, is a graduate of Midwestern University College of Pharmacy - Glendale and currently works as a pharmacist at Express Scripts.

Steven Morrison, PharmD, is a graduate of Midwestern University College of Pharmacy - Glendale and currently works for Walgreens Pharmacy in Glendale, AZ.

The authors wish to acknowledge the editorial assistance of Abeba Berhane. 

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

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