Renal nutrition is concerned with the special dietary needs of kidney patients.
According to the National Kidney Foundation, more than 20 million Americans, one in nine adults, have chronic kidney disease, and an additional 20 million others are at increased risk. Kidney disease is a consequence of damaged nephrons, the tiny structures inside the kidneys that function as filters to remove wastes and extra fluids from the blood. It takes a long time to damage the kidney's nephrons, and the process usually occurs gradually over years. The most common causes of kidney disease include:
(Illustration by GGS Information Services/Thomson Gale.)
Kidney disease interferes with the vital function of the kidneys. The kidneys are bean-shaped organs located near the middle of the back, just below the rib cage. Kidneys filter blood, removing waste products and extra water, which become urine. They are very efficient filtering units, processing some 200 quarts of blood and producing about 2 quarts of urine per day in a healthy adult. The wastes in the blood result from the normal breakdown of active muscle and from digestion. After the body extracts nutrients from ingested food, the resulting waste is sent to the blood which is filtered by the kidneys. The kidneys also release three important hormones:
Damaged kidneys do not clean the blood efficiently. Instead, waste products and fluid build up in the blood leading to kidney disease that often cannot be cured. In the early stages of a kidney disease, treatment may be able to make the kidneys last longer. Eventually, kidneys may stop working altogether (kidney failure), and the body fills with extra water and waste products (uremia), which may lead to seizures or coma, and ultimately to death. When kidneys stop working completely, dialysis or a kidney transplantation is required.
Dialysis is an artificial way to filter blood after the kidneys have failed. With hemodialysis, the blood travels through tubes to a dialyzer, a machine that removes wastes and extra fluid. The cleaned blood is then returned to the body. The procedure is usually performed at a dialysis center three times per week for 3-4 hours. In peritoneal dialysis, a fluid (dialysate) is dripped into the abdomen to capture the waste products from the blood. After a few hours, the dialysate is drained out, and a fresh bag of dialysate is dripped into the abdomen. Patients can perform peritoneal dialysis themselves.
Renal nutrition is concerned with ensuring that kidney patients eat the right foods to make dialysis efficient and improve health. Dialysis clinics have dietitians on staff who help patients plan meals. Standard guidelines are: eating more high protein foods, and less high salt, high potassium, and high phosphorus foods. Patients are also advised on safe fluid intake levels. The National Kidney Foundation offers the following dietary advice to adults starting hemodialysis:
Most dairy foods are very high in phosphorus and intake of milk, yogurt, and cheese should be limited to 1/2 cup milk or yogurt or 1 ounce of cheese per day. Dairy foods low in phosphorus include:
All fruits have some potassium. Some fruits however, have more than others. Star fruit (carambola) should be always avoided. Other fruits that should be limited or totally avoided are:
All vegetables contain some potassium, but some have more than others and should be limited or totally avoided. Examples are:
Patients are advised to eat 2-3 servings of the following low-potassium vegetables each day. One serving =1/2 cup.
Since dialysis patients must avoid several types of foods, their diet may be missing important vitamins and mineral micronutrients. Dialysis also removes some vitamins from the body. The treating physician may prescribe a vitamin and mineral supplement designed specifically for kidney failure patients. The physician may also prescribe vitamin C and a group of vitamins called B complex. A calcium tablet may also be given to bind the phosphorous present in food and provide the extra calcium needed by the body. Patients should never take off-the-counter supplements since they may contain vitamins or minerals that may cause harmful interactions.
Kidney patients on dialysis have very special dietary needs that exceed restricting foods, because eating poorly can increase the risk of complications. This is why a dietitian is such a crucial member of the healthcare team. The dietitian will keep track of the fat and muscle stores in a patient's face, hands, arms, shoulders, and legs. The dialysis care team will look for changes in the blood level of proteins, especially the albumin level, as a change in this protein can be indicative of body protein loss. Special blood tests are also done on a monthly basis. They include Kt/V and urea reduction ratio (URR) tests. The tests are used by the care team to evaluate the appropriate course of dialysis required to help patients feel best. A change in any of these tests could mean that a patient is not getting enough dialysis. The tests also provide information about a patient's protein intake and on the protein equivalent of nitrogen appearance (PNA). Using the PNA, the albumin results and any changes in patient appetite, the dietitian can determine if the intake of the right foods is adequate.
Kidney patients are at risk of developing complications such as high blood pressure, anemia (low blood count), weak bones, poor nutritional health and nerve damage. Also, kidney disease increases the risk of heart and blood vessel (cardiovascular) disease.
In patients receiving dialysis, a type of protein called beta-2-microglobulin builds up in the blood. As a result, beta-2-microglobulin molecules tend to join together to form aggregated molecules (amyloids). These aggregates can form deposits and eventually damage the surrounding tissues while causing significant discomfort. This condition is called dialysis–related amyloidosis (DRA). DRA is relatively common in patients, especially older people, who have been on hemodialysis for more than five years. This is because dialysis membranes after being used for several years do not effectively remove the beta-2-microglobulin amyloids from the bloodstream. New hemodialysis membranes, as well as peritoneal dialysis, remove beta-2-microglobulin more effectively, but not enough to keep blood levels normal. As a result, blood levels remain elevated, and deposits formin bone, joints, and tendons.
The two major problems faced by children with kidney failure are poor growth and weight gain, so their diet is usually not restricted unless needed. Children grow fastest during the first two years of life and the earlier the age at which kidney failure occurs, the more likely is growth to be affected. The goals in feeding a child with kidney failure are to balance nutrition for normal growth and protect health as well. The treating physician works with a dietitian to monitor possible problems and suggests, if needed, a diet that will try to take into account the child's food likes and dislikes.
Parents should learn as much as they can about a child's kidney disease and its treatment, encouraging the child to ask questions not only to family members but also to doctors, nurses, and other members of the care team. This also includes explaining the special nutrition restrictions of kidney disease. If explained clearly and simply, even very young children can understand special dietary needs. It is found on the whole that children are in general more compliant with dietary restrictions than adults. One way to help children develop a sense of control over the illness is to have a child make a list of favorite foods and take him or her along to dietitian appointments to see if these foods can be incorporated into the diet plan. Trying to bribe or force a child to eat is ill-advised and counterproductive. Helping a child understand kidney disease, its treatment and the purpose of the special diet is the only way to ensure dietary compliance while maintaining a positive climate of support and encouragement.
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Mitch, W. E., Klahr, S. Handbook of Nutrition and the Kidney. Conshohocken, PA: Lippincott Williams & Wilkins, 2005.
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Pennington, J. A. T., Douglass, J. S. Bowes and Church's Food Values of Portions Commonly Used. Philadelphia, PA: J.P. Lippincott Co., 2004.
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Wiggins, K. L., ed. Guidelines for Nutrition Care of Renal Patients. Chicago, IL: American Dietetic Association, 2002.
American Association of Kidney Patients (AAKP). 3505 E. Frontage Rd., Suite 315, Tampa, FL 33607. 1-800-749-2257. <www.aakp.org>.
American Dietetic Association (ADA). 120 South Riverside Plaza, Suite 2000, Chicago, IL. 60606-6995. 1-800/877-1600. <www.eatright.org>.
American Society for Nutrition (ASN). 9650 Rockville Pike, Bethesda, MD 20814. (301) 634-7050. <www.nutrition.org>.
National Kidney Foundation. 30 East 33rd Street, New York, NY 10016. 1-800-622-9010. <www.kidney.org>.
National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC). 3 Information Way, Bethesda, MD 20892–3580. <kidney.niddk.nih.gov>.
Renal dieticians (RPG). 120 South Riverside Plaza, Suite 2000, Chicago, IL. 60606-6995. 1-800-877-1600 ext. 4815. <www.renalnutrition.org>.
Monique Laberge, Ph.D.
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