Renal nutrition refers to the special dietary needs of people with kidney disease. The types of foods prescribed depend on the level of kidney failure the patient is experiencing, but generally the diet involves controlling sodium, potassium, phosphorus, protien, and fluid intake.
Renal nutrition is necessary to help limit the symptoms of kidney disease and slow the development of kidney failure. For people with kidney disease, it is important to maintain a proper balance of electrolytes, minerals, and fluid in the bloodstream, and for patients undergoing dialysis treatments, this becomes even more important. When a person's kidneys are not functioning properly, these substances can build up in the body and cause symptoms such as nausea, vomiting, tiredness, weakness, sleepiness, and other problems. Symptoms can be controlled by limiting certain foods and substances, but if limited too severely, patients can suffer from malnutrition. Renal nutrition is concerned with helping kidney patients eat the right foods to maintain their health in the short and long term.
According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), more than 20 million people in the United States, or at least 10% of the adult population, have chronic kidney disease. Kidney disease is a consequence of damaged nephrons, the tiny structures inside the kidneys that function as filters to remove waste 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 diabetes mellitus and high blood pressure (hypertension). Diabetes results from the body's inability to use the sugar glucose efficiently, either because it lacks insulin, the hormone that controls the level of glucose in the blood, or because it cannot use the available insulin. The glucose stays in the blood and over time, the high blood sugar levels can damage the kidneys. High blood pressure can damage the small blood vessels of the kidneys with the result that the kidneys can no longer filter wastes from the blood very well. Some kidney diseases have no direct cause but result from hereditary factors and run in families.
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 (189 L) of blood and producing about 2 quarts (1.89 L) 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: erythropoietin, which stimulates the bones to make red blood cells; renin, which regulates blood pressure; and the active form of vitamin D, required to regulate calcium for bone health and for normal chemical balance in the body.
Damaged kidneys do not clean the blood efficiently. Instead, waste products and fluid build up in the blood leading to kidney disease, which often cannot be cured. In the early stages of kidney disease, treatment may be able to make the kidneys last longer, but eventually, the kidneys may stop working altogether (kidney failure). If the kidneys fail, 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.
The National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC) of the NIDDK offers general guidelines on foods that kidney patients should limit or avoid.
POTASSIUM. Potassium is a mineral found in varying amounts in almost all foods, including fruits, vegetables, meat, and dairy products. The body uses potassium for a variety of important functions, such as maintaining fluid and electrolyte balance, sending nerve impulse to muscles, aiding in metabolic processes, and keeping heart rate steady. When a person's kidneys are functioning normally, they filter out excess potassium, helping to maintain the proper amount in the body. Without properly functioning kidneys, patients need to control their potassium intake. Foods like oranges, bananas, tomatoes, potatoes, and dried fruits must be avoided. Some potassium can be removed from potatoes and other vegetables by a process called leaching, which involves peeling and soaking them in a large container of water for several hours before cooking them in fresh water.
SODIUM. Sodium (salt) is a mineral found in many foods and is important to many different bodily functions. One of the most important functions of sodium involves maintaining fluid levels. Too much sodium causes thirst and leads to increased fluid intake, which makes the heart work harder to pump the fluid through the body. Over time, this can cause high blood pressure and congestive heart failure. Normally functioning kidneys help to maintain proper levels of sodium in the body, but if the kidneys begin to fail, it becomes more important to maintain appropriate levels of sodium intake.
One way to cut sodium intake is to stop putting extra salt on foods. People frequently add salt during cooking and at the table. Even just stopping these behaviors will usually cut back significantly on the amount of sodium ingested. It is also important to avoid particularly salty foods such as foods canned in brine, soup stocks, soy sauce, theater popcorn, potato chips, salted pretzels, bacon and other cured meats, salted fish, salted nuts, and processed cheese. Checking the ingredients list of processed, frozen, and canned foods is a good way to see how much sodium the product contains; most canned foods (including soups) and frozen dinners contain excess sodium.
FLUIDS. Though patients with kidney disease will want to drink enough water to prevent dehydration, extra fluid can raise blood pressure, make the heart work harder, and increase the stress of dialysis treatments. If patients are experiencing kidney failure, their kidneys are no longer removing water from the body with proper efficiency. Water retention can cause swelling in the feet and ankles, as well as other parts of the body. Many foods, such as soup, ice cream, and fruits, also contain plenty of water, and a dietitian is the best person to provide advice on controlling thirst; he or she will help dialysis patients determine how much fluid to drink each day. Monitoring sodium intake will also help prevent water retention.
PROTEIN. Proteins are complex organic molecules made of amino acid chains. The body uses these chains to build and maintain muscles, organs, and glands. When the body breaks down protein, it produces a waste product called urea. This waste product is usually filtered out by the kidneys and expelled from the body in urine. Patients experiencing kidney failure must be careful about the amount and type of protein they consume, because the kidneys are no longer doing a good job of removing urea. If too much urea builds up in the body, it can cause serious illness. Patients must be sure to get enough protein, however, because without protein, the body is not able to perform proper muscle maintenance.
Before kidney patients begin dialysis treatments, they are usually told to follow a low-protein diet. However, once dialysis treatment begins, most kidney patients are encouraged to eat as much high-quality protein as they can, such as lean meat, fish, poultry, and eggs. High-quality proteins produce less waste than other types of protein, and obtaining dietary protein from these sources can reduce the amount of urea in blood.
CALORIES. Maintaining daily calorie intake is a concern for patients following a kidney diet. With the great number of restrictions placed on the kinds of foods a person can eat, it can be difficult for a patient to eat enough calories each day. If overall caloric intake is not maintained at high enough levels, the patient can suffer body tissue breakdown. Vegetable oils, such as olive, canola, and safflower oils, are good sources of calories and healthy fats. Hard candy, sugar, honey, jam, and jelly also provide calories and energy. However, kidney patients with diabetes must follow the guidance of a dietitian.
Dialysis clinics have dietitians on staff who help patients plan meals. The National Kidney Foundation provides lists of acceptable foods for adults starting hemodialysis:
SODIUM AND SALT. Because salt contributes so much to the flavor of foods, it may be helpful to find replacements for it. Lemon juice, herbs, spices, and low-salt flavor enhancers add flavor without adding sodium; however, mixtures such as garlic salt should not be used because they still contain a high level of sodium. Artificial salt substitutes should also be avoided because they usually contain high levels of potassium, another mineral that people following a kidney diet need to monitor.
PROTEIN AND MEAT. Patients should try to eat a high-protein food (meat, fish, poultry, fresh pork, or eggs) at every meal, for a total of 8–10 ounces of high protein foods every day. Even though peanut butter, nuts, seeds, dried beans, peas, and lentils have protein, they are also high in both potassium and phosphorus and are thus not generally recommended.
GRAINS AND CEREALS. Suggested grains and cereals include:
MILK, YOGURT, AND CHEESE. 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:
FRUITS AND JUICES. All fruits have some potassium, but some have less than others. Star fruit (carambola) should be always avoided. Other fruits that should be limited or totally avoided are:
Because fruit is an important part of a healthy diet, 2–3 servings of the following low-potassium fruits can be eaten each day:
VEGETABLES. All vegetables contain some potassium, but some have more than others and should be limited or totally avoided, including:
Patients are advised to eat 2–3 1/2 cup servings of the following low-potassium vegetables each day:
DESSERTS. Depending on calorie needs, a dietitian may recommend high-calorie desserts such as pies, cookies, sherbet, and cakes. Dairy-based desserts and those made with chocolate, nuts, and bananas should be limited.
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 patients with kidney failure. Alternatively, the physician may 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 over-the-counter supplements since they may contain vitamins or minerals that may cause harmful interactions.
Kidney patients on dialysis have very special dietary needs beyond just 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 (PIA). Using the PIA, the albumin results, and any changes in patient appetite, the dietitian can determine if the intake of the right foods is adequate.
Patients undergoing dialysis can also experience side effects, caused by rapid changes in the body's fluid and chemical balance during treatment. Two common side effects are muscle cramps and low blood pressure (hypotension). Hypotension can make the patient feel weak, dizzy, or nauseous. Fortunately, dialysis side effects often can be treated quickly and easily.
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 form in 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, so the earlier the age at which kidney failure occurs, the more likely growth is to be affected. The goals in feeding a child with kidney failure are to balance nutrition for normal growth and protect health. 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.
See also Diabetes mellitus ; Low-sodium diet .
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Monique Laberge, PhD