A low-protein diet, a diet in which people are required to reduce their intake of protein, is used by people with abnormal kidney or liver function to prevent worsening of their disease.
The low-protein diet was developed by dietitians and nutritionists in response to adverse effects that protein can have on people with kidney or liver disease. Proteins are required for growth, upkeep, and repair of body tissues. They also help the body fight infections and heal wounds. Protein contains 16% nitrogen, which the body eliminates in the urine as urea. In cases where liver or kidney function is impaired, urea, ammonia, or other toxic nitrogen metabolites may build up in the blood. The low-protein diet is designed to reduce these nitrogen metabolites and ammonia in individuals with liver disease or kidney failure and to reduce the workload on the kidney or liver. If the kidneys, which are responsible for excretion of urea, are not functioning properly (renal failure), or if high levels of protein are continually present in the diet, urea and other toxic nitrogen compounds build up in the bloodstream, causing loss of appetite, nausea, headaches, bad taste in the mouth, and fatigue, as well as possibly further adversely affecting the kidney or liver.
Other conditions that require low-protein diets include tyrosinemia, phenylketonuria (PKU), and maple syrup urine disease (MSUD). In these conditions, the body cannot breakdown certain amino acids properly, causing them to accumulate in the body. These conditions can have serious consequences if left untreated.
The low-protein diet focuses on obtaining most of a person's daily calories from complex carbohydrates rather than from proteins. There are two main sources of protein in the diet: higher levels are found in animal products, including fish, poultry, eggs, meat, and dairy products, while lower levels are found in vegetable products such as breads, cereals, rice, pasta, and dried beans. Generally foods in the high-protein group contain about 8 grams of protein per serving. Cereals and grains have about 2 grams of protein in 1/2 cup or 1 slice. Vegetables have about 1 gram of protein in 1/2 cup, while fruits have only a trace amount of protein in 1/2 cup. To control protein intake, foods such as starches, sugars, grains, fruits, vegetables, fats, and oils should be eaten at levels sufficient to meet daily energy needs. If a person has diabetes, the diet must also be designed to control blood sugar.
Protein should never be completely eliminated from the diet. The amount of protein that can be included in the diet depends on the degree of kidney or liver damage and the amount of protein needed for an individual to maintain good health. Laboratory tests are used to determine the amount of protein and protein waste breakdown products in the blood. A suggested acceptable level of protein in a low-protein diet is about 0.6 g/kg of body weight per day, or about 40 to 50 grams per day. A person suffering from a kidney disease such as nephrotic syndrome, where large amounts of protein are lost in the urine, should ingest moderate levels of protein (0.8 kg per kg of body weight per day).
A sample menu for one day might include:
This sample menu contains about 1850 calories, with a protein content of 8%.
Special, low-protein products, especially breads and pastas, are available from various food manufacturers for people who need to follow a low-protein diet. Specific information on the protein content of foods can be found on food labels. Books that list protein contents of various foods as well as low-protein cookbooks are also available.
In addition, it is recommended that fat calories be obtained from monounsaturated and polyunsaturated fats. Some people may also be required to reduce their sodium and potassium ingestion in foods. Sodium restriction improves the ability to control blood pressure and body fluid build-up as well as to avoid congestive heart failure. Foods with high sodium contents, such as processed, convenience and fast foods, salty snacks, and salty seasonings, should be avoided. Potassium is necessary for nerve and muscle health. Dietary potassium restriction is required if potassium is not excreted and builds to high levels in the blood, which may result in dangerous heart rhythms. At very high levels, potassium can even cause the heart to stop beating.
As kidney function decreases, the kidneys may reduce their production of urine, and the body can become overloaded with fluids. This fluid accumulation can result in swelling of the legs, hands, and face; high blood pressure; and shortness of breath. To relieve these symptoms, restriction of fluids, including water, soup, juice, milk, popsicles, and gelatin, should be incorporated into the low-protein diet. Liver disease may also require dietary fluid restrictions.
The purpose of a low-protein diet is to prevent worsening of kidney or liver disease. The diet is effective because it decreases the stress on the kidney or liver.
Protein restriction lessens the protein load on the kidney or liver, which slows down the continued development of disease.
A person requiring a low-protein diet should consult a dietitian familiar with liver or kidney disease to provide guidance on developing an appropriate diet as well as to learn how to follow the diet effectively. The diet must meet the person's nutritional needs, cut down the work load on the kidneys or liver, help maintain the kidney or liver function that is left, control the build-up of waste products, and reduce symptoms of the kidney or liver disease. Strict adherence to the diet can be difficult, especially for children. Small amounts of protein-containing food combined with larger amounts of low or no-protein foods can be used to make the diet more acceptable. Some people eliminate meat, eggs, and cheese from their diets rather than measure the amounts of protein from these foods. However, care must be taken to make sure that some protein is included in a vegetarian diet to provide for growth and development, including building muscles and repairing wounds. Another approach, since it is difficult to manage portion sizes of foods other than milk, is to omit meats, fish, and chicken from the diet and use milk as the primary source of protein.
A person with both kidney disease and diabetes must be careful to eat only low-to-moderate amounts of carbohydrates along with monounsaturated and polyunsaturated fats.
The human body reacts to protein deficiency by taking amino acids (the building blocks of proteins) away from muscle tissue and other areas of the body. The process, in which the body basically metabolizes itself, is called catabolism and leads to muscle loss and weakness. The use of exercise and strength training is recommended to counter the effects of muscle loss.
A low-protein diet may also be deficient in some essential amino acids (which are the building blocks of protein); the vitamins niacin, thiamin, and riboflavin; and the mineral iron (most people with advanced kidney disease have severe anemia). Vitamin supplementation is dependent on the amount of protein restriction, the extent of kidney damage, and the vitamin content of food that is eaten. A person with kidney failure may have decreased urine output. The amount of fluids a person needs to drink is based on the amount of urine produced daily, the amount of fluid being retained, the amount of sodium in the diet, the use of diuretics, and whether the person has congestive heart failure.
In people with advanced kidney disease, a low-protein diet may lead to malnutrition. The person may lose muscle and weight, lack energy, and have difficulty fighting infections. Daily calorie intake is dependent on the amount needed to prevent breakdown of body tissues. Body weight and protein status should be monitored periodically, which in some cases may be daily. Extra calories can be added to the diet by increasing the use of heart-healthy fats and eating candy or other sweet foods, such as canned or frozen fruits in heavy syrup.
Very low-protein diets coupled with amino acid supplements have been shown to slow down the progression of and even cure certain types of kidney disease in people in early stages of the disease. In adults with moderate-to-severe chronic renal failure, reduced protein intake has also been shown to decrease the risk of end-stage renal disease, based on a systematic review of eight randomized trials with 1,524 patients who were followed for at least one year. Renal death was defined as initiation of dialysis, kidney transplant, or patient death. The incidence of renal death was 13.5% in patients following the low-protein diet compared with 19.4% in patients receiving the higher-protein diet. However, there was insufficient evidence to determine the optimal level of protein intake.
Although the low-protein diet may help those with chronic kidney or liver disease, it is known to lead to muscle loss. In 2004, researchers in the Nutrition, Exercise Physiology, and Sarcopenia Laboratory of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University (HNRCA) in Boston, Massachusetts, reported on a study involving a group of volunteers with chronic kidney disease who consumed a low-protein diet. About half the group engaged in resistance training, while the other half served as a control group. Among the strength-training participants who exercised for 45 minutes (including warmup and cool-down) three times per week for 12 weeks, measurements showed that, on average, total muscle fiber increased by 32%, and muscle strength increased by 30%. Those who did not exercise lost on average about 3% of their body weight, or about 9 pounds.
Researchers studying a group of vegetarians who had maintained a diet relatively low in protein and calories found that they had lower blood levels of several hormones and other substances that have been tied to certain cancers. Additionally, it has been shown that a low-protein diet protects against gout, which is caused by too much uric acid in the blood. The excess uric acid forms crystal deposits in joints, particularly in the big toe, feet, and ankles, resulting in episodes of pain.
See also Carbohydrate addict's diet ; Carbohydrates ; Maple syrup urine disease ; Protein .
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National Kidney Foundation, 30 East 33rd St., New York, NY, 10016, (212) 889-2210, Fax: (212) 689-9261, (800) 622-9010, http://www.kidney.org .
Tish Davidson, AM
Revised by Laura Jean Cataldo, RN, EdD