Proteins
From Wiki.Nephron.org
Contents |
Dietary protein for the person with chronic kidney disease
As a person's kidney function diminishes, the dietary restrictions needed to compensate increase. One of the most difficultdietary challenges to master is the restriction of protein. To understand why protein protein requirements change, we need to first understand how the body treats the proteins we eat.
What are proteins?
The story of proteins must begin with amino acids, the structural units of proteins. Protein is formed by linking amino acids together - not unlike pearls on a necklace.There are about 20 amino acid building blocks (depending how they are defined); out of these twenty, ten are considered essential amino acids and the other ten non-essential.
An essential amino acid is one that the body cannot make and therefore must be supplied by the diet. While non-essential amino acids are necessary for health and life, the body can manufacture them from the other essential amino acids. Foods containing the essential amino acids are called "high biologic value" or HBV protein foods.
Dietary Protein
The protein we eat is digested and broken down into its amino acids,which our body then uses to build its own proteins. These proteins perform thousands of functions, including the formation of connective and muscle tissue, producing enzymes to promote chemical reactions, or building larger substances needed for life, like the hormone insulin or the hemoglobin in red blood cells.
Protein digestion begins in the stomach and intestines. The amino acids then go into the blood stream and are transported to other parts of the body where they are absorbed and used as needed. The body needs different amounts of the various amino acids and it is possible to eat more protein than the body needs. When this happens, the excess amino acids are removed as waste (urea is an example of a protein "waste product"). One of the most important methods of removal is through the kidneys and out in the urine.
Proteins and the Kidney
The more amino acids that need removing, the harder the kidney filters must work. This is overtaxing and leads to kidney damage. It may accelerate kidney disease in patients who already have altered function.
It does not mean one must stop eating protein, because proteins are necessary for life. Not getting enough protein leads to malnutrition. The solution is to eat enough protein to maintain health, but minimize excess amino acid intake and spare the kidneys. The best way to do this is to eat foods that contain the proteins causing the least waste - that is foods that have the right ratios of different amino acids the body needs, and will be used most efficiently. Since we are in the animal kingdom, foods that come from animals (dairy foods, eggs, meat, poultry, fish) have the best combination of amino acids and produce the least waste. These are the "high biologic value" or HBV foods we referred to above.
It is important to understand that grains, legumes (beans), fruits and vegetables all have varying amounts of protein made of the same amino acids in animal foods. They are not "bad foods"- its just that the amino acids they have are not in the same amounts that the human body can use most efficiently - that is, they produce more waste. This is not universally true; there are good quality vegetable sources such as soy beans that can also be used. People with kidney disease need to pay attention to both the amount and type of protein they eat.
How much protein is enough?
A healthy, active person needs about 0.36 grams of protein per pound of body weight. The average American consumes over 100 grams of protein a day, or about 0.67 grams of protein per pound (1.47 grams/kilogram) or almost twice what is needed. So, a place to start for the person with kidney disease is to stop eating their usual amounts of protein and restrict their intake to only what their body needs. If the average American man of 70 kg (154 pounds) eats 80 grams of protein per day, but only needs 54 grams, he should cut back to 37-41 grams of protein per day. To reduce protein waste, it is recommended that 60% of the protein should be from a HBV source. Animal or HBV foods (meat, fish, poultry) have 7.0 grams of protein per ounce. Milk has 4 grams per half-cup. Fruits (trace), vegetables (1.0 grams), and grains such as pasta,breads, cereals (2.0 grams) have lower amounts of protein perserving. Fats such as butter and oils have no protein- but they do have calories!
We now have enough information to work out a diet for our average 70 kg (154-pound) man:
Target protein intake: 40 grams of protein per day with (for example) 70% coming from HBV foods: 40 times 70% = 28 grams or 4 ounces (28 divided by 7) 40 times 30% = 12 grams of protein from other foods. These 12 grams can be supplied with different combinations, but variety in the diet is important, both for maintaining health and also to keep the diet interesting. An example could be 3 servings of vegetables (3 grams), 5 grain servings (10 grams), and 3 fruit servings (0 gram) for a total of 41 grams.
| Body weight (pounds): | Protein (grams) | Calories |
| 100 | 25-27 | 1600 |
| 125 | 31-34 | 2000 |
| 150 | 38-41 | 2400 |
| 175 | 44-47 | 2800 |
| 200 | 50-54 | 3200 |
It is difficult to get exact intake, the goal is to consistently be close to your goals. In this case, we came up with a total of 41 grams- tomorrow it may be 39. It is also important to make sure you are eating enough calories.If the calorie intake is too low, your body will use theprotein you eat for energy, rather than maintenance. Rememberthese numbers will change depending on your weight, gender, and the stage of your kidney disease.
The dietitian can determine the best amounts for you. It's important to be sure the proteinrestricted diet is still providing enough protein to maintain muscles and general health. This is determined by reviewing your blood tests.
This type of diet is probably different from what patients are used to and will take practice and experimenting to master. But modifying the diet is one area where patients can take charge of their medical treatment. The dietitian as will as Recipe Club can help with recipes and menus to keep protein intake at a healthy level, yet keeping the diet interesting.
Determining the Amount of Dietary Protein: A Formula
To assist the patient in complying with his/her prescribed amount of dietary protein, it is important to tell the patient exactly how much protein is contained in their diet. Fortunately, this is possible using three measurements, the body weight in kilograms (N.B., one kilogram = 2.2 lbs), the BUN and a 24 hour collection of urine (to measure the content of urea nitrogen). The basis for determining the amount of protein in the diet is directly proportional to the amount of urea nitrogen being made each day.
The following steps will provide an exact amount of protein in the patient's diet:
- if the BUN and weight are constant, then the urea nitrogen excreted in 24 hours equals the urea nitrogen produced.
- the amount of nitrogen in all other forms (i.e., the nitrogen in urine creatinine, uric acid, peptides, etc. plus the nitrogen in stool) is equal to 0.031 g nitrogen per kg body weight per day.
- The amount of protein in the diet is equal to the amount of urea nitrogen excreted each day plus 0.031 g nitrogen per kg per day. The amount of protein is then calculated as the total amount of nitrogen times 6.25 g protein per g nitrogen.
EXAMPLE: A 70 kg (154 pound) man has a 24 hour urea nitrogen excretion of 6 g nitrogen/day. His protein intake (assuming that the BUN and weight are constant and hence, he is in the steady-state) is equal to 6 gN/day + (70 kg times 0.031 gN/kg/day) equals 6 g urea nitrogen/day + 2.17 g non-urea nitrogen/day = 8.17 g nitrogen/day. The amount of protein eaten = 8.17 x 6.25 g protein/g nitrogen = 51 g protein per day. If the man was prescribed a diet of 0.6 g protein per kg per day (i.e., 42 g protein/day), he needs to visit with the dietician!
The protein intake formula can be found on the web at http://nephron.org/protein_intake.
