According to the National Kidney Disease Education Program, more than 20 million Americans may have kidney disease, and many more are at risk for it. Diabetes is the leading cause of kidney disease in the United States. The type of kidney disease associated with diabetes, called diabetic nephropathy, develops as a result of damage to the kidneys caused by elevated blood glucose levels over time.
It is unknown how many years of high blood glucose levels it takes, or exactly how high these levels must be, to cause nephropathy in a given person, but research has established a relationship between suboptimal blood glucose control and the development of kidney disease. In one of the most famous research studies — the Diabetes Control and Complications Trial, which ended in 1993 — intensive blood glucose control among study participants led to a 50% reduction in the risk of developing kidney disease.
While it is still not completely understood why some people with diabetes develop nephropathy and others do not, there is evidence that factors other than blood glucose control also contribute to kidney disease. High blood pressure is one such risk factor, as is having African-American or Hispanic ancestry. Diabetes and high blood pressure are the leading causes of kidney disease, so if you have both of these conditions, it is very important to talk to your doctor about preventive measures you can take to reduce your risk of nephropathy.
What the kidneys do
The primary functions of the kidneys are to regulate body fluids and eliminate waste products. The kidneys also have a role in maintaining the proper balance of sodium and potassium in the body, and they produce important hormones as well. The kidneys contain special working units called nephrons that are responsible for filtering the blood. There are around one million nephrons in each kidney. These tiny structures retain the water the body needs and reject waste products by producing urine for excretion. A very small amount of fluid is excreted as urine compared with the total amount of fluid from the blood filtered by the kidneys. In one day, the kidneys can filter about 200 quarts of fluid — meaning that the liquid content of all the blood in the body is filtered about every 45 minutes.
Because of the heavy workload that nephrons have, there are a lot of blood vessels in the kidneys to support them. Therefore, good blood flow is important both to deliver the blood for filtration and to support the nephrons in performing the filtration process. In diabetic nephropathy, damage to the blood vessels supplying the nephrons leads to the shutting down of some nephrons. Nephrons cannot be regenerated by the body.
Detecting kidney disease
The good news is that if diabetes-related nephropathy is detected in its early stages, it can be managed. This is why people with diabetes should have their kidney function assessed regularly. If left untreated (or inadequately treated), kidney disease will ultimately progress to a point where dialysis — artificial filtration of the blood — is needed for the person to survive.
There are usually no symptoms associated with early kidney disease, but as the function of the kidneys deteriorates, a fluid imbalance can occur. This may show up as sudden weight gain, swollen hands or feet, or problems with blood pressure.
Fortunately, kidney disease can be detected in the early stages by some standardized laboratory tests that your doctor can order. One of the tests, for microalbuminuria, measures how much protein is in your urine. People with kidney disease will have an abnormal amount of protein in the urine, since it is wasted as a result of improper filtration rather than kept in the body. Another way to screen for kidney disease is to measure the level of creatinine in the blood. Creatinine is a waste product, normally found in the blood at low levels, that should be eliminated in the urine. However, if the kidneys are not working properly, creatinine begins to build up in the bloodstream.
A more specific measure of kidney function is the glomerular filtration rate (GFR), which can be estimated using laboratory tests and information about a person’s age, sex, race, and other factors. This measurement is used to determine what stage of kidney disease a person has. There are five defined stages of kidney disease. Stage 1, or early, kidney disease is diagnosed when the GFR is still fairly high but no longer in the normal range. Stage 5 is diagnosed when the GFR is extremely low and dialysis is required.
Some dietary measures may be prescribed as a way to prevent kidney disease or as a treatment for high blood pressure. If you already have kidney disease, management of your diet is essential to your treatment. Research has shown that in the early stages of kidney disease, you may be able to slow the pace of damage to your kidneys by changing the way you eat. In later stages of kidney disease, dietary modifications become essential. These changes may include modifications for weight loss and restriction of sodium, potassium, protein, phosphorus, and sometimes fluids.
Sodium is a nutrient that is essential to the body, but most Americans consume several times more sodium than their body actually needs. When there is an excess of sodium in the body, the kidneys retain extra fluid, which puts an extra burden on the blood vessels, the heart, and the kidneys themselves. Guidelines from the American Diabetes Association recommend that people with diabetes consume no more than 1500 milligrams of sodium each day to help prevent or slow the progression of kidney disease. The American Heart Association recommends the same restriction for people with high blood pressure. This translates into less than about three-fourths of a teaspoon of salt per day (table salt is approximately 40% sodium, 60% chloride).
Eliminating table salt from one’s diet, however, is usually not enough to reduce sodium intake to the recommended level. This is because most Americans frequently consume processed and prepared foods, many of which contain a high level of sodium. Examples of high-sodium processed foods include canned and dry packaged soups, macaroni and rice mixes, frozen entrées, canned meats and fish, packaged sandwich meats, salted snack foods, and pickled, smoked, or cured items. Cutting back on these foods can be hard because our taste buds are simply accustomed to high levels of sodium. However, as millions of Americans have become more conscious of the need to reduce sodium in their diet, food manufacturers have made a variety of lower-sodium options available.
To find lower-sodium food choices at the store, read the labels. “Salt-free” means that a product has less than 5 milligrams of salt per serving. “Sodium-free” means that there are fewer than 5 milligrams of sodium per serving. “No salt added” does not always mean that there is no sodium in the product; it simply means that none was added during processing, so check the sodium content in the Nutrition Facts panel when considering a product with this claim. Other phrases used to indicate sodium content include “very low sodium” (35 milligrams or less per serving), “light in sodium” (50% less sodium than is found in the regular version of the product), and “less sodium” or “reduced sodium” (at least 25% less than in the regular product).
There are also sodium-free seasonings that can be used in place of salt to add flavor to foods. Try using spices, fresh herbs, lemon juice, vinegar, and aromatic vegetables like onions and garlic. It will take time for your taste buds to get used to less salt, but it is possible, and eventually you may grow to prefer less salty tastes.
Once kidney disease reaches stage 2 or 3, additional dietary restrictions will most likely be recommended. These may include restrictions on potassium, a mineral found in many foods. The heart and other muscles depend on potassium to function properly. If the kidneys are not working well, however, too much potassium can build up in the body. This can create significant health risks, including an increased risk of heart attack.
Foods that are high in potassium include oranges, bananas, tomatoes, dried beans, peas, chocolate, and bran and bran products. A registered dietitian is often the best resource for determining which foods to avoid or limit — and how much to limit them — because of their potassium content. Food manufacturers are not required to list the amount of potassium on nutrition labels, so it is necessary to seek that information elsewhere. One place to look is the searchable USDA National Nutrient Database for Standard Reference, located online at http://ndb.nal.usda.gov.
When high-potassium foods must be limited, eating more lower-potassium foods is encouraged. These include green beans, apples, cucumbers, and watermelon. But even when eating lower-potassium foods, portion control is essential. If you eat more than the amount recommended by your dietitian, you may be consuming too much potassium.
Protein restrictions can be prescribed at any stage of kidney disease, depending on the medical needs of the individual. The decision to recommend a protein-restricted diet is based on a person’s overall health condition, laboratory results, and current dietary practices.
Protein is found in both animal products (meat, eggs, dairy products, fish) and plant products (nuts, seeds, beans, grains, and most vegetables). Protein is made up of molecules known as amino acids, and humans need certain specific amino acids to stay healthy. Animal protein is considered to have “high biological value” because it contains all of the essential amino acids that humans require. Most sources of plant protein lack one or more essential amino acids, but those gaps can be filled by eating a variety of plant foods every day.
It is possible, therefore, to obtain adequate protein from plant foods while reducing overall protein intake by restricting animal protein. This restriction is often challenging for people who have made meat the centerpiece of their meals or who prefer large portions of meat. Many meal plans, however, allow the inclusion of meat and protein-rich plant foods in reduced quantities. A good way to reduce the quantity of meat in a meal is to cut up a small piece of meat and mix it into a stir-fry, pasta dish, casserole, salad, or stew. In this way, the smaller amount is not so obvious because the taste is spread out.
When kidney disease reaches stage 3 or higher, it may be necessary to add another restriction: the mineral phosphorus. Untreated high phosphorus levels in the blood will cause an imbalance with another essential mineral in the body, calcium. As the blood phosphorus level rises, calcium gets pulled out of the bones, resulting in weakened bones and teeth and other health problems.
Foods that are high in phosphorus include dark colas, dairy products, bran cereals, dried beans, seeds, and chocolate. Some recommended dietary modifications may involve replacing cream- or milk-based soups with water-based ones and using nondairy creamer as a milk substitute. Some people at this stage of kidney disease are prescribed a drug called a phosphate binder if dietary restrictions alone do not adequately regulate the level of phosphorus in the blood. This drug works by absorbing some of the phosphorus from the food that you eat. If you are prescribed a phosphate binder, it is important that you take this drug consistently and according to your doctor’s instructions.
If kidney function has deteriorated to the point that the kidneys cannot excrete excess fluid in the body, it may be necessary to restrict your fluid intake. This means monitoring your intake not just of beverages, but also of foods with a high water content such as soups, stews, gelatins, puddings, frozen desserts such as sorbet, and certain fruits. This restriction is often prescribed to people who are already on dialysis, as their success on dialysis depends on a very accurate monitoring of the fluid content of the body before, during, and after dialysis treatment sessions.
(Click here for a sample renal meal plan.)
Getting individualized help
Because the exact dietary restrictions necessary will vary based on an individual’s condition — that is, his lab results as they relate to overall kidney function and blood levels of different minerals and waste products — it is essential to get individualized guidance. Coming up with a comprehensive kidney care plan will require cooperation among the members of your health-care team: your doctor or doctors, your pharmacist, your dietitian, and others.
In addition to discussing your condition, treatment options, and needs with your health-care providers, you may also want to seek out general information about kidney disease and its care from organizations such as the National Kidney Foundation, the National Kidney Disease Education Program, and the National Institute of Diabetes and Digestive and Kidney Disorders. These organizations can provide fact sheets, newsletters, other educational materials, and even kidney-friendly recipes and cookbooks.
At any stage of kidney disease, you may benefit from the services of a nephrologist, a doctor who specializes in the care of disorders of the kidneys. But even if you don’t have kidney disease, you can be proactive about your kidney health by asking your doctor to give you a yearly update on your risk factors for kidney disease, including your blood pressure, HbA1c level (a measure of blood glucose control), and microalbuminuria test results. By knowing where you stand, you can take appropriate action to improve your health and prevent or limit kidney dysfunction. If that action includes changing your diet, it may feel challenging at times, but be assured that it can be done in a way that leaves you satisfied and in control — and ready to work toward being the healthiest you can be.