Diet, Kidney Disease and the Urinary Tract
|Written by Elisa Katz, DVM CVA|
|Friday, May 20, 2011 12:35 PM|
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There is a connection between what cats are fed and what diseases they might get. This is an idea that is becoming much more widely accepted. Diet plays a role in disease syndromes such as kidney disease, urinary problems such as stones and crystals, Feline Lower Urinary Tract Disease and gastrointestinal problems such as IBD.
The kidneys regulate the water and salt balance in the body, maintaining hydration, electrolyte levels and regulating blood pressure. As proteins are metabolized by the body for energy, by-products are produced and circulated in the blood. It's the kidney's job to remove these toxic substances. Waste products such as urea nitrogen, creatinine and phosphorus, as well as certain drug metabolites, are all filtered from the blood and excreted in the urine. You may be familiar with the names of these by-products of protein breakdown if your cat has had blood work done. These are what are measured in the blood to detect declining kidney function – high levels mean that the kidneys aren't working normally. A complication is that the kidneys possess an amazing capacity for compensation. As much as 75% of kidney function must be lost before we can detect abnormally high blood values for these substances. Sometimes we see increased thirst and urination before the blood values rise above normal, as the kidneys become less able to conserve water, but not always. Since so much kidney function is lost by the time disease is usually detected, we need to do everything we can to help our cats maintain good kidney health in the first place.
There are only a few definitively known causes for kidney disease. Genetic conditions such as polycystic kidney disease in Persian breeds, toxins such as anti-freeze and lilies, infections and cancer are all known to cause kidney disease. Much of the time, the exact cause is unknown, and a number of things may be contributing to impaired kidney function, including diet. Low-grade persistent bacterial infection in the bloodstream, such as that which occurs with advanced dental disease, may injure the kidneys over time. Preventing dental disease can be an important factor in the long-term health of your cat's kidneys. Other conditions that can cause harm to the kidneys include hypertension due to hyperthyroidism or other diseases.
The type of diet you feed your cat can directly affect your cat's kidneys. Dehydration in cats causes the kidneys to concentrate urine to try to maintain the body's water balance. Concentrating urine predisposes a cat to renal injury.¹ The chronic, mild dehydration that cats experience when fed dry foods exclusively can cause increased stress on the kidneys, leading ultimately to decreased kidney function. Also, the low magnesium content in diets designed to decrease urinary stone and crystal formation may adversely affect the kidneys over time.²
³ There is evidence that restricting protein may actually slow down the filtering action of the kidneys.⁴ It is important that cats receive good quality protein in appropriate amounts without excessive levels of phosphorous to help maintain kidney function. This means that the protein source should be from actual meat and not a meat meal. Meat meals can consist of mostly ground up connective tissue and bones. Usable muscle meat is removed before rendering, and so meat meals may contain high levels of calcium and phosphorous, which can harm the kidneys.⁵You may have been told to feed your kidney-compromised cat a diet that has a reduced protein content. Should you do it? Recent research demonstrates that diets high in protein have no detrimental effect on the kidneys, and animals with mildly decreased kidney function do not benefit from reduced protein diets.
Rather than restricting protein that cats depend upon for their energy requirements, reducing phosphorus in the diet can help many cats with kidney disease. Phosphorous restriction is important in order to prevent the development of renal secondary hyperparathyroidism, a condition where excess phosphorous leads to an altered calcium/phosphorous balance. The end result of this imbalance causes calcium to be drawn from the cat's bones and deposited into the tissues and organs, including the kidneys, further impairing their function.⁶
Phosphorous limitation can be accomplished through the substitution of cooked egg whites for a portion of the meat in the diet, which dilutes the overall amount of phosphorus in the serving. Cooked egg whites are high in protein and very low in phosphorus. The phosphorous content of 100 grams of cooked egg white is 15 mg. For comparison, 100 grams of raw chicken has 198 mg of phosphorus and 100 grams of raw beef has 177 mg of phosphorus.⁷ Phosphorous binders can also be used. Binders are added to the food to prevent phosphorus from being absorbed into the body and bloodstream.⁸ A blood test is required to determine blood phosphorus levels, so you will need to work with your veterinarian to choose the best course. It is best to try to keep the phosphorous level in the blood to within the normal range or only slightly above.
Limiting protein is still sometimes used in cats with advanced kidney failure, but it has been my professional experience that diuresis helps these cats much more than protein restriction. Diuresis can be accomplished either at home with subcutaneous fluids or in the clinic with intravenous or subcutaneous fluids. Good hydration is a key element in helping cats with kidney disease. You can tell a human with kidney disease to be sure to drink plenty of extra water, but unfortunately you can't do that with your cat. We have to resort to other ways to accomplish extra fluid intake. Increasing water intake through food or, if necessary, through fluids given under the skin, can go a long way towards decreasing the stress on the kidneys and slow the progression of the disease.
Feline Lower Urinary Tract Disease/ FLUTD
Water intake affects more than just a cat's kidneys, and kidney function plays a role in other urinary problems. The normal instinctual diet of the feline consists of prey animals that are high in protein, have moderate amounts of fat, and little if any carbohydrate. The typical prey animal for a housecat, usually a rat or mouse, consists of approximately 50 to 60% protein, 15 to 30% fat on a dry matter basis and 70 to 80% water. This indicates that 70 to 80% of the cat's natural instinctive diet should consist of water.⁹
A quick search of any pet food website regarding the moisture content of most dry cat foods will tell you that they are generally only 8 to 10% moisture. Cats fed a dry diet exclusively are at a significant water deficit compared to cats eating a natural diet. They are only consuming about 12 to 15% of their ideal daily water intake in their food. You may think that the cat can make it up by drinking more water, but cats innately have a low thirst drive, as they evolved to eat prey consisting of so much moisture.¹⁰