Nutritional Management of Uroliths
Sean J. Delaney and Andrea J. Fascetti
Uroliths are commonly referred to as "stones" and can occur in any section of the urinary tract with bladder stones being the most common. Regardless of specific type, uroliths occur when the urine becomes too concentrated with urolith precursors and the environmental conditions are appropriate for stone formation. Strategies to prevent future formation are aimed at reducing the concentration of urolith precursors and changing the environmental conditions to an unfriendly one for urolith formation. Despite extensive research in both human and veterinary medicine, perfect dietary strategies for the prevention of some uroliths have not been found and dietary efforts are geared toward reducing the frequency of recurrence.
Increasing Water Consumption
A common treatment strategy employed for all types of uroliths not caused by a bacterial infection is to increase the pet's water consumption. Increasing the amount of water that a pet consumes each day dilutes the urine or makes it less concentrated with urolith precursors. Effectively increasing a pet's water consumption can be achieved by switching pets that are on dry food to wet food or, for pets already on wet food, adding more water to the food. Although pets appear to drink more water when eating dry food, it is known that they do not drink enough water to make up for the difference in water content between dry (usually <12% water) and wet (around 75% water) diets. Another strategy that can be employed, when feeding wet food is not practical, is the use of salt. Increasing the amount of salt in the diet can increase thirst and cause the pet to drink more water. Specific recommendations from your veterinarian should be obtained before adding salt to any animal's food or using a high salt diet. A high intake of salt can be harmful if a pet is also suffering from heart, liver or kidney disease. Other less proven strategies to increase a pet's water consumption include placing multiple bowls of fresh water throughout the house and yard daily; using special water bowls that constantly re-circulate water with an electric pump; and adding ice or meat flavoring to water. Most veterinarians will recommend that pet's urine concentration be tested periodically to test the effectiveness of any of these approaches. This test result is referred to as a urine specific gravity, and the lower the number, the less concentrated the urine.
Struvite uroliths are comprised of magnesium, ammonium and phosphate and especially like to form in urine that is alkaline (higher pH). In dogs, struvite uroliths commonly occur when there is a concurrent urinary tract infection (UTI). In dogs that formed struvite uroliths due to a UTI, the prevention of recurrence is aimed at avoiding future UTI's with the use of antibiotics, and the dog's diet is usually not changed. In most cats and in some dogs, there is no UTI and the primary management of the disease is dietary. Dietary strategies to prevent struvite uroliths are primarily focused on decreasing the concentration of struvite precursors and acidifying the urine. Decreasing the struvite precursors is first accomplished by increasing water consumption by the methods discussed above. In addition, the amount of magnesium, ammonium and phosphate in the diet are reduced. Since the majority of ammonium and phosphate in the urine comes from the protein in the diet, the diets designed to prevent or even dissolve struvite stones are low in protein. The amount of protein in foods designed to dissolve struvite uroliths is so low that careful monitoring of the patient is necessary when dietary dissolution of struvite uroltihs is employed instead of surgical removal. This may include frequent and repeated radiographs (x-rays) to ensure that the diet is actually causing the stone to dissolve as well as blood tests. Unfortunately, low protein diets cause the urine to be alkaline which increases the likelihood of struvite formation. To counter this effect, diets designed to prevent or dissolve struvite uroliths have urine acidifiers added to them such as the amino acid methionine. Close monitoring for a low urine specific gravity, low urine pH and lack of struvite crystals with frequent urinalysis is crucial to effectively preventing recurrence of struvite uroliths.
Urate uroliths are primarily comprised of uric acid or urate and usually form in pets that have a liver shunt, with the exception of Dalmatians and, possibly, English bulldogs. In Dalmatians and English bulldogs, there appears to be a genetic defect in their ability to normally handle dietary purine that results in the formation of urate uroliths. Treatment for Dalmatians and possibly English bulldogs is with both diet and medication (i.e. Allopurinol). The amount of medication needed is dependent upon the amount of purine in their diet. Thus, adjustments in medication must be made based on urine collection following any dietary changes. For other animals that have urate uroliths, veterinarians will perform relatively extensive diagnostic testing to determine if the underlying cause is a liver shunt. A liver shunt occurs when blood from the intestinal tract bypasses the liver and goes directly into the systemic bloodstream. This bypass prevents the normal by-products from digested and absorbed food from going to the liver for removal from the circulation. These by-products remain in the circulation and can adversely affect some animals. These adverse effects are frequently manifested as neurologic signs, referred to as hepatic encephalopathy (HE). HE can be manifested as lethargy, "spacy" looks or "star gazing" or even seizures. The onset of these clinical signs is frequently associated with the consumption of a meal. Foods high in protein are frequently associated with the development of clinical signs because one of the break-down products of protein is ammonia. Ammonia is hypothesized to be a key trigger of HE. Pets with urate uroliths may never show signs of HE. Some shunts can be corrected surgically. If the surgery is successful and the shunt is corrected, some of these patients will not need long-term dietary changes. For dogs with a genetic predisposition and for dogs and cats where surgery is not effective or not an option, the treatment of urate urolithiasis relies on the use of low purine diets. Purines are a component of DNA. DNA is found in high concentrations in common protein sources, although some types of proteins contain more than others. Organ meats have the highest purine content, and vegetable and dairy proteins have the lowest. Therefore, cottage cheese or egg-based diets are commonly utilized, as are vegetarian diets for dogs. Cats have too many special nutritional needs as carnivores to be able to do well on a vegetarian diet. Another method of reducing the amount of purine in the diet is to decrease the amount of protein in the diet. This strategy can be effective, but can lead to protein malnutrition if the animal is not eating enough food to meet its daily energy needs. Your veterinarian can measure blood concentrations to help monitor for this potential problem.
The selection of low purine diets is often similar to the diets selected for treating HE, but again precautions are taken to avoid protein malnutrition. Most pets with signs of HE have a liver that is not as efficient at using dietary protein and, thus, actually need more protein and not less. This is why protein is not restricted in pets with liver disease unless they have signs of HE. Even when pets have signs of HE, many veterinarians will slowly increase the amount of protein in the diet (for example, by adding small amounts of cottage cheese) in an attempt to maximize the amount of protein the pet is getting without causing signs of HE.
Calcium Oxalate Urolithiasis
Calcium oxalate uroliths are comprised of calcium and oxalate and especially like to form in concentrated urine. The best nutritional approach remains to be determined and is somewhat controversial; however, certain dietary strategies appear to help reduce the frequency of recurrence and will be briefly mentioned here. As with most uroliths, increasing water consumption is important for prevention and, with calcium oxalate uroliths, it is vital. The above discussed methods for increasing water consumption can all be employed, but there is some debate as to whether salt should be used or not as a method due to the frequent existence of kidney disease in patients with calcium oxalate uroliths. Careful screening and monitoring of a pet's urine specific gravity (along with blood tests) should resolve this as a concern. Pets with kidney disease lose their ability to concentrate their urine; therefore, there is no point to adding salt to a pet's diet when they already have a low urine specific gravity from renal failure. Thus, patients who would be harmed from the addition of salt to their diet would not be appropriate candidates for this strategy. Reducing calcium and oxalate in the diet is another strategy that is used for prevention. Decreasing the amount of calcium in the diet must be done carefully. Decreasing calcium to very low levels can actually cause oxalate in the diet to be absorbed at even greater amounts since calcium can bind to oxalate in the gut and prevent its systemic absorption. Decreasing oxalate in the diet is an effective strategy, but much of the oxalate in a pet's body is not from the diet but from the natural processing of other nutrients in the diet. In any event, avoidance of high oxalate containing treats such as many vegetables and fruits is an important preventative step. Ask your veterinarian about specific foods to avoid if you have questions about particular treats you like to feed. One commonly employed dietary strategy that may not be as effective as originally proposed is decreasing dietary protein. Decreasing protein in the diet causes the urine to become more alkaline. A more alkaline urine was thought to discourage calcium oxalate formation in the urinary tract. There is growing, but somewhat debated evidence, that this is not the case. For now, focusing on prevention of calcium oxalate crystals in the urine and a low urine specific gravity appear to be the best methods of decreasing the risk for recurrence of calcium oxalate uroliths.
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