Cosmo’s Urinary Stones – Part 2

Miniature schnauzers are one of several breeds that tend to form oxalate stones. This may be due to some physiological abnormality, but little is known about the underlying cause of stone formation. The problem is most significant in male dogs since the urethra is relatively narrow and can become blocked with oxalate crystals, causing a life-threatening situation. Unlike some other types of urinary crystals, calcium oxalate cannot be dissolved. So it is best to prevent them by addressing the factors that appear to lead to stone formation: diluting the urine, reducing calcium and oxalate levels in the urine, and increasing urine pH. These goals can be accomplished through diet, but little evidence supports this approach. However, the alternative is to do nothing and risk yet another recurrence of stones and subsequent surgery. So a special diet seems worth a try.

One of the keystones of any diet for stone-forming dogs is increased fluid intake since the more water ingested, the more dilute the urine will be with respect to calcium and oxalate. This is why canned food is usually recommended for stone-forming dogs since it is usually 70-80% water. Cosmo’s ineffective diet contained about 72% water overall, which provided about 12 oz (~375 g) of water from his meals alone. He also drinks some water between meals. I would guess his total water intake is about 13 oz per day. For perspective, I would have to drink almost 3 liters of water per day to match this intake.

Dietary changes can come with potential problems. One needs to make sure the diet change does not cause harm. The amounts of the various essential components of the diet are important (e.g., is the RDA of calcium being met; is too much vitamin A being provided), but also the interplay between some nutrients can be important (e.g., the balance between calcium and phosphorus), which adds to the complexity of the analysis. I should note that this sort of analysis would not be possible without software that can quickly analyze diets for the various essential nutrients. Without it, one is basically “flying blind,” with no firm idea what changing a diet constituent will do to nutrients and calories. Fortunately, software like Pet Diet Designer exists.

Looking at Cosmo’s ineffective diet, several things stand out. First, the protein level was about 300% of his RDA. Some canine nutritionists believe a low-protein diet may help prevent these stones. In humans, a high-animal-protein diet can cause metabolic acidosis, which leads to acidic urine and increased calcium excretion by the kidneys. Whether this is true in dogs remains to be determined.

Probably the most important change in his diet was to increase his water intake to keep his urine dilute. Most authorities recommend a urine specific gravity below 1.020. Without intervention, it is typically above 1.040. He rarely met this goal with his previous diet. So we increased his “soup” intake to 3 times per day (11 am, 2 pm, and 5 pm). Each soup course contains 30ml of low-sodium bouillon, 60ml of water, and 15 grams of cooked rice. The soup, along with these two meals (7:30 am and 8 pm), has increased his daily water intake to about 600ml.

Cosmo’s urine is normally very acidic (~pH5), which may contribute to his problem with oxalate stones in his bladder. This acidic pH may inhibit certain macromolecules that naturally inhibit stone formation. There are ways of increasing the pH of a dog’s urine without reducing protein intake. Potassium citrate at 75mg/kg twice daily has been shown to increase urinary pH. Citrate is metabolized to bicarbonate, which is then excreted in the urine to increase pH. If citrate is excreted in dog urine to any extent, and there is some debate on this point, the citrate may also have the benefit of competing for calcium with oxalate, thus hopefully reducing the amount of calcium oxalate available for stone formation. Cosmo’s ineffective diet did not include citrate.

Another potential problem with his ineffective diet was the calcium level, which was about 150% of his RDA. Some miniature schnauzers absorb too much calcium from the gut, resulting in excessive urinary calcium. Cosmo’s urinary calcium has not been determined, but his blood calcium has been within the normal range on two occasions (January 2015 and July 2016). Only about 4% of stone-forming dogs have hypercalcemia. It would be useful to know if his urinary calcium is also normal.

Sodium is another potential problem. His ineffective diet provided almost 630% of his RDA, and most of this came from the Urinary SO. Some veterinary nutritionists recommend high sodium levels to increase thirst and, thus, water intake, which produces more dilute urine. However, no studies have shown this effect, and giving too much salt carries risks.

Cosmo’s ineffective diet did not contain sufficient vitamin D. It contained only about 60% of his RDA for this vitamin. In humans, vitamin D forms in the skin when exposed to sunlight, but dogs do not form significant amounts because of their fur. Without sufficient vitamin D, which is important for calcium absorption, the body may activate mechanisms to mobilize more calcium from the bones, again resulting in more calcium being secreted by the kidneys. This is obviously contrary to the hyper-absorption of calcium mentioned above. Regardless, his diet does need additional vitamin D.

Finally, the diet he was on had way too much vitamin A, almost 1000% of the RDA for Cosmo. Even though we did not see any adverse effects from ingesting this amount of vitamin A, it makes sense to reduce it to a more normal level. A lot of the vitamin A came from the Urinary SO moderate-calorie component of his diet.

Which brings up another point: why did we include the Urinary SO food in his ineffective diet? About 1/3 of the calories came from Urinary SO. This food provides many vitamins and other nutrients that would not be present in sufficient quantities without it. Many leafy vegetables, which are high in vitamins, especially the B vitamins, cannot be used in a low oxalate diet. So including the Urinary SO avoided the need to add vitamin and mineral supplements to his diet. Without Urinary SO, there were about 20 nutrients that were not at 100% of his RDA or above. With the Urinary SO, only 5 were. This review using PDD was only possible because Royal Canin provides a typical analysis of their foods with detailed levels for amino acids, vitamins, and minerals. Other commercial dog food providers, such as Hill’s and Purina, are less forthcoming with such data.

Obviously, his diet had to change if there was any hope of preventing a third occurrence of his stones. But what to do? I thought looking at commercial diets for stone-forming dogs might help. At least three are on the market in the US, as shown below. The protein values below are in grams fed per day to Cosmo to yield about 550 calories, and the fat values are too. RDAs are from Pet Diet Designer and are for Cosmo.