Diet

Perhaps the most important dietary modification that can be made is to increase water intake and urinary volume while decreasing urine specific gravity. Retrospective studies of cats (Lekcharoensuk et al, 2001) and dogs (Lekcharoensuk et al, 2002) with CaOx urolithiasis found a significantly lower risk of CaOx formation with higher dietary moisture content. Feeding a canned diet is the best way to increase water intake, but some dogs and cats will not eat canned food. In these cases, water or broth can be added to dry food, or broth can be added to the water supply. Water fountains also may be helpful to increase water intake in cats. Appropriate targets for specific gravity are less than 1.025 in cats and less than 1.020 in dogs; achieving dilute urine can be very difficult in cats.

Supplementation of sodium chloride has been investigated as a means of increasing water consumption but has been a point of controversy. Increased sodium consumption increases urinary calcium excretion and may increase the risk of CaOx urolithiasis. However, prospective studies have shown that increasing dietary sodium content significantly decreased the CaOx RSS in healthy and CaOx stone–forming dogs (Lulich et al, 2005; Stevenson, Hynds et al, 2003) as well as in healthy cats. The total daily urinary calcium excretion increased in these studies, but apparently the effect on CaOx RSS is offset by the increase in water intake and urine volume. These findings suggest a benefit to NaCl supplementation, but long-term studies still are needed. Sodium supplementation can be considered if there is an inadequate response to dietary therapy and the urine is not dilute, but patient selection must be done carefully. Short-term studies in cats have shown no adverse effects on kidney function or blood pressure, but caution is required when considering adding salt to the diets of dogs or cats with kidney disease or hypertension until longer-term studies are done. Additionally, high-sodium diets are contraindicated for animals with heart disease.

Higher dietary protein historically has been associated with an elevated risk of CaOx formation because it can promote acidosis and hypercalciuria. However, retrospective studies in dogs and cats have found a lower risk of CaOx formation with higher dietary protein (Lekcharoensuk et al, 2001, 2002). Overall, the exact amount and type of protein that is ideal has yet to be determined, but most diets designed to reduce CaOx urolithiasis have reduced protein levels.

The importance of the calcium and oxalate content of food was demonstrated by a study in healthy dogs (Stevenson, Hynds et al, 2003). In these dogs, urinary oxalate excretion and CaOx RSS increased when oxalate intake was increased, but only when the intake of calcium was low. The lowest CaOx RSS was found in dogs fed a diet that was lowest in both calcium and oxalate. If only calcium or only oxalate was decreased, the CaOx RSS increased. This emphasizes the need for a balanced amount of calcium and oxalate in the diet, but in all cases high calcium or oxalate content should be avoided. Examples of high-oxalate foods are leafy green vegetables and nuts. For a more complete list of foods with high oxalate content, see the Oxalosis and Hyperoxaluria Foundation website (www.ohf.org/diet.html).

Vitamin intake also can be important: hypercalciuria can result from excessive vitamin D intake because of enhanced intestinal absorption, and excessive vitamin C may promote hyperoxaluria, although this has yet to be proven in dogs and cats. A deficiency in vitamin B₆ (pyridoxine) also may play a role in urinary oxalate excretion. A study in healthy kittens showed that urinary oxalate excretion was higher in those fed a diet deficient in pyridoxine than in those fed a normal amount (Bai et al, 1989). Supplementation with vitamin B₆ (2 mg/kg q24-48h PO) thus can be considered as adjunct medical treatment.

Three studies have prospectively evaluated the effect of commercially available urinary diets on dogs and cats with CaOx urolithiasis. The diets evaluated were canned Royal Canin Canine SO Lower Urinary Tract Support, canned Hill’s Prescription Diet Feline c/d-oxl, and canned Hill’s Prescription Diet u/d. A fourth diet is available for cats (Purina Veterinary Diets UR Urinary St/Ox Feline Formula). Unpublished work indicates that feeding this diet to normal cats yielded urine that was metastable for CaOx, which suggests that it would be an appropriate option for cats with CaOx urolithiasis. However, published studies still are needed for a more complete evaluation.

In dogs fed canned Canine SO, the CaOx RSS was reduced significantly by 63% compared to that in dogs fed a maintenance diet (Stevenson et al, 2004). This group of dogs was followed for 12 months, during which time no recurrence of CaOx stones was noted.

In cats fed Prescription Diet Feline c/d-oxl, the activity product ratio (measure of CaOx supersaturation) was reduced significantly by 59% compared with that in the same group of cats fed a variety of dry maintenance diets (Lulich et al, 2004b). This diet is no longer available, but Hill’s Prescription Diet c/d Multicare Feline has similar efficacy according to the manufacturer.

A third study evaluated the effect of feeding canned Prescription Diet u/d to dogs with CaOx urolithiasis (Lulich et al, 2001). Compared with dogs fed a maintenance diet, dogs fed Prescription Diet u/d had significantly lower urinary concentrations of both calcium and oxalate. Protein malnutrition is rare but possible during long-term feeding of Prescription Diet u/d; dogs fed this diet had a significantly lower serum albumin level than controls, although values remained in the normal range. The low protein levels make this diet contraindicated in dogs with dilated cardiomyopathy or in breeds that are predisposed to this disease. In recent years Hill’s has supplemented Prescription Diet u/d with taurine and carnitine, but concerns still remain. In general, veterinary nutritionists recommend against using Prescription Diet u/d as a maintenance diet. Furthermore, this diet tends to be higher in fat and may not be the best choice for dogs with a history of pancreatitis or hyperlipidemia.

As mentioned earlier, acidifying diets have been associated with a higher risk of CaOx formation in both dogs and cats, but further studies have yielded conflicting results regarding the importance of daily urinary pH in CaOx prevention. Overall, it can be said that pH generally appears to be less important in controlling CaOx stone formation than in controlling formation of other stones such as struvite. Given the available information it seems prudent to avoid significant acidification of the urine. An appropriate initial target urine pH is approximately 7.0; however, with this in mind, the target urinary pH values achieved by feeding almost all of the diets listed in Table 197-1 are acidic. The only diet designed to produce a urinary pH of more than 7.0 is Hill’s Prescription Diet u/d, with the others aiming for an acidic pH. This is because the goal of the Royal Canin Urinary SO formulation and Hill’s Prescription Diet c/d Multicare is to prevent both CaOx and struvite uroliths. Since urine pH control is more important for dissolution and prevention in treating struvite stones than in treating CaOx stones, these diets target a lower pH as part of the strategy to prevent struvites. Despite causing a mildly acidic urine, all the listed diets are effective in producing urine with a low CaOx RSS. This apparent paradox exemplifies the complex nature of CaOx urolithiasis and the many factors must be considered in designing an appropriate diet. Of course, if CaOx uroliths continue to be a problem despite feeding an appropriate diet, alkalinization of the urine can be considered (see later) to potentially improve control.