Stubborn stones: calcium oxalates

Next to struvite stones, calcium oxalates are the second most common type of stone found in dogs, and with struvites make up 85% of all uroliths found in dogs.

Unfortunately for nutrition nerds like myself, calcium oxalate stones cannot be dissolved like struvites can and need to be surgically removed. But does that then mean nutrition can’t help calcium oxalate stones? Although we can’t dissolve them, there’s a number of things we can do to prevent calcium oxalates from forming and once we’ve surgically removed them, stop them from ever forming again.

How does it happen?
Calcium oxalate stones form in an acidic and supersaturated environment. Often a urinary tract infection can trigger the formation of stones in dogs as the presence of bacteria in the bladder can upset the delicate urine pH. Unfortunately, a lesser known causes of calcium oxalates is the overuse of antibiotics! So when treating a recurrent urinary tract infection, we may inadvertently contribute to the issue. Antibiotics reduce the numbers of the bacteria Oxalobacter formigenes whose sole source of nutrition is oxalates. In dogs with low numbers of this bacteria, large amounts of oxalates are excreted into urine which increases the likelihood of developing calcium oxalate stones.

There are other risk factors too; male dogs are more prone to forming calcium oxalates than females, and breeds such as Shih Tzus, Bichon Frise, Miniature Schnauzer and Yorkshire Terriers are commonly affected.

The pet’s diet also plays a huge role in both the cause, and prevention of stones. Diets that are acidifying, contain excessive amounts of protein, sodium or calcium, have a low moisture content or contain ingredients that are high in oxalates (a particular concern if clients are homecooking or feeding a fresh food diet and using fruits or vegetables that have a high oxalate content). In addition to this, if clients are supplementing with calcium, vitamin D, vitamin C, magnesium or an ingredient that interferes the body’s absorption or excretion of these nutrients, this can also increase the pet’s likelihood of developing calcium oxalate urolithiasis.

Small fluffy breeds like Bichon Frise and Shih Tzu are overrepresented when it comes to forming calcium oxalate stones

Nutrition fun facts

• Uroliths sent to University of Minnesota Urolith Center from 1981 to 2007 have shown an increase in calcium oxalate content with them now being the most predominate stone type at 60% of all stones submitted. Looking at diets from 1981 to 2007, the trend towards high protein diets has grown (and continues to grow) which has likely impacted the health of our pets. This is another reason I caution against high protein diets – see my blog post on the topic here.

• Pyroxidine (aka Vitamin B6) deficiencies promote the production of oxalic acid. Pyroxidine increases the transamination of glyoxylate (an important precursor to oxalic acid) to glycine, thus reducing the level excreted in urine. Commercial diets are fortified with Vitamin B6 so would not be deficient, whereas a homecooked diet is likely to be deficient if a multivitamin supplement is not included.

• We still don’t really know the mechanism behind the formation of calcium oxalate. It is thought that within the next ten years we will finally be able to identify and modify the underlying mechanisms involved in calcium oxalate urolithiasis and create more effective ways of treating them.

So what can nutrition do?
We all know we can’t dissolve calcium oxalate stones like struvites, so then what is the role of nutrition?

Sometimes, urinary stones are actually compound stones. This means, the stone may not be entirely calcium oxalate but a combination of struvite or some other type of stone. If the stone is identified as likely to be a compound stone (usually from x-ray imaging), the stone may actually be shrunk slightly before the procedure if it’s coated in a struvite “shell”. This can help relieve some of the discomfort associated with urolithiasis and make it easier to remove during surgery. Smaller stones can also be passed, further relieving some of the symptoms. Placing a pet on a dissolving diet for a minimum of six weeks is generally recommended to help reduce the size of the stones, if a compound stone is suspected. A repeat x-ray can help confirm the effectiveness of the diet.

Recurrence of calcium oxalates was observed in 25% of dogs after surgical removal of the stones, this also increases with every year after surgical intervention; 100% of pets four years after their surgery were observed to have a recurrence (Lulich et all, 2004). So, the prevention of recurrence is very high on the list of priorities when managing calcium oxalate stones.

The goals of dietary management are:

1. Reducing the calcium content of urine
2. Reducing oxalic acid in the urine
3. Promoting high concentration and activity of the inhibitors of calcium oxalate crystals growth in urine
4. Reducing the concentration of urine

The following Key Nutritional Factors for the prevention of calcium oxalate uroliths are listed below:

Key Nutritional Factors in the prevention of calcium oxalate stones (Source: Small Animal Clinical Nutrition)

What should I be looking for in a diet?

For pets that are an at risk cohort, my recommendation is always first and foremost to feed a complete and balanced diet from a reputable science-based company that research their diet formulations, and avoid any additional supplementation. In my experience, this is the most protective thing you can do to prevent calcium oxalates occuring at all. Many of the patients I’ve seen with calcium oxalates have been fed homecooked diets or some form of alternative or high protein diet.

Due to the very high probability of recurrence, it is recommended that these patients remain on a therapeutic diet for life. Foods designed to dissolve struvite stones can be beneficial, but they actually can present some significant risks to patients with calcium oxalate stones; given these diets are designed to create an acidic urinary pH and calcium oxalates form in an acidic urinary pH, it may not be practical to use these diets. In addition, reduction in the phosphorus content of the food, which is another feature of acidifying urinary diets, this can create hypercalciuria thus again increasing the potential to form calcium oxalates.

So, when looking for a diet for calcium oxalates, you want to select a diet that matches the above Key Nutritional Factors. In conjuction with blood testing and counteracting any nutrient deficiencies or excesses that may already be present, selecting a diet that meets these requirements is important to manage the likelihood of relapse. Diets that are designed specifically for urinary stones such as Hills Prescription Diet U/d are typically the first choice, but it’s important to monitor the pet closely for signs of protein deficiency as the diet is low protein. You can also use diets designed for kidney disease, as these are also lower in protein and elicit a slightly alkaline urine – ProPlan NF or Royal Canin Renal are also good choices.

If the client wants to home cook for their pet, I’d recommend advising them of the importance of ongoing monitoring; it’s a good idea to check the urinary pH daily (or multiple times per day), and to have annual blood testing to ensure there aren’t any deficiencies or excesses that can increase the pet’s risk of oxalates. It’s important that a low purine diet recipe is used from a Board Certified Veterinary Nutritionist, and specific ingredients that are very high in oxalates are avoided if the owner is creating a recipe through or some other formulation software (see table below).

Human foods to avoid or limit in patients with calcium oxalate uroliths (Source: Small Animal Clinical Nutrition)

What else can I do?

It’s important to also consider treats and supplements in your management of these patients. The high sodium content of commercially available treats have been associated with calcium oxalate uroliths in pets, so if owners want to include treats in the diet, I recommend using low oxalate vegetables or fruits or the pet’s own food as treats.

As for supplements, the only additive I’d recommend for these patients would be a probiotic. Urinary tract infections, particularly in female dogs, often occur as a result of ascending infection from the gastrointestinal tract. Probiotic supplements that are designed to out-compete this microbe can prevent the recurrence of infections which can help reduce stone formation. As mentioned above regarding the overuse of antibiotics impacting the pet’s helpful microbiota that feed off oxalates, a probiotic would be a very good idea in these pets. The products I typically recommend for these pets are CystoPro and SynBiotic DC.

What do you find challenging when treating calcium oxalate stones? Have you ever had a pet or patient with these stones? Let us know your experience below!

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Small Animal Clinical Nutrition, Chapter 40 – Canine Calcium Oxalate Urolithiasis: Changing Paradigms in Detection, Management and Prevention.