Oxalate in pet food – is it dangerous for dogs and cats?

Everyone seems to have a friend or family member with a history of kidney stones. The dangers of dietary oxalate (or oxalic acid) are often discussed, and it’s something we get asked about a lot as nutritionists. We routinely include ingredients like leafy greens, berries and kiwifruit in our recipes, and some of these foods have been shown to be high in oxalate.

So, is this safe for dogs and cats? Should we be excluding all oxalate-containing ingredients from their diets? Does feeding oxalate-rich foods increase the risk of calcium oxalate bladder stones in our pets?

What is oxalate?

Oxalate, or oxalic acid, is an anti-nutrient present, commonly in trace amounts, in fruits, grains, nuts, fungi, and vegetables. No doubt about it, plant-derived ingredients are the main sources of oxalate in our diet.

Simple but potentially troublesome: an oxalic acid molecule

What’s also important to know though, is that mammals can also produce oxalates in small amounts. In this case, oxalate is produced endogenously (within the body), from ascorbate (vitamin C), hydroxyproline (an amino acid), glyoxylate (an intermediate in the metabolism of glycine), and glycine (another amino acid).

Research has shown that urinary oxalate mostly consists of endogenous oxalate, as opposed to exogenous dietary oxalate.

Are leafy greens bad? What foods are high in oxalate?

It is surprisingly hard to find reliable and consistent information on the oxalate content of many foods. This is because often there are differences between oxalate values for the same food, because these values can vary according to growth, ripeness, climate, region, soil conditions, and time of harvest. Sample preparation and analytical methods can also affect oxalate results.

Green leafy vegetables are considered high-oxalate foods. For example, published oxalate values are 329.6–2350 mg total oxalates per 100 grams of spinach, and 1458.1 mg total oxalates per 100 grams of swiss chard. Green kiwifruit contains from 12.7 to 84.3 mg total oxalates per 100 grams, and raspberries, almonds, potatoes, sweet potatoes and carrots are all moderate to high in oxalate.

Cabbage, broccoli, cauliflower, cucumber, kale, and pumpkin have all been shown to be low in oxalate, and many fruits like apple, strawberries, banana and melon are as well.

White rice and corn are low in oxalates; many whole grains like buckwheat, brown rice and quinoa are moderate to high. If your dog has a history of calcium oxalate stones and you are feeding a homemade diet, it's really important to make sure it is properly formulated by a Board Certified Veterinary Nutritionist, as dietary factors can worsen the risk of recurrence.

Why can oxalate be a problem?

In human medicine, oxalates have been a potential concern for a long time due to their anti-nutritive effects and potential nephrotoxicity. As anti-nutrients, oxalates can restrict the bioavailability of some nutrients since they can bind to minerals, reducing their absorption and use. Absorbed dietary oxalates may contribute to calcium oxalate kidney stone formation in some people; although research on this is quite conflicting.

But what about dogs and cats? Let’s consider them separately, since the research shows some different things.

Is it safe to feed my dog foods that contain oxalate?

For healthy dogs with no history of urinary stones, it's fine to feed small amounts of foods that contain oxalate. Even better if you combine this with a high moisture, complete and balanced diet, as this may help to reduce any risk.

For dogs with a history of calcium oxalate stones, the answer is less clear. However, research has shown that for some dogs, reducing oxalate in the diet may be a sensible precaution, especially if dietary calcium is low.

Calcium can play an important role in reducing absorption of calcium in the gut, by binding oxalate and increasing its loss in stools. This is reflected in human research, which shows that consuming dairy products with oxalate-rich foods like spinach can reduce the availability of oxalate.

It is far too simplistic to just blame dietary oxalate as the sole cause of calcium oxalate stones in dogs though. There are many other factors that can contribute, and these can relate to the individual animal (e.g. genetic factors), or the diet (e.g. the water content of the food).

New research has shown that abnormal vitamin D metabolism is some dogs may contribute to stone risk, and another study demonstrated that the microbiome may play a role, with Acinetobacter overrepresented in stone formers (and is associated with calcium oxalate in humans). Research is definitely ongoing; it will be great to see what further potential causes or solutions are discovered.

Is it safe to feed my cat foods that contain oxalate?

We sometimes have cat owners react with horror when they see that our recipes often include a tablespoon of finely chopped fresh leafy greens like kale, spinach, silverbeet or even cat nip.

So, the key question is, does feeding oxalate-rich foods to a cat increase oxalate in the urine?

Short answer: No.

An important study by Dijcker et al. (2014) showed that increasing oxalate intake from 13 to 93 mg/100 g dry matter basis did not affect urinary oxalate excretion but instead resulted in an increase in faecal oxalate output (the cats pooped it out).

What matters much more for cats in terms of the risk of calcium oxalate stones is the moisture content of the diet (high moisture diets dilute the urine, reducing the concentration of compounds that form crystals), encouraging drinking, feeding a diet that contains omega-3 fatty acids, and potentially feeding a high protein diet.

Additionally, routine bloods are a sensible idea, especially for older cats. Elevated calcium in the blood (hypercalcemia) can increase the risk of calcium oxalate stones, so it is important to diagnose and treat this, if present.

So, what's the plan?

1. For dogs and cats with a history of calcium oxalate bladder stones: minimise oxalate in the diet. Even though oxalate in the diet may not affect cats as much, it's still a sensible precaution. This can be achieved by including low oxalate fruits, vegetables and grains in the diet, such as: White rice, pumpkin, cauliflower, broccoli, cabbage, capsicum and kale. Fruits that are safe to include are: apples, pears, bananas, strawberries, blueberries, melon, papaya, mango, peaches, nectarines and plums. All meats and fish are fine; oxalate is only found in plant-derived ingredients. Again, make sure the diet is properly formulated, or feed a commercial urinary diet (as per your vet's recommendations). For treats, be careful to avoid treats with plant-derived ingredients (grains, vegetables, nuts, seeds). Meat and seafood-derived treats are safest.
   

2. For healthy dogs and cats: there is no need to avoid leafy greens or higher oxalate fruits like raspberries. These ingredients can provide lots of beneficial nutrients and fibre, and are typically only included in the diet in small amounts anyway. Additionally, vegetables are rich in potassium, magnesium, and phytate, which all decrease kidney stone formation through an array of mechanisms.
   

3. Ensure any diet (commercial or homemade) is complete and balanced, with adequate calcium. Feeding a calcium-deficient diet may increase oxalate absorption from the gut.
   

4. Depending on preference, leafy greens for homemade diets can be boiled or steamed. Steaming has a lesser effect than boiling, though it has been shown to reduce oxalate in green Swiss chard and spinach by 46% and 42%, respectively. Grains can be soaked before cooking. This may help to reduce the amount of soluble oxalates.
   

5. Routine bloods in cats may help to detect hypercalcemia, which is a risk factor for calcium oxalate urolithiasis. It’s important that ionised calcium is checked if total calcium is elevated.

Thank you for reading! If your dog or cat has calcium oxalate stones and you need help with diet, or if you have any questions, please get in touch: info@vngpets.com

Did you know? High oxalate pasture can also cause problems for horses, by binding to calcium in the gut and causing calcium deficiency. This can lead to oxalate toxicosis or 'big head' disease.