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Calcium tartrate tetrahydrate stones

Minnesota Urolith Center photo

Calcium tartrate tetrahydrate stones are shown magnified. For scale, the marks at the bottom of the image represent millimeters.

The largest laboratory in the United States that analyzes urinary tract stones in veterinary patients is seeing a sharp rise in a rare stone type among dogs that might be attributable to their diets.

Researchers at the Minnesota Urolith Center have found the stone to be associated with consumption of a particular form of choline used in many fresh and homemade dog foods. Choline is an essential nutrient that is present in whole foods, such as liver and egg yolks, and is commonly added to pet foods as a powdered supplement.

The urolith center, which is part of the University of Minnesota College of Veterinary Medicine, identified 159 cases of dogs with these stones in 2025, more than five times what it saw the year before, according to Dr. Jody Lulich, the center's co-director. The stone type, composed of calcium tartrate tetrahydrate, or CTT for short, was recorded in a dog for the first time ever in 2008 in South Africa, Lulich said.

In the early 2020s, the Minnesota center tallied no more than 10 such stones in dogs each year. In 2023, the number rose to 11. Then, in 2024, to 28. The rise was eye-catching, prompting Lulich and colleagues to take a closer look.

Examining the cases of 29 affected dogs for whom they were able to obtain diet history, Lulich and team found a link between CTT stones and consumption of tartaric acid, mostly in the form of choline bitartrate — a source of choline favored for use in commercial fresh and homemade food for dogs. The research findings were published on April 28, 2025, in the Journal of Veterinary Internal Medicine.

While the study was small and only suggestive, not conclusive, about the provenance of CTT stones in dogs, one leading maker of fresh foods intends to stop using choline bitartrate and switch to a choline form without tartrate that has long been used in kibble and canned food — choline chloride. Just Food for Dogs has spent much of the past year working through the formulation adjustments needed to make the swap, according to Dr. Chris Margrey, the company's director of veterinary nutrition.

Recognizing that the number of stones detected is relatively low — a mere 0.2% of the 68,657 stones in dogs analyzed by the Minnesota Urolith Center last year — Margrey said he nevertheless felt that a change was called for, and company executives supported that decision.

"The general takeaway we got was, this seems like a thing of concern," Margrey said. "It's not a widespread concern for every single dog ... but there is potentially a high-risk subpopulation that this nutrient might pose problems for, and since we think we have a viable solution, let's make efforts to change it."

About tartaric acid

In veterinary toxicology, tartaric acid is known as the agent in grapes and raisins that, as identified in research published in 2022, is likely responsible for making the fruit poisonous to some dogs, potentially injuring the kidneys.

No one is saying that choline bitartrate is poisonous like grapes are to some dogs, but research suggests that other sources of tartaric acid might have health implications, depending on variables such as dose, form and body chemistry.

According to the study by Lulich and colleagues, tartaric acid was first identified as a component of urinary stones around 2001 in lab rodents that were fed a diet formulated with synthetic choline bitartrate. Analysis showed the stones were composed of CTT. Later, CTT stones turned up in four people who had taken a supplement with tartrate constituents. The stone type has not turned up in cats, at least not at the Minnesota lab. CTT is an unusual stone composition all around.

The Minnesota canine urolith study identified a source of tartaric acid in 86% of the 29 dogs for which it had diet information. Most were in commercial fresh diets or nutrient supplements added to homemade diets. (Tartrate was a listed ingredient in two medications consumed by some affected dogs, as well.)

Among 63 dogs found to have CTT stones between Jan. 1, 2020, and Dec. 31, 2024, the center reported that 84% were male and that the average age was just under 10 years. The breeds most represented were small: shih tzu, Pomeranian, Chihuahua, Maltese, Yorkshire terrier and miniature schnauzer.

Regardless of type, uroliths can be extremely painful, depending on their size, and they can lead to infection and harm the kidneys and other parts of the urinary tract. Stones that don't pass naturally might require surgical removal.

A history of mistaken identity?

It's possible that CTT stones aren't actually new in dogs but only newly recognized, Lulich says, noting that on the face, they resemble calcium oxalate stones, which are far more common. Moreover, according to the Minnesota paper, many dog breeds with CTT stones are the same ones at risk for calcium oxalate uroliths. It might also be the case that certain biochemical reactions result in either or both calcium oxalate and CTT stones.

Calcium oxalate stones (common)

Minnesota Urolith Center photo

Calcium oxalate stones, shown magnified, superficially resemble calcium tartrate tetrahydrate stones but are harder. The marks at the bottom of the image represent millimeters.

However, Lulich said technicians at the Minnesota lab have become proficient in distinguishing between the two. Calcium oxalate stones are "very hard," he said, "and the tetrahydrate stones aren't as hard. They get an idea when they crack it."

For positive identification, the lab uses an infrared spectrometer, which measures the interaction of infrared radiation with an object, enabling the user to determine the object's composition.

The Minnesota Urolith Center is not the only veterinary urolith laboratory in North America, but it is the largest, analyzing nearly 100,000 cases a year from around the world, a service that it provides free to veterinarians. (Its operation is supported by donations, including from the pet food company Hill's.)

A laboratory at the University of Guelph in Ontario has not seen any CTT stones in dogs to date among the approximately 10,000 cases it analyzes annually, according to its manager, Andrew Moore. The University of California, Davis, also operates a veterinary stone lab, but its director is out of the office and unable to provide up-to-date information.

The Minnesota center study identified seven commercially sold diets that were fed to dogs that had had CTT stones. A majority of the dogs consumed foods with choline bitartrate. It was identified in home-cooked diets and in varieties made by three brands: Just Food for Dogs, The Farmer's Dog and Nom Nom. (Lulich noted that not all varieties sold by the identified brands necessarily contain choline bitartrate.)

Fresh dog foods — generally identifiable as those needing refrigeration even in sealed packages — constitute a growing market in the U.S. currently worth nearly $1.8 billion and making up 8.5% of the dog food market as a whole, according to the trade group Pet Food Institute (PFI), citing data from the market research company Circana.

'A fulfilling journey'

When the Minnesota center paper about CTT stones came out last April, Margrey at Just Food for Dogs had just become aware of a dog with a history of CTT stones. The owners came to Margrey to create a custom diet, a service his company provides.

At the time, Margrey was mystified. While training to become a veterinary nutritionist, he'd never heard of CTT stones. Reading the new study over breakfast, he was both relieved to have an answer and dismayed to learn that the suspected factor is a choline source in common use.

"Thank goodness Dr. Lulich published this," he said. " ... I mean, what a way to start your morning, but I'm glad we're at least learning something new."

In the study's list of foods consumed by a sampling of dogs with CTT stones, Just Food for Dogs products reportedly were fed to 13 of 29 dogs whose diet histories were collected. "OK," Margrey remembers thinking, "we have some homework to do."

CTT and other urolith types in dogs

The challenge in eliminating choline bitartrate was to avoid introducing new problems or interfering with the palatability of the brand's nearly 20 formulations. Even though choline chloride has long been used in pet food, it has a characteristic that makes it challenging to handle, especially in fresh food. Namely, it absorbs moisture from the air — the technical term is "hygroscopic" — which causes the powder to clump.

"It turns into these very dense, hard rocks that you can't break up," Margrey said. "... It's just a block of nutrient powder. It's not tasty. It's not well distributed."

As Margrey and team experimented with the ingredient, they encountered more issues. Describing a sample kept in a vial on the counter, Margrey said: "Within a day, it clumped up and looked like a really big snowball. The next day, it was kind of like a rock. And the day after that, it turned green like Play-Doh and started smelling like rotten eggs."

That was early on. Over time, the group developed ways to work with choline chloride, taking care to use a human-grade product. The company has already been using choline chloride in a couple of formulations, Margrey said, but expanding to the rest is a tedious process. "It's all percentages. As you potentially need more of this, you need to figure out what other ingredient you can reduce while still making the diet work. That's a diet-by-diet decision as we go through the portfolio," he said.

Just Food for Dogs plans to introduce the reformulated products gradually; it hasn't yet determined when. Margrey is upbeat about the effort. "It's been a very fulfilling journey, I would personally say, as a veterinarian — getting to see this and evolve with it and learn with it and find ways to improve what we do every day," he said.

Watching and waiting

Other companies whose brands reportedly were consumed by dogs with CTT stones are taking a different tack. In an article on its website, The Farmer's Dog emphasizes the "infinitesimally low" prevalence of CTT stones and notes that "choline bitartrate's presence in dog food far exceeds its super-rare presence in bladder stones."

The company declined an interview with the VIN News Service. It emailed this statement:

"CTT stones are an extremely rare finding, and importantly, do not appear to pose greater health risk than calcium oxalate stones. Close to 70% of all uroliths are calcium oxalate or struvite compared to just 0.02% being CTT, and many of the dogs with CTTs were already at risk of developing oxalates. While CTT development continues to be investigated, preventing and reducing oxalates and struvites seems to deserve more attention given the overwhelming disparity in incidence rates."

The Farmer's Dog statement goes on to say that its formulations have a relatively low risk of promoting oxalate and struvite stones and crystals.

(For context, the 0.02% figure cited by The Farmer's Dog reflects the 2020-24 period. In 2025, CTT uroliths constituted 0.2% of canine uroliths analyzed by the Minnesota center.)

Royal Canin, a division of Mars Petcare that oversees the Nom Nom brand, did not comment but referred an inquiry from VIN News to the trade group PFI, of which it is a member. The statement reads in part:

"This study represents ongoing scientific research that PFI and U.S. pet food makers are watching and considering closely to best support the health and safety of dogs and cats through their diets."

Pondering the variables

Not every fresh dog food maker uses choline bitartrate. The first commercial brand on the market, Freshpet, has used choline chloride since the company's founding in 2006, according to Dr. Lisa Weeth, its head of veterinary research and communications.

On the Freshpet website, Weeth writes that choline chloride "is the more established, well-studied, and biologically reliable form of this essential nutrient" and that its tendency to absorb moisture from the air "does not affect the nutrient's stability or bioavailability."

At the same time, Weeth cautions that correlating CTT stones to choline bitartrate doesn't mean the supplement is causing the stones. "Additional controlled research is needed ..." she concludes.

In an interview, Weeth described personally encountering cases of CTT stones. Until last June, she practiced at a specialty referral hospital in Los Angeles, running its nutrition department for eight years. Owners of patients who'd had stones surgically removed were sent to her for counseling on dietary considerations for lowering the risk of forming more stones. When one patient came in with a history of CTT stones (which had been identified by the Minnesota lab), Weeth was stumped.

She said to the dog's owner, "I've never seen it before, which means it's a really rare stone type. So I don't know what to tell you in terms of recurrence."

That was last April. Soon after, Weeth read Lulich's study.

In addition to her day job, Weeth formulates home-cooked diets for pet owners through her own consultancy. She is very familiar with choline bitartrate, having used it for 22 years.

Upon seeing it associated with CTT stones, she thought: "Oh no, is there something about the diets we're creating that could be increasing the risk for some patients? Even though it's a small percentage, it's not zero, and if it's a diet-induced condition and we have the ability to prevent it, we should."

Then she pondered the constellation of factors that influence the formation of urinary tract stones. Take struvites, the most common stone type seen in dogs at the Minnesota center last year. Magnesium is a component of struvite stones, Weeth said, but its presence is not a problem in acidic urine. "These stones, these crystals, tend to form in more alkaline urine. So is there a urinary pH factor?" she mused. "If the urine is more acidic, does that increase the risk of the CTT crystals forming?

"If there are other compounds in the diet that interfere [with] or increase the absorption of calcium or excretion of calcium, is that a risk factor?" she wondered further. "We have to think about what other aspects of the diet could be influencing [stone formation], because it's usually not just one problem."

While such questions are as yet unanswered, Weeth still uses choline bitartrate in home-cooked diets, but she takes extra care to portion it conservatively.