Costco Is Being Sued Over Its Nature's Domain Grain-Free Dog Food — And Every Dog Owner Needs to Pay Attention
If you are one of the millions of American men who buys dog food at Costco — tossing a massive bag of Kirkland Signature Nature's Domain grain-free kibble into the flatbed cart alongside your case of sparkling water and your rotisserie chicken — this lawsuit is for you. A newly filed proposed class action alleges that Costco has spent years marketing a product as healthy and nutritious while a growing body of scientific evidence suggests it may be quietly damaging dogs' hearts. The stakes could not be more personal: this is about the animal sitting at your feet right now.
The class action suit alleges that Costco has deceptively marketed its Kirkland Signature Nature's Domain grain-free dog food as healthy and safe, despite scientific research suggesting that grain-free diets high in legumes and pulse ingredients may increase a dog's risk of dilated cardiomyopathy (DCM), a fatal heart disease. The complaint is 51 pages long and pulls no punches about what it believes Costco has done and, more importantly, what it has failed to do for its customers.
What the Lawsuit Actually Claims
The lawsuit was filed in federal court for the Western District of Washington by California resident Taylor West, who claims he purchased the grain-free dog food for his dogs because Costco advertised it as a healthy choice. West's experience is far from unique — millions of Americans have made the same decision at the same warehouse, drawn in by the same reassuring packaging and the implicit trust that comes with the Kirkland Signature brand.
West discovered that the dog food, in addition to other grain-free brands, has been associated with dilated cardiomyopathy in dogs, and that there is "no medical or scientific justification to feed a dog a grain-free diet high in legume or pulse ingredients," his attorneys wrote in the complaint. Four different law firms, including Seattle-based Tousley Brain Stephens, represent West in the lawsuit.
The product packaging markets Nature's Domain grain-free dog food as "formulated to meet the nutritional needs" of dogs and says it provides "great nutrition for overall health and vitality of all dogs," according to the claim. That is the kind of language that sells a product — and, the plaintiffs argue, the kind of language that locks a consumer into a false sense of security about what he is actually putting in his dog's bowl every morning.
The 51-page complaint contends that Costco has failed to disclose the "contributory and causal connection" between grain-free diets and the development and exacerbation of DCM while continuing to advertise the Nature's Domain products as nutritious and safe, particularly for dogs with food sensitivities.
The suit further alleges that Costco has not provided any supporting evidence for its pro-grain-free diet claims, ostensibly because it has yet to conduct adequate feeding studies to assess the safety or efficacy of the Nature's Domain dog food. That is an especially damning allegation. Asking consumers to trust a health claim without having done the science to back it up is, at a minimum, irresponsible. In the context of a product that may be linked to a fatal cardiac condition, the plaintiffs argue it crosses into something far worse.
"No reasonable consumer would expect that Nature's Domain grain-free dog food marketed as healthy and safe would pose a risk to their dogs' cardiac health, safety, and well-being, or that it would cause or contribute to a fatal heart condition," the complaint stresses.
The lawsuit emphasizes that pet owners have paid a price premium for products they believed were safe, healthy, and supported by science when they otherwise might have chosen a different dog food. The filing also charges: "In misrepresenting its product and failing to inform consumers of the implications of consuming Nature's Domain grain-free dog food, while simultaneously marketing the products as safe and healthy, Defendant abuses the public's trust."
What Is Dilated Cardiomyopathy, and Why Should It Terrify You?
According to the complaint, dilated cardiomyopathy is a heart disease that causes enlargement and weakening of the left ventricle, which reduces the heart's ability to pump blood effectively. For a dog, that slow mechanical failure is as grim as it sounds. The heart labors harder and harder to compensate for its own diminished capacity, often with no obvious outward signs for months.
DCM is a type of canine heart disease that affects the heart muscle. The hearts of dogs with DCM have a decreased ability to pump blood, which often results in congestive heart failure. When that failure arrives, it can arrive fast — and it can arrive without warning.
According to the lawsuit, DCM was historically believed to be a primarily inherited condition mainly affecting certain large dog breeds, such as Great Danes, Irish wolfhounds, and Doberman pinschers. Some breeds, especially large and giant breeds, have a predisposition to DCM. These breeds include Doberman Pinschers, Great Danes, Newfoundlands, Irish Wolfhounds, and Saint Bernards. While DCM is less common in medium and small breeds, English and American Cocker Spaniels are also predisposed to this condition. In other words, if you own a mixed-breed dog, a Golden Retriever, a Lab, or a Bulldog, DCM should historically have been the furthest thing from your veterinarian's worry list. That assumption began to unravel around 2018.
In 2018, veterinarians noticed that an increasing number of dogs were showing signs of dilated cardiomyopathy. This condition occurs when the heart muscles weaken and can't pump blood as effectively throughout the dog's body. These dogs didn't belong to breeds known to be at risk for DCM, but they did seem to have one thing in common — they were eating either grain-free diets or "boutique" diets, often with unusual, grain-free, or legume-rich ingredient lists.
From January 2018 through April 2019, the FDA received reports of 553 dogs with DCM, compared with previous years, in which reports received of dogs with DCM ranged from zero to three. That is an extraordinary spike in adverse event reports in any regulatory context. For veterinary cardiology, it was a five-alarm signal.
The Disease's Devastating Progression
In light of increased reports of diet-induced DCM, veterinary cardiologists began promulgating warnings against such diets and recommending immediate feeding changes to reduce the risk of disease progression. According to the complaint, a growing body of peer-reviewed research has not only linked grain-free diets to an elevated risk of DCM and related cardiac damage, but has also shown that dogs diagnosed with DCM experienced improvements in heart function after being switched off a grain-free diet and to a traditional one. That latter finding carries weight: if heart function improves upon diet change, the diet connection becomes difficult to dismiss as pure coincidence.
Unlike primary DCM, which seems to run in certain breeds and has a hereditary component, the cases being reported are consistent with secondary DCM, which is associated with nutrition. These diet-associated cases can occur in any breed of dog. In addition, they can sometimes be reversed if caught in time simply by changing the dog's diet. Unfortunately, the improvement may take a very long time, and sometimes dogs that seem to be improving die suddenly. That last sentence describes the experience of real dog owners who thought they had caught the problem in time — and then didn't.
The FDA Investigation: Eight Years of Scrutiny
In July 2018, the FDA announced that it had begun investigating reports of canine dilated cardiomyopathy in dogs eating certain pet foods, many labeled as "grain-free," which contained a high proportion of peas, lentils, other legume seeds (pulses), and/or potatoes in various forms as main ingredients, listed within the first 10 ingredients in the ingredient list, before vitamins and minerals.
Many of these case reports included breeds of dogs not previously known to have a genetic predisposition to the disease. That is what put the FDA on alert. Golden Retrievers, Labrador Retrievers, Shih Tzus, Bulldogs — these are the dogs of American households, the mutts and mixes sitting on couches across the country, and they were turning up with a disease that was supposed to belong to a completely different category of animal.
In the FDA's July 2019 update on diet and canine heart disease, they examined labels of dog food products reported in DCM cases to determine whether the foods were "grain-free" and whether the foods contained peas, lentils, chickpeas, beans, or potatoes. Their report states that more than 90 percent of foods reported in DCM cases were grain-free, 93 percent of reported foods contained peas and/or lentils, and 42 percent contained potatoes/sweet potatoes.
According to the complaint, FDA data demonstrated that Nature's Domain grain-free dog food was ranked the seventh most commonly cited brand in DCM reports — a specific, damning detail that puts Costco's house brand squarely in the middle of a federal health investigation.
The FDA found 16 dog food companies that had ten or more cases of DCM associated with their food. More than 90% of the diets were grain-free, and 93% of the diets contained peas or lentils. A far smaller proportion contained potatoes.
The Science Gets Complicated
The FDA's investigation did not produce a clean, unambiguous verdict, and that complexity is important to understand. Based on the data collected and analyzed, the agency believes that the potential association between diet and DCM in dogs is a complex scientific issue that may involve multiple factors. One major area of focus has been taurine, an amino acid critical to cardiac function.
Nutritional research indicates that taurine is an essential amino acid for cats, but it is generally not considered an essential amino acid for dogs, because dogs are able to synthesize taurine from dietary cysteine and methionine. Golden retrievers and American cocker spaniels, however, can develop DCM secondary to taurine deficiency. Taurine deficiency does not appear to be the primary cause of the diet-related cardiomyopathy the FDA is investigating.
When these foods were tested, they contained the same average percentages of protein, fat, taurine, and taurine precursors as products containing grain. That finding complicated the narrative considerably — the problem, if there is one, may not be as simple as a taurine shortage. In 2021, Dr. Freeman and fellow researchers published a study comparing diets associated with DCM and diets not associated with DCM. They found that the inclusion of peas not only represented the greatest difference between the two diets, but their results indicated that peas were also associated with higher and lower concentrations of certain compounds compared to the diets not associated with DCM.
The FDA published a Q&A after a 2020 scientific forum stating that "emerging science appears to indicate that non-hereditary forms of DCM occur in dogs as a complex medical condition that may be affected by multiple factors such as genetics, underlying medical conditions, and diet." Since July 2018, a substantive number of peer-reviewed research papers have been published on the topic. However, a specific cause of non-hereditary DCM in dogs still has not been identified.
A Disputed Investigation With Industry Complications
Not everyone accepts the grain-free-to-DCM connection at face value, and it would be irresponsible to leave that out. According to one update on the FDA's website, there is no scientific evidence to support the claim that feeding your dog grain-free or BEG (boutique, exotic ingredient, grain-free) diets puts them at increased risk for dilated cardiomyopathy. Critics of the investigation have raised concerns about how it was conducted and who shaped its direction.
An investigation by 100Reporters found that veterinarians who prompted the FDA to consider diet have financial and other ties to the leading sellers of grain-inclusive pet foods. Additionally, agency records show that for the initial study, some vets were instructed to submit only DCM cases that implicated grain-free, "exotic," or "boutique" pet foods. Those are serious allegations about the integrity of the data that launched a multi-year federal investigation — and they deserve scrutiny alongside the lawsuit itself.
The FDA's report states that grain-free diets generally contain high levels of legumes, pulses, or potatoes, but that it is not known if there is any connection between these ingredients and canine heart disease; the report cautions that the correlation might simply be a result of the increasing market share of grain-free dog food products. That caveat matters. Correlation is not causation, and the FDA has been careful not to overstate its findings.
Most of the diets associated with the reports of non-hereditary DCM have legume seed ingredients, also called "pulses" — peas, lentils, etc. — high in their ingredient lists, although soy is a legume that did not generate a signal. These include both grain-free and grain-containing formulations. Legumes, including pulse ingredients, have been used in pet foods for many years, with no evidence to indicate they are inherently dangerous, but analysis of data reported indicates that pulse ingredients are used in many grain-free diets in greater proportion than in most grain-containing formulas.
The Price Premium Problem: What Costco Customers Actually Paid For
There is a straightforward consumer protection argument at the heart of this case that has nothing to do with whether science has definitively proven that grain-free food causes DCM. The argument is this: Costco charged a premium for a product it claimed was safer and healthier than conventional dog food. That premium was justified, on the label and in the marketing, by health claims. If those health claims are not supported by evidence — if the company never ran the feeding studies needed to back them up — then customers were overcharged for something they never actually received.
The lawsuit emphasizes that pet owners have paid a price premium for products they believed were safe, healthy and supported by science when they otherwise might have chosen a different dog food. This is not a trivial grievance. Grain-free formulas have historically commanded meaningful price premiums over standard kibble, and for many buyers, the purchase was motivated entirely by a belief that they were doing the best thing for their dog's health.
The class action suit stresses that because there exists no established scientific or medical justification to feed dogs grain-free diets, pet owners are "needlessly exposing their dogs to cardiac risks," while Costco has allegedly failed to conduct any clinical feeding studies to substantiate its grain-free marketing claims.
This Is Not Costco's First Rodeo With Nature's Domain
The DCM-related lawsuit is not the first time Kirkland Signature Nature's Domain grain-free food has found itself at the center of legal trouble. A separate, earlier class action raised a different and equally striking allegation: that the food labeled "grain-free" was not actually grain-free at all.
A prior proposed class action claimed that the supposedly "grain-free" Nature's Domain dog and puppy food sold under Costco's Kirkland brand may contain significant amounts of wheat and other fillers that consumers would never expect to be in the products. According to that case, an independent analysis of the products' ingredients found "significant amounts" of wheat that by any scientific standard would be considered more than just a trace amount.
Costco faced a class action from consumers who claimed Kirkland's Diamond pet foods were misleadingly marketed as "grain-free." Despite being marketed as "grain-free" and containing "limited ingredients," Diamond pet food products sold under Costco's Kirkland brand allegedly contained "cheap fillers." Independent testing done by the plaintiffs allegedly revealed that the Kirkland products contained "material amounts of wheat." Although the exact testing method or details about how much wheat was found in the products were not disclosed by the plaintiffs, the purchasers note that their testing used the "industry standard" DNA testing to detect the wheat levels.
The earlier case was filed against Costco Wholesale Corporation and Schell and Kampeter, Inc., which does business as Diamond Pet Foods Inc. The involvement of Diamond Pet Foods is worth noting: it is a major private-label manufacturer, and the relationship between a retailer's brand and its contract manufacturer is one that rarely gets scrutinized in the marketing materials handed to consumers.
The Grain-Free Boom and the Pet Industry's Reckoning
To understand how we got here, it helps to understand the cultural moment that made grain-free dog food a multi-billion-dollar category in the first place. The grain-free trend in pet food closely tracked the human wellness movement — Paleo diets, gluten-free eating, ancestral nutrition philosophies. Pet food brands positioned grain-free as more "natural," more aligned with what a dog's wild ancestor would supposedly eat. That story was compelling, and it sold.
Overall, the pet industry brings in more than $74.6 billion per year, with natural and grain-free foods a growing trend. The grain-free segment alone has represented hundreds of millions of dollars in sales annually at peak, making this not just a health story but an enormous market story. Brands — including Costco's house label — built entire product lines around this positioning and, critics now argue, failed to subject those products to the rigorous nutritional science that the marketing implied.
In the Journal of the American Veterinary Medical Association, Dr. Lisa M. Freeman, DVM, Ph.D., DACVN, provided an update to the research on DCM and emphasized the issue is not just grain-free diets. She calls the suspected diets "BEG" diets — boutique companies, exotic ingredients, or grain-free diets. "The apparent link between BEG diets and DCM may be due to ingredients used to replace grains in grain-free diets, such as lentils or chickpeas, but also may be due to other common ingredients commonly found in BEG diets, such as exotic meats, vegetables, and fruits," Freeman wrote.
Within months of the FDA announcement, the story had simplified dramatically: veterinarians warned clients off grain-free diets, brands lost market share by the hundreds of millions of dollars, and a generation of dog owners began associating peas and lentils with cardiac death. That simplification is now itself a subject of debate, but for practical purposes, it reshaped an entire industry segment — and left millions of pet owners in the middle, unsure whom to trust.
What Veterinary Cardiologists Are Saying
Dr. Steven Rosenthal, a veterinary cardiologist and co-founder of Chesapeake Veterinary Cardiology Associates, has noted that diet formulations often change, which makes evaluating patients for diet-associated DCM a "constant battle with this moving target." He has also observed encouraging signs: "From my personal impression, I believe the incidence of disease related to DCM has reduced over the past couple of years, and there's a number of reasons why that may be the case." He suggested reasons could include changes in the diets themselves, increased public awareness, and reduced frequency of feeding diets associated with this disorder.
Overall, reports of DCM, which had totaled 1,382 dogs, have diminished, with only 255 cases having been reported since July 2020. It's not known whether the decreased amount is because fewer dogs are being affected — most of the diets previously labeled "at fault" have since changed their formulations — or because people have ceased reporting cases as the publicity has worn off.
The AKC's Chief Veterinary Officer has framed the issue with appropriate measured concern: "At this time, there is no proof that these ingredients are the cause of DCM in a broader range of dogs, but dog owners should be aware of this alert from the FDA. The FDA continues to work with veterinary cardiologists and veterinary nutritionists to better understand the effect, if any, of grain-free diets on dogs."
What You Should Actually Do Right Now
The honest answer is not to panic, but it is also not to do nothing. The science remains genuinely unsettled, the FDA's investigation has been ongoing for years without a definitive finding, and a lawsuit is an allegation — not a verdict. But several practical steps make sense regardless of how the litigation resolves.
If your dog is eating a grain-free or non-traditional diet, talk to your veterinarian and ask if your dog should be referred to a board-certified veterinary cardiologist for an evaluation of heart muscle function, a test called an echocardiogram. That is especially true if your dog is a breed with any known cardiac predisposition, or if you have been feeding a high-legume, grain-free diet for an extended period.
Keep in mind that the most rigorous test of nutritional adequacy is a feeding trial. Not all foods undergo this type of testing, but the label on the bag or can will indicate the testing method used. If your current dog food has not undergone a feeding trial — or if it cannot demonstrate that it has — that is worth factoring into your next purchase decision.
Because every dog is unique and has their own nutritional needs, your veterinarian is in the best position to advise you on how to best feed your dog. Consult your veterinarian before making a change in your dog's diet. That is not boilerplate caution — it is genuinely the best available guidance given how much the science is still in flux.
The Bigger Picture: Trust, Transparency, and Pet Food Labels
This lawsuit, whatever its ultimate outcome, raises a broader and lasting question about the standards American consumers have a right to expect from pet food manufacturers — particularly those operating under the banner of a brand as trusted as Kirkland Signature. Costco has built an extraordinary amount of loyalty on the promise that its house brand products represent value without compromise. That promise is what brought millions of dog owners to Nature's Domain in the first place.
The filing charges: "In misrepresenting its product and failing to inform consumers of the implications of consuming Nature's Domain grain-free dog food, while simultaneously marketing the products as safe and healthy, Defendant abuses the public's trust." Whether a jury ultimately agrees with that characterization or not, the allegation itself signals something important: health claims on pet food packaging carry real moral weight, because the animal consuming those products has no voice in the matter. Its owner does — and that owner trusted the label.
The men who shop at Costco, who buy in bulk, who pride themselves on making smart decisions for their households, deserve complete information when they make those decisions. A dog is not a line item in the budget. For most of us, he is a member of the family. That is exactly why the gap between what is printed on a bag of dog food and what the science actually supports is not a minor marketing grievance — it is a question of character, and of accountability.
