While there is yet no conclusive evidence that dog foods high in legumes or potatoes cause the heart condition known as dilated cardiomyopathy, some dogs diagnosed with DCM have improved simply by changing their diet, according to a report posted today by the U.S. Food and Drug Administration.

However, the phenomenon of pets — chiefly dogs, plus a few cats — developing DCM possibly because of what they eat is a problem not given to one easy solution.

"Based on the data collected and analyzed thus far, the agency believes that the potential association between diet and DCM is a complex scientific issue that may involve multiple factors," the FDA said in the update on its investigation.

The agency announced in July that it was looking into a possible connection between diet and heart disease in dogs. The suspect diets are formulations containing as main ingredients peas, lentils or other legumes or seeds of legumes; or potatoes. Some, but not all, are labeled "grain-free."

In its update, the FDA says that between Jan. 1, 2014 and Nov. 30, 2018, it's received reports of 325 dogs and 10 cats diagnosed with DCM. (The agency notes that the tally does not include reports received in December and January owing to the partial federal government shutdown that occurred Dec. 22 to Jan. 25.)

The figures include 74 dogs and two cats that have died.

DCM is a disease in which the heart muscle weakens and cannot pump blood efficiently.

Dogs with DCM may tire quickly, cough, or struggle to breathe. They may have episodes of sudden weakness, fainting or collapse, or die suddenly.

DCM is recognized as a genetic condition in particular dog breeds — doberman pinschers, great Danes, Irish wolfhounds and cocker spaniels among them. But the illness suspected to be linked to diet has been found in dogs of a variety of atypical breeds.

According to the FDA update, the affected dogs range in age from puppies to seniors, weighing from eight pounds to 212 pounds.

The affected cats range in age from kittens to seniors, weighing seven to 13 pounds.

And while the common dietary thread is legumes, seeds of legumes (called pulses) and potatoes as the main ingredients, no single brand or formulation has been found to blame. Instead, implicated foods "include both grain-free and grain-containing diets in all forms (kibble, canned raw, home-cooked)," the FDA reports.

In pet foods sold commercially, the agency considers "main ingredients" to be those listed before the first vitamin or mineral ingredient.

In most cases, the implicated foods are dry formulations, but there are a few instances involving semi-moist and wet foods.

In 196 reports of DCM, dogs were fed only a single diet. Of those diets, 90 percent were said to be labeled as "grain-free." The other 10 percent ate diets containing grains, some of which were vegan or vegetarian.

In answers to frequently asked questions posted by the FDA, the agency declines to identify which commercial diets haven't been linked to DCM.

"FDA does not have a comprehensive list of all foods on the market and ... the investigation has not yet identified a root cause for the reports of DCM," it explains. "... Additionally, any reports of illness thought to be connected to food products are voluntary. We rely on pet owners and veterinarians to provide reports of illness, as well as clinical evidence to help document the case."

Another complicating aspect of the investigation is the role of the amino acid taurine.

In the 1980s, researchers found DCM in cats to be associated with taurine deficiency; supplementing with taurine reversed the disease. Subsequent research found that certain breeds of dogs appeared prone to taurine deficiency, as well, and that supplementation likewise was helpful.

With the current cases of DCM, taurine appears to play a role sometimes, but not always.

Researchers in the FDA-coordinated Veterinary Laboratory Investigation and Response Network found that 37 dogs and two cats with DCM had low taurine levels. However, 29 dogs with DCM had normal or high taurine levels.

"According to recheck echocardiograms in the medical records," the FDA says, "some pets with DCM improved after veterinary treatment, diet change, and taurine supplementation, while others improved with appropriate veterinary care and diet change alone."

As an example, researchers offer the case of a 3-year-old male beagle mix who presented with a cough of six weeks that was treated with 30 days of doxycycline, an antibiotic.

"The dog had been eating a limited ingredient grain-free diet containing a novel protein source and six legume-based ingredients. The whole blood and plasma taurine levels were above normal reference values," the researchers report. (Novel protein sources are something other than the usual chicken, turkey or beef; meats such as kangaroo, venison or alligator.)

The dog's first echocardiogram showed classic features of DCM found in measurements of the heart's ability to pump blood efficiently. After two years on a chicken-and-rice diet, multiple measures of the dog's cardiac function showed marked improvement. The numbers suggest the patient had gone from having severe DCM to having a normal heart.

The FDA said it also is investigating whether changes in the heart short of DCM are part of a disease continuum or have other important significance.

In a paper, "Diet-associated dilated cardiomyopathy in dogs: what do we know?" published Dec. 1 in the Journal of the American Veterinary Medical Association, researchers from the veterinary schools at Tufts University, the University of California, Davis, the University of Illinois and North Carolina State University discuss the complicated nature of pet food and implications on health.

"The complexity of pet food manufacturing is often underestimated," they write. "Pet foods must contain all required nutrients in the right amounts and right proportions. Nutrient standards (minimums and, for some nutrients, maximums) are established by the Association of American Feed Control Officials. However, the effects of processing (or not processing) the ingredients must also be considered, along with nutrient bioavailability and the effects of all other ingredients in the food. Unfortunately, this may not always be done. In addition, extensive testing is needed on an ongoing basis to ensure rigorous quality control. Inclusion of exotic ingredients, such as kangaroo, alligator, fava beans, and lentils, adds another level of complexity to ensuring the diet is nutritious and healthy. Exotic ingredients have different nutritional profiles and different digestibility than typical ingredients and have the potential to affect the metabolism of other nutrients. For example, the bioavailability of taurine is different when included in a lamb-based diet, compared with a chicken-based diet, and can be affected by the amount and types of fiber in the diet."

The authors conclude: "Pet food marketing has outpaced the science, and owners are not always making healthy, science-based decisions even though they want to do the best for their pets."

At the same time, as the FDA notes, diet-associated reports of DCM have affected a very small proportion of the estimated 77 million pet dogs in the country. "Tens of millions of dogs have been eating dog food without developing DCM," the agency says, adding, "If you are concerned about the diet you are currently feeding your dog, FDA recommends consulting with your veterinarian or a veterinary nutritionist to discuss the best and safest diet for your dog."

Editor's note: This story has been changed from the original to delete references to the beagle-mix patient's echocardiogram results that are of minor relevance to DCM.