Data Intuition Case Study: Grain-free Dog Food

My vet told me I should stop feeding my dog grain-free dog food. Apparently, grain-free dog food is linked with a heart condition called Dilated Cardiomyopathy (DCM). This set off my bullshit detector, so I decided to dig deeper.

The FDA has a great document explaining their investigation here. It's very approachable. I encourage you to give it a read. But to save you some time, here's their summary of their investigation:

In July 2018, the FDA announced that it had begun investigating reports of canine dilated cardiomyopathy (DCM) 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 (whole, flour, protein, etc.) as main ingredients [...]. Many of these case reports included breeds of dogs not previously known to have a genetic predisposition to the disease.

In short: we heard more dogs are being diagnosed with DCM. Dogs who are diagnosed with DCM are mostly eating grain-free dog food, which is odd, so we're digging in deeper.

Let's review the data they present.


The FDA started looking into the link between DCM and grain-free dog food in July 2018. By April 2019 they had 515 reports of canine DCM. Interestingly, 91% of the dogs reported with DCM were eating a grain-free diet:

ingredient prevalence for dogs with reported

That's definitely suspicious. I don't have any data to back this up, but I suspect most dogs are not eating a grain-free diet. It's strange that these dogs are almost uniformly eating grain-free. Maybe it's that simple?

But how, though?

It doesn't look that simple. The FDA says DCM is usually either genetic or caused by a taurine deficiency. The FDA's tests show the grain-free dog foods aren't missing any important nutrients. Even weirder, Google says taurine comes primarily from meat and definitely not from grain. How could removing grain cause a taurine deficiency? The opposite seems more likely.

It sounds like the current hypothesis is that replacing corn and wheat with peas and lentils somehow interferes with how the dog digests the food. Even that hypothesis doesn't make much sense given the data. Only about half of the dogs tested for taurine deficiency actually had a taurine deficiency (source).

Overall, it sounds like we don't really know how grain-free dog food could cause DCM. But hey, that doesn't mean it isn't happening. So far, this still feels worthy of further investigation.

Another interpretation

I have a different hypothesis for what's going on here. I suspect this link between grain-free dog food and DCM is entirely caused by a good old-fashioned sampling bias.

The FDA notes: "We suspect that cases are underreported because animals are typically treated symptomatically, and diagnostic testing and treatment can be complex and costly to owners." (bolding mine).

Aha! Cases are being underreported, but they're also being selectively reported. The FDA notes many of these reports include: "echocardiogram results, cardiology/veterinary records, and detailed diet histories". Sounds expensive. I wouldn't be surprised if most dog owners say, "Nah, just treat the dog. No need to call the FDA."

And there's the bias. We're only seeing dogs that belong to owners with enough free time and money to go through the rigamarole of getting their dog diagnosed with DCM. It's no surprise to me that these dogs are more likely to be eating a (more expensive) grain-free diet.

So far, this is just a theory. I can see some evidence for my theory in this chart, though:

brand prevalence for dogs with reported DCM

I don't know much about dog food, but I know Acana is expensive. Acana's website suggests their dog food is not-quite-human-grade, but is made with human-grade ingredients. Look... humans should definitely not eat dog food, but the fact that Acana needs to set the record straight tells me this is some gourmet shit. I've probably eaten cans of Progresso with worse ingredients.

It's strange that an expensive food like Acana is the most commonly reported dog-food brand. I'd expect that a less expensive and more accessible dog food (like Blue Buffalo) would be more common in practice.

Maybe the pattern we're seeing isn't "grain-free dog food is associated with DCM" and is instead "dog owners willing to pursue a DCM diagnosis also tend to buy expensive dog food". Maybe I'll call this an "Affluence Bias".


Zooming out, this looks to be a rare disease. The FDA announced they were investigating grain-free dog food in July 2018. Up to April 2019 the FDA received 515 reports of canine DCM and they estimate there are 77 million pet dogs in the US. That's ~0.0007% of dogs per year.

Even if the real case count is 100x bigger than the reported case count, that's still less than a tenth of a percent of all dogs. I think it's safe to call that "rare".

In Summary

I don't plan on changing my dog's food - at least not given these data.

It looks like the FDA is demonstrating an abundance of caution by investigating this link. From what I understand, that's what the FDA does.

However, my vet shouldn't cite these results as fact given this level of evidence. Vets probably shouldn't cite this relationship at all given the effect size of 0.0007% of dogs per year. We've got bigger things to worry about.

When all is said and done, I expect we'll all be left with a vague sense that grain-free dog food causes DCM. In reality, I suspect it's just a quirk of the data collection.

© Ryan T. Harter. Built using Pelican. Theme by Giulio Fidente on github.