Grain-Free Diets in Pets: Where are we at with the evidence?
Most veterinary professionals (myself included) have long since felt that grain-free diets are a fad, with no real evidence that they are better than grain-inclusive diets. Then, in 2018, the FDA released a statement that they were investigating grain-free diets in connection with a rise in cases of dilated cardiomyopathy (DCM), and grain-free diets became the subject of a lot of speculation. Four years later, in Dec 2022, they announced they had found no causal link and would not be providing further updates unless new evidence came to light.
Following on from this, grain-free diet brand KetoNatural opened a class action lawsuit against Hills and associated organisations and veterinary nutritionists, accusing them of making the whole thing up to reduce competition. The legal battle over that is still ongoing, but for most vets, the question is simple: do grain-free diets cause DCM? Where are we at with scientific evidence of a connection?
In this article, we’re going to summarise the latest thinking on grain-free diets – their pros, their cons, and whether there’s a link with DCM in dogs.
So, the FDA has all-but dropped the investigation after being unable to find a causal link between grain free diets and DCM. But that doesn’t mean there isn’t any evidence that something was amiss.
After the FDA released their statement saying they were investigating a possible causal link between grain free diets and DCM, over 1000 DCM cases were reported in the US, mostly over 2019 and 2020 – levels were back to almost-normal by 2021. Although vets in other countries also saw cases of possible diet-associated cardiomyopathy, they did not report a large number of additional cases.
How do we know these cases aren’t ‘normal’ DCM?
Well, we don’t!
It’s possible that this ‘spike’ of reports was down to over-reporting due to a hyper-awareness of the problem – this is supported by research showing no significant increase in DCM cases across the US from 2000-2019, despite a huge rise in grain-free feeding during this time period. (That said, this study was on a national scale – it’s possible that a complex link between grain-free and DCM [including geographical factors] has confounded these results, with a true increased incidence being hidden in the large volumes of data).
The cases reported to the FDA at the time did appear to be different to ‘normal’ DCM. They were reported in non-DCM-prone breeds and – unlike normal DCM (which is considered irreversible) – some of these dogs improved once their diet was switched. It’s therefore possible that the spike was real, but the levels of reported DCM reverted to normal due to actions taken by manufacturers, or even that it was a few contaminated batches that caused the major spike in cases, and once these were out of circulation, the cases reduced again.
Is there evidence that grain-free diets are harmful?
So, while there’s no definitive evidence of a causal link between grain-free diets and a spike of DCM cases, some research has looked into what the differences are between diets implicated in DCM cases and normal diets.
One study looked at the molecules present in higher or lower amounts in diets that were associated with DCM cases vs those that weren’t. They spotted 111 statistically significant differences between DCM diets and non-DCM diets and found that most of these were attributed to four ingredients – the presence of peas (and lentils to a lesser extent), or the absence of chicken/turkey and rice. If there’s a link to these causing DCM, it’s likely that it’s a complex combination of molecules – they couldn’t identify which exactly.
The same researchers also did another study where they looked at the metabolites in the blood of dogs being fed DCM-associated diets vs traditional diets, and found 17 metabolites that differed significantly. Again, no definitive cause was found.
Going from molecules and metabolites to actual clinical impact is tricky. One recent study found measurable changes in dogs’ heart function after just 28 days on a diet formulated with pea flour, but not when they were fed diets formulated with lentil flour, rice flour, or a generic commercial food. It was a very small study, with just eight dogs, but does add weight to the link between peas and non-hereditary DCM.
Bakke et al fed six healthy labradors a high-legume, grain-free diet for 30 days. They noted a significant decrease in red blood cells, and a significant increase in plasma phosphorus, compared to the five dogs fed a normal diet. These changes were consistent with the records of dogs diagnosed with DCM, again suggesting that a high-legume diet might cause clinically significant changes.
And, while it’s a lot less worrying than potentially fatal heart disease, it’s worth noting that total digestible fibre appears to be much lower in grain-free foods, which can lead to poor faecal consistency.
We know taurine deficiency causes DCM, so is it taurine?
Unlike cats, dogs can metabolise taurine from methionine and cystine, making it a non-essential amino acid for them. That said, a deficiency in taurine or its precursors can cause DCM in dogs. According to the FDA’s investigation, the DCM-implicated diets had methionine and cystine in the normal ranges, although this doesn’t tell us anything about the bioavailability of these nutrients.
With a short feeding trial, Bakke et al noted that blood taurine concentrations didn’t change in dogs fed a grain-free diet – which is consistent with Donadelli et al. In contrast, Quillam showed reduced plasma taurine in her two feeding trials, although the taurine did remain in the reference limits. Bakke et al did note a decrease in urine taurine:creatinine concentrations, which they thought could be an indication that biosynthesis of taurine couldn’t keep up with the bodily demands during their short feeding trial, possibly showing that low methionine was a problem – Quillam et al also found that plasma methionine was lower in dogs fed grain-free diets. This contrasts with Donadelli, who concluded that urine concentrations of taurine hadn’t changed and that low taurine wasn’t the reason for DCM in dogs fed grain-free diets, although they didn’t comment on methionine.
In other words, it’s unlikely that these diets were formulated without enough taurine (or its precursors), but possibly that bioavailability was lower than expected for some reason. The presence of ‘anti-nutritional factors’ has been floated, but nobody has been able to pinpoint what these are in these diets – or indeed why the DCM cases suddenly dropped again, despite many dogs remaining on grain-free food.
Can grain-free diets be healthy?
A lot of the proponents of grain-free feeding claim that grain-free diets are more biologically appropriate, easier to digest, and better for health. Many also claim that dogs are grain-allergic. But is there any evidence?
A couple of small-scale studies (1, 2) have shown that grain-free diets are more palatable than grain-inclusive diets. There’s also some evidence that they reduce postprandial glucose, which may be of benefit in diabetic dogs. Of course, gluten sensitivity is very rare in dogs, but in cases of paroxysmal diskinesia in Border Terriers, grain-free diets are likely to help.
Otherwise, any benefit caused by a switch to a grain-free diet is likely to be coincidental.
What next?
Research into diet-associated dilated cardiomyopathy is continuing. The FDA are continuing to monitor the situation but don’t believe there’s enough evidence to make any diet recalls. Early diagnosis improves outcomes, especially in dogs with non-hereditary DCM, so keep an eye out for cases. Remember to take a nutritional history at every routine appointment, as recording diet choice in the history may help these cases be recognised sooner.
What should I tell pet owners?
It’s important to remember that pet diet choice is rarely simple, and owners are always doing the best they can with the information they have. Nobody feeds their dog a diet they believe is bad – the decisions are always born from love, even if they don’t seem like a good decision in the end. Therefore, empathy is essential.
Generally, most pets do not need a grain-free diet, as there’s little evidence for benefits, except in the rare cases where a true grain allergy is present. If owners would like to feed a grain-free diet, help them find one made by an established, reputable brand – the WSAVA guidelines can be helpful with this. With the current evidence, they could also aim for one with lower levels of peas and/or containing chicken, as – while the complex interplay of ingredients is hard to predict – there’s some evidence that these are less likely to cause DCM. Dogs prone to hereditary DCM probably shouldn’t be fed a grain-free diet as this increases their risk, but if owners can’t be persuaded, switching to a non-pea formula could be a sensible precaution.
Getting help with veterinary nutrition CPD
Nutrition can be a challenging conversation in any veterinary practice, but having some in-house expertise can really help. We offer the training for the Nurse Certificate in Small Animal Nutrition for those nurses that want to help pets, consult with pet owners, and be their practice’s go-to source of information on issues like grain-free dog diets, raw diets, and more. The CPD course is made up of four two-day attendance sessions in our training facilities in Swindon, and nurses can then take the online assessments for the ISVPS Nurse Certificate – a globally respected qualification with postnominals as evidence of their achievements.
Conclusion
There’s no conclusive evidence that grain-free diets cause DCM, and the case numbers seem to have returned to normal. That said, while nobody has pinpointed the exact issue, some dogs were being diagnosed with DCM that improved when they started a grain-inclusive diet. It’s therefore worth looking out for cases and taking a nutritional history regularly.
