Now, this is dietary science, and it’s not even the type of dietary science that actually gets funded in significant amounts (i.e. human dietary science). So I’m taking all of this with a grain of salt; it’s worth holding these conclusions loosely.

General studies on vegan diets

Domínguez-Oliva et al 2023, The Impact of Vegan Diets on Indicators of Health in Dogs and Cats: A Systematic Review

(adelaide uni rep!!)
In this review, we conducted a formal assessment of the evidence in the form of a systematic review. We found that there has been limited scientific study on the impact of vegan diets on cat and dog health. In addition, the studies that have been conducted tended to employ small sample sizes, with study designs which are considered less reliable in evidence-based practice. Whilst there have been several survey studies with larger sample sizes, these types of studies can be subject to selection bias based on the disposition of the respondents towards alternative diets, or since answers may relate to subjective concepts such as body condition.
However, there is little evidence of adverse effects arising in dogs and cats on vegan diets. In addition, some of the evidence on adverse health impacts is contradicted in other studies. Additionally, there is some evidence of benefits, particularly arising from guardians’ perceptions of the diets.
Given the lack of large population-based studies, a cautious approach is recommended.
notably if you look at figure 10, the overall findings about taurine are that there’s an observed increase in plant-based diets, but n = extremely low (37) and the certainty of the evidence is graded as “low”. somewhat provocative given below stuff on cardiomyopathy.

Knight et al 2024, Vegan versus meat-based dog food: Guardian-reported health outcomes in 2,536 dogs, after controlling for canine demographic factors

we surveyed 2,536 dog guardians who provided data and opinions about dogs fed conventional meat (1,370 = 54 %), raw meat (830 = 33 %) or vegan (336 = 13 %) diets for at least one year. We examined seven general indicators of illness: increased numbers of veterinary visits, medication use, progression onto a therapeutic diet after initial maintenance on a vegan or meat-based diet, reported veterinary assessment of being unwell, reported veterinary assessment and guardian opinion of increased illness severity, and number of health disorders per unwell dog. We also considered the prevalence of 22 specific health disorders, based on reported veterinary assessments.
In each dietary group the proportions of dogs considered to have suffered from health disorders were: conventional meat – 49 %, raw meat – 43 % and vegan – 36 %. Probabilities of suffering from a disorder respectively appeared highest in dogs fed conventional meat for 11 disorders, raw meat for eight disorders, and vegan diets for three disorders.
Dogs fed vegan diets had the best health outcomes. This trend was clear and consistent, with dogs fed vegan diets usually having substantial and statistically significant decreases in risks of these seven general indicators of illness. These ranged from 14.4 % to 51.3 % compared to dogs fed conventional meat-based diets.
For six specific disorders, vegan diets were associated with statistically significant risk reductions of 50 %–61 % compared to dogs fed conventional meat.
After pooling our results with related studies published to date, vegan dog food was consistently associated with lowered risks of multiple specific health disorders. No health disorder was consistently more prevalent in dogs fed vegan diets.

Specific studies on dilated cardiomyopathy from grain-free diets

One of our vets expressed a (very mild and non-urgent) concern that grain-free diets might be associated with higher dilated cardiomyopathy and recommended that we consider this at some point.

McCauley et al 2020, Review of canine dilated cardiomyopathy in the wake of diet-associated concerns

Future studies are needed to evaluate one variable at a time and to minimize confounding variables and speculation
Furthermore, to prevent sampling bias with the current FDA reports, the veterinary community should be asked to provide information for all cases of DCM in dogs. This should include cases during the same time period, regardless of the practitioner’s proposed etiology, due to no definitive association between diets with specific characteristics, such as, but not limited to, grain-free diets and those containing legumes, novel protein diets, and those produced by small manufacturers to DCM in dogs. In summary, in order to determine if certain ingredients, categories of diets, or manufacturing processes are related to an increased risk of DCM, further studies investigating these variables are necessary
Deficiencies in certain nutrients are known to play a role in the development of DCM. These include taurine, carnitine, and their precursors, such as methionine and cysteine (taurine) and lysine and methionine (carnitine)
A review of the current literature reveals gaps within DCM studies in dogs, including sampling bias, inconsistencies in sampling parameters, confounding variables, and lack of complete data for case studies on DCM and known genetic predisposition in certain dog breeds.
studies listed in page 11 (left column) indeed have very small sample sizes, double digits. and these studies and others listed in subsequent pages are clearly confounded.
Recently, the FDA has stated that there is an increase in the prevalence of DCM in dogs. Based on the limited literature regarding DCM incidence rate, prior to the FDA reports, it is unknown whether this represents a true increase in cases.

Freid et al 2020, Retrospective study of dilated cardiomyopathy in dogs

n = 71
Medical records of dogs diagnosed with DCM between January 1, 2014 and September 30, 2018 were reviewed. Dogs were grouped into “traditional” or “nontraditional” diet categories and whether or not diet was changed after diagnosis
still confounded, huge variance
Our results showed that for dogs eating nontraditional diets, those that had their diets changed had a significantly longer survival time compared to those that did not have their diets changed. Although this observation supports a benefit of diet change and a possible role of diet in the etiology of some cases of DCM, additional research is needed to show a causal association.
At the time of diagnosis, no significant differences were found between dogs eating traditional and nontraditional diets for age, sex, breed, weight, body condition score, muscle condition score, CBC (performed in 45 dogs; data not shown) or serum biochemistry profile variables (performed in 63 dogs; data not shown), murmur grade, presence of any arrhythmia or presence of supraventricular arrhythmias, presence of CHF, or echocardiographic measurements (Tables 1 and 2). Dogs eating nontraditional diets were more likely to have CHF (P = .02) and less likely to have ventricular arrhythmias (P = .02) compared to dogs eating traditional diets (Table 1).
The reason for the higher prevalence of CHF at the time of diagnosis in dogs eating nontraditional diets is unclear but could be related to diagnosis later in the course of the disease, to faster progression of the disease, or to other factors. Dogs eating nontraditional diets also were less likely to have ventricular arrhythmias compared to dogs eating traditional diets.
In 2018, grain-free diets were reported to represent 46% of the US pet food market.24 In a retrospective study of dogs with DCM diagnosed between January 1, 2015 and May 1, 2018, 75% were eating grain-free diets.14 Overall, 79% of dogs in our study (diagnosed between January 2014 and September 2018) were eating nontraditional diets. A recent abstract reported that 95% of dogs with DCM seen between May and September 2018 were eating nontraditional diets compared to 47% of dogs with myxomatous mitral valve disease (P < .001; Freid et al. Retrospective investigation of diet and dilated cardiomyopathy [DCM] in dogs. J Vet Intern Med 2020, abstract). The abstract represented only 5 months of data from a single hospital, and thus further research is warranted to investigate a possible role of diet in some dogs with DCM.
(^ that is all still confounded too)

Quest et al 2022, Incidence of canine dilated cardiomyopathy diagnosed at referral institutes and grain-free pet food store sales: a retrospective survey

Recent reports suggest an increased incidence of DCM in atypical breeds eating grain-free and/or legume-rich diets.
To address the gap in the literature, this project examines the DCM incidence rate, over time, by retrospective polling of veterinary cardiologists across the US. Further, grain-free brick-and-mortar pet food market share data from 2011 to 2019 were presented. Fourteen US hospitals, out of 88 that were contacted to participate, provided all cardiology canine cases and of those, which specifically were diagnosed as DCM cases, for as many years as were available (1–20 years, average 8.1 years). This included a total of 68,297 canine patients evaluated by a cardiologist.
Nationally, the data did not support a significant change in percent DCM over time, from 2000 to 2019.
A Poisson regression analysis revealed a statistically significant positive trend in the Mixed Breed group (P = 0.025, RC = 0.082), indicating that this group of dogs had an increased trend of DCM diagnoses over the past 15 years
The average incidence rate of DCM, amongst referral cases seen in the participating hospitals, was 3.90% (range 2.53–5.65%), while grain-free diet sales increased from 2011 to 2019. There was no significant correlation between the national DCM incidence rate or the individual breed groups (P > 0.05) in relation to the grain-free pet food sales.
if you look at figure 6, grain-free pet food sales have increased quite substantially (~2% in 2011 to ~8% in 2019, increases every year) while the reported incidence of DCM has remained relatively steady (a few percent, some fluctuations from year to year but seems like a steady line with a slope of 0, and the statistical analysis in the paper supports this conclusion). if DCM was caused by grain-free pet food sales, even if other factors were involved, you’d expect a 4x increase in the share of grain-free pet food sales to show up in the DCM prevalence data to some extent.
it’s possible that there could be a lagging relationship, i.e. if grain-free pet food sales now cause higher DCM in five years time or whatever. the data doesn’t really have a wide enough range to show this. While I live in the future, there’s no systematic and ongoing sampling effort, so I can’t find any more updated prevalence estimates.

Fisher et al 2023, Pit bull–type breeds with dilated cardiomyopathy eating nontraditional diets improve after diet change (2015–2022)

pit bull–type breeds diagnosed between 2015 and 2022 with dilated cardiomyopathy (DCM) or with DCM diagnosed by a cardiologist but that did not meet all study echocardiographic criteria (DCM-C).
91 dogs with DCM and 11 dogs with DCM-C.
For dogs with diet information available for time of diagnosis, 64/76 (84%) dogs were eating nontraditional commercial diets, while 12/76 (16%) were eating traditional commercial diets. There were few differences between diet groups at baseline, with congestive heart failure and arrhythmias common in both groups. Thirty-four dogs with known baseline diet and diet change status had follow-up echocardiograms between 60 and 1,076 days later (traditional diet, n = 7; nontraditional diet that changed diets, 27; and nontraditional diet group without diet change, 0). Dogs in the nontraditional diet group that changed diets had a significantly greater decrease in normalized left ventricular diameter (diastolic, P = .02; systolic, P = .048) and the left atrium-to-aorta ratio (P = .002) and a significantly greater increase in fractional shortening (P = .02) compared to dogs eating traditional diets. Dogs eating nontraditional diets with diet change (n = 45; P < .001) and dogs eating traditional diets (12; P < .001) had a significantly longer survival time compared to dogs eating nontraditional diets without diet change (4). Dogs with DCM-C also had significant echocardiographic improvements after diet change.
(slightly more compelling but strictly speaking still confounded because we don’t have a population prevalence of nontraditional diets across all pit bull-type breed dogs; very small sample size in some cells, e.g. n=4 for dogs with nontraditional diets without diet change, which is just not big enough to make any claims; and “diet change” is also certainly confounded by many variables that there isn’t data on, e.g. guardian’s responsiveness to vet recommendations could plausibly be correlated with general level of care for the dog)

https://www.fda.gov/animal-veterinary/animal-health-literacy/questions-answers-fdas-work-potential-causes-non-hereditary-dcm-dogs

Reports of DCM diagnosis in dogs submitted to FDA, basically zero until 2018, then a huge spike, then declined again from 2021 onwards to a very low level
Obviously this isn’t the same as an empirical sampling effort
1. Is this an issue with only grain-free diets or diets containing legumes or pulses? No. FDA has received reports of non-hereditary DCM associated with both grain-free and grain-containing diets. Most of the diets associated with reports of non-hereditary DCM have non-soy legumes and pulses (e.g., peas, lentils, etc.) high in their ingredient lists. However, it is important to note that legumes and pulses have been used in pet foods for many years, with no evidence to indicate they are inherently dangerous. CVM’s data show that pulse ingredients are likely used in many “grain-free” diets in greater proportion than in most grain-containing formulas.