The debate about raw diets for pets is an ongoing “hot button” topic in veterinary circles. There is also a less heated but still vigorous debate about cat nutrition in particular and what form and composition constitutes the optimal diet for our pet cats. I have previously written about raw diets extensively, and I still hold the opinion that there is little reason to believe they are superior to cooked diets, including commercial pet foods (1, 2, 3, 4, 5, 6, 7).
The theoretical arguments advanced for using these diets are weak and often simply a form of the appeal to nature fallacy. “Natural” is not synonymous with “optimal,” so even for cats, which are obligate carnivores and often still actively predatory, actual experimental evidence is needed to justify the claim that the natural diet of raw, live small prey, or other raw diets promoted as equivalent, is better than the alternatives. This is even more true for dogs, who have been so deeply altered by breeding and association with humans from their wild ancestors that it is a stretch even to claim there is a “natural” diet that domestic dogs should be eating.
While the benefits of raw diets remain unproven, there are legitimate concerns about risks, including infectious disease, parasites, and potential dangerous from specific components of some diets, such as bones. These risks do not yet appear dramatic, but they need to be weighed against benefits which can be substantiated by real research, which does not yet exist.
One recent study has compared a commercial kibble with both cooked and raw alternatives in a few cats, and some raw diet advocates are suggesting this is evidence in favor of the benefits of raw diets. Having looked at the study, I think it is an interesting beginning in terms of comparing different feeding options, but I am not convinced it provides significant evidence to support most of the claims made for raw pet diets.
K.R. Kerr, B.M. Vester Boler, C.L. Morris, K.J. Liu, and K.S. Swanson. 2012. Apparent total tract energy and macronutrient digestibility and fecal fermentative end-product concentrations of domestic cats fed extruded, raw beef-based, and cooked beef-based diets. Journal of Animal Science: 90: 515-522.
The study was nicely designed, though this also meant labor intensive, which likely contributed to the small sample size. Nine cats were each fed three different diets in rotation; a beef-based raw diet, the same diet cooked in a microwave to the standards for food safety recommended by the USDA, and a high-protein commercial dry diet. The cats ate each diet for 21 days. The last 4 days of each period, all feces and urine was collected for analysis, and on the last day a blood sample was taken.
The nutritional composition of the three diets was complete and roughly equivalent, though the ingredients of the commercial kibble were significantly different from the ingredients of the other two diets. And of course the moisture content of the dry diet was significantly less than that of the others.
The only difference among the diets in terms of the bloodwork was a higher creatinine (a measure of kidney function, hydration, and protein metabolism) and triglyceride (a fat) in cats eating the cooked and raw fresh diets than in those eating the commercial dry diet. Though these values differed between the diets, they were within the normal range for all cats.
Interestingly, there were no differences in the urinalysis values between any of the diets, and all the cats had very high urine specific gravity measurements (>1.064). One of the arguments for feeding fresh or canned diets is that the increased fluid content should improve hydration and reduce urine specific gravity, which is hypothesized to be useful in preventing and managing kidney disease. However, despite having water available at all times, the urine specific gravity was extremely high in all of these cats, and there was no difference between the kibble and the fresh diets.
Any interpretation of these numbers is speculative given the small size of the study, but one would expect the fresh diets to lead to lower renal bloodwork values and lower urine specific gravity than the kibble, particularly if they are to have any protective benefits against kidney disease. This data seems to undermine this hypothesis.
The amount the cats were fed was adjusted regularly to maintain an ideal body condition, so no differences in the effect of diet on body condition could be evaluated. Cats on the dry diet did eat more, in terms of volume and calories, than cats on the other two diets (which were not different from each other). This agrees with the finding that the dry diet appeared to be less digestible than the other two diets (which again did not differ from each other).
The various measures of digestibility showed the dry diet to be between 4.2% and 11.7% less digestible than the cooked and raw fresh diets. However, as the authors state, “All diets tested in this experiment were highly digestible,” so the real-world significance of this isn’t clear. Presumably, a cat would have to eat a greater quantity of a less digestible diet and would produce more stool, but it is unclear how great a difference this might be or whether it would have any health implications.
Stool quality is often a marketing point for raw diet advocates. In this study, stool quality was evaluated on a 5-point scale:
1 = hard, dry pellets
2 = dry, well-formed stools
3 = soft, moist, formed stool
4 = soft, unformed stool
5 = watery liquid that can be poured
Cats on the dry diet had a higher score (average =3.3) compared to the other diets (which did not differ significantly at averages of 2.8 and 2.9). The ideal score is 3.0, so all of the diets generated a normal fecal consistency. Oddly, the dry diet appeared to lead to moister stools than the higher moisture content diets, which is the opposite of what one might expect.
A number of fecal compounds were measured, but the only conclusion drawn from the differences seemed to be again that the dry diet was less digestible and so led to greater colonic bacterial metabolism than the other two diets. However, the measurements were consistent with normal values seen in healthy cats. The health implications of this are unclear.
So what conclusions did the authors draw from these data?
Although the raw and cooked beef-based diets were more digestible than [the dry diet], all diets were highly digestible in this experiment.
Few differences in serum metabolites were detected
Urine variables did not differ among diets.
All scores of fecal consistency were within a desirable range, but cats fed [the dry diet] had greater scores
Carbohydrate fermentation was similar for all diets.
Fecal putrefactive compounds…were increased in cats fed [the dry diet] but were similar to values reported in the literature for healthy cats.
Because cooking may minimize risk of microbial contamination, and the results from the cooked beef-based diet tested herein were not different than the raw diet, cooking may be an appropriate modification to this feeding strategy
And, finally, the authors argued that more research is needed, which is certainly the case.
Overall, this study is interesting but doesn’t seem to offer much support to the claims made for raw, or even fresh high-moisture diets. No difference of any kind was reported between the raw and the cooked diet, and the vast majority of the measurements made did not differ between these and the dry kibble. And even the high moisture diets led to a high urine specific gravity, which would suggest less than optimal fluid intake.
The dry was slightly less digestible than the fresh diets, but all were easily and thoroughly digested. But as the authors state, “Because the ingredient composition of [the dry diet] was different than that of [the other diets], we acknowledge that the influence of the dietary composition and extrusion cannot be separated.” So it is not even clear if the difference found has to do with the form of the diet or simply the particular ingredients used. Even less clear is whether these differences have any implications for health or disease.
I certainly support further research into different pet feeding methods. My personal guess is that raw will not prove to have any benefits sufficient to outweigh the risks, though I do think feeding moist diets may turn out to be better for cats than feeding dry diets. Homemade diets may have some advantages, but these are probably outweighed by the persistent problems with nutritional adequacy and quality control unless a nutritionist is involved in formulating and supervising them.
I look forward to studies which either confirm these guesses or prove me completely wrong. However, until there is adequate research, these are simply guesses, not facts. The biggest concern I have about raw diet advocates is that they tend to make grand health claims far beyond anything supported by evidence. I wouldn’t be surprised if this paper is held up as evidence for those claims, but a close reading of it does not support that interpretation.