Among the timeless questions debated endlessly from generation to generation, such as “What is the meaning of life?” and “Does the refrigerator light go off when the door is closed?,” is one veterinarians and cat owners are particularly familiar with: “Is feeding canned or dry food better for cats?” Cat lovers—and many veterinarians—often have strong and absolute opinions on this question, and disagreements on the subject tend to be about as amicable as those on politics and religion.
The most widely held belief seems to be that canned foods are healthier for cats than dry diets. This is predicated on two key arguments:
1) As obligate carnivores, cats cannot thrive on dietary carbohydrates, and canned diets are low-carb compared with kibble. Therefore, dry diets are more likely to lead to obesity and diabetes mellitus (DM), two common and important feline health problems.1–5
2) Cats eating dry food don’t drink enough water, and this increases the risk of chronic kidney disease (CKD) and lower urinary tract diseases, such as feline interstitial cystitis (FIC) and urolithiasis. Feeding canned foods maintains better hydration and promotes dilute urine, reducing the risk of these diseases.4,6,7
These are plausible and logically sound arguments, but the history of medicine is full of reasonable hypotheses that turned out to be wrong. It is not enough to make a good argument based on general principles. To know what is really best for our patients, we must test such arguments experimentally and follow the evidence, whether or not it supports seemingly obvious, “common sense” beliefs.
Not cut and dry
The question of whether cats are healthier when fed canned or dry food is, unfortunately, far more complicated than it seems. This is because there are many nutritional variables affecting health and disease that are not determined solely by whether the food is dry or canned.
Macronutrient content can vary dramatically between diets, and while dry foods are typically higher in carbohydrates than canned diets, they can be higher or lower than canned foods in fat or protein. The diet’s micronutrient content also is critical, as the consequences of feeding taurine-deficient diets to cats illustrate. Other variables, such as calorie density, the amount of food fed, and the feeding pattern (e.g. number of meals per day) can have as much or more impact on health than the diet’s general form.
Even the basic assumptions of the arguments in favor of canned diets are not always as clear as they might seem. Cats certainly process carbohydrates differently from more omnivorous species, but that doesn’t translate into a simple equation that “carbohydrates = bad for cats.” The type and relative amount of carbohydrates fed make a great deal of difference. And while canned food unquestionably contains more water than dry food, the idea that cats eating canned food are better hydrated and have more dilute urine turns out not to be consistently true.
Cats and carbs
The debate about the effect of dietary carbohydrates on cats has raged for decades. A couple of reviews have recently summarized the evidence, and simplistic, general conclusions are difficult to justify.1,8 It is clear cats do metabolize carbohydrates differently than dogs and humans. However, the type of carbohydrate (simple or complex) and the feeding pattern significantly affect postprandial glucose levels and other measures. Cats can readily and effectively utilize carbohydrates as an energy source, and they can adapt metabolically to different macronutrient ratios in the diet, so the simplistic notion of carbohydrates as “toxic” to cats isn’t supported.
Research evidence generally shows no adverse effects on resting glucose or insulin sensitivity in cats fed typical types and levels of dietary carbohydrates. Diets with greater than 50 percent of calories from carbohydrates, especially when fed once daily rather than ad lib or as multiple meals, can generate higher and more prolonged spikes in blood glucose. However, even this does not appear to achieve levels associated with harm in experimental studies. Such extreme carbohydrate levels are only found in research diets, not in typical commercial cat foods.
Though there is some inconsistency among studies, most research has failed to find dietary carbohydrates are a significant risk factor for DM in cats.1,8 One study even found cats who developed diabetes were less likely to be fed dry foods than cats without DM.9 Obesity, of course, is a very important risk factor for DM in cats, but the research evidence is clear that dietary carbohydrates are not a cause of feline obesity.8–11 There is evidence that reduced-carbohydrate diets may be useful in management of feline DM, though such diets can be counterproductive and promote obesity if they are very high in fat.1,12
Cats and water
Cats can produce more highly concentrated urine than dogs and have lower weight-specific water requirements.6 It has been argued dry diets are associated with less overall water intake, they promote dehydration, and may increase the risk of CKD and other urinary tract diseases.4 Some research has found cats drink less water when eating low-moisture diets.4,6 However, other studies contradict this finding and identify no difference in water turnover and intake or body water content between cats fed dry and canned diets.6 There are many factors affecting water intake in cats other than the form of the food, including protein and mineral content, as well as energy density. Therefore, simply feeding a canned diet is not guaranteed to increase water intake or reduce urine specific gravity.6
Some studies have identified consumption of dry diets as a risk factor for FIC and urolithiasis, while others have not confirmed this link.6 Other research has even found that cats who develop FIC are more likely to be fed canned food than control cats, suggesting canned foods could increase FIC risk in some cases.13 Similarly, while dry diets are often cited as a risk factor for the development of CKD, research has consistently failed to support this purported association.6,14,15 And while canned diets certainly have a role in the management of CKD and urolithiasis, moisture content is not the only relevant variable, and dry diets can have benefits in patients with these conditions as well.6,16
Higher-moisture diets are typically less calorie-dense than lower-moisture diets, and it has been suggested canned diets may help prevent or treat obesity in cats. Other factors also are clearly relevant, of course, such as the diet’s specific composition, the amount fed, and the feeding pattern. It is clearly possible to maintain cats in a lean body condition with dry foods and to develop and perpetuate obesity while feeding canned diets. Overall, however, it is likely high-moisture diets, including canned foods or dry diets with added water, may be beneficial in preventing and managing feline obesity.6
Regular readers will not be surprised to see the simple answer—canned foods are better/worse than dry foods for cats—is not the true answer. “It depends” is a much less satisfying response to questions about the relative merits of canned food and kibble, but it is more likely to lead us to the best dietary strategy for individual patients.
Overall, concerns about the health effects of dietary carbohydrates in cats are typically exaggerated, and dry diets should not be avoided on the basis of the idea they have too much carbohydrate and promote obesity and DM. The diet’s specific nutrient and calorie content is more important than the form in which it comes.
Canned diets are higher in moisture than dry foods, and while it is unclear how much impact this has on the risk of urinary tract disease, it does seem likely this characteristic may help owners control calorie intake and weight in their cats.
So when our clients ask whether they should feed canned or dry food to their cats, rather than giving a satisfying, simple answer that is probably wrong, we should be prepared to discuss the evidence and the nuances of the issue in the context of the individual pet. This is the essence of evidence-based practice.
Brennen McKenzie, MA, MSc, VMD, cVMA, discovered evidence-based veterinary medicine after attending the University of Pennsylvania School of Veterinary Medicine and working as a small animal general practice veterinarian. He has served as president of the Evidence-Based Veterinary Medicine Association and reaches out to the public through his SkeptVet blog, the Science-Based Medicine blog, and more. He is certified in medical acupuncture for veterinarians. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News.
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