Introduction

Veterinarians and practice support staff encounter questions and comments on a daily basis from clients regarding various aspects of petfoods, many of which are based on hearsay, popular belief and even falsehoods. In this paper Dr. Heinze considers some of the more frequently posed questions and offers both some background to the issues and reasoned science-based answers.

Q. Are home-cooked diets healthier for my pet than commercial diets?

In some countries, home-prepared diets have always been popular, while in other countries (such as the U.S.), home-cooking for pets has become much more popular in the past ten years. Many pet owners who cook for their pets cite a distrust of commercial pet foods, the perception that home-prepared foods are healthier, or the desire to include or exclude certain ingredients. They may also believe that making food for their pets at home allows for increased palatability or decreased costs.

In theory, home-prepared pet foods, if done correctly, can be nutritionally balanced and healthy, although whether they are healthier than commercial diets remains debatable due to the lack of evidence for or against this idea. Unfortunately, most home-cooked diets either put together empirically by pet owners (or even veterinarians), or made from recipes obtained from books or the internet, have serious nutritional concerns. Several recent studies have investigated recipes for home-prepared diets for dogs and cats in books and on websites and have found that the vast majority of them contain lower than recommended amounts of essential nutrients 1 2 3.

While a detailed analysis of a home-cooked recipe requires either laboratory or computer analysis using diet formulation software, some basic components should always be included in a home-prepared diet recipe. Recipes without the following components are more likely than not to be unbalanced; however, even recipes that contain all these components can have serious nutritional imbalances.

1. An animal protein source – home-cooked recipes that do not contain animal protein are often inadequate in protein or amino acids.
2. UA calcium source – either as calcium carbonate, calcium phosphate or bone meal. Typically inorganic calcium sources are best, as bone meal has variable digestibility in the author’s experience.
3. A source of linoleic acid – typically this is corn, canola, or safflower oils or meals, although chicken fat or oats can sometimes provide adequate levels.
4. UA source of vitamins and minerals – this typically needs to be a complete vitamin/mineral human supplement designed to be taken once daily. Pet supplements usually do not contain adequate amounts of nutrients to balance home-prepared diets. However, there are some supplements available designed specifically (and only) for balancing home-prepared pet diets and they may fulfill this purpose.
5. A taurine source (for cats) – cooking meat reduces the concentration of taurine, so it should always be supplemented.

Because of the significant risk of inadequate nutrients, home-cooked diets should never be used in growing, gestating or lactating animals.

These life stages are very demanding nutritionally and inappropriate nutrient levels can have dire consequences.

Pet owners interested in feeding home-cooked diets should be referred to board-certified veterinary nutritionists or individuals with an advanced degree (e.g., a PhD) in animal nutrition to ensure that they have the best recipe possible. All recipes should be followed exactly and re-evaluated yearly to ensure that the recipe still meets appropriate nutritional guidelines as well as the needs of the pet.

Q. Does my dog or cat need any extra vitamins or supplements?

If a pet is fed a diet that meets minimal nutrition guidelines [such as the Association of American Feed Control Officials (AAFCO) nutrient profiles 4], then it is unlikely to benefit from added vitamin and mineral supplements unless it has a specific health problem. Most vitamin and mineral supplements marketed for pets assume that the pet is getting the nutrients that it needs from a balanced diet and therefore include only small amounts of vitamins and minerals which are unlikely to have much benefit in a healthy animal.

On the other hand, if the supplement manufacturer is not knowledgeable or careful, it is possible to end up with excessive, potentially toxic, amounts of nutrients when fed alongside a balanced diet. Examples that the author has found are supplements marketed for large breed puppies that contain added calcium (excessive calcium is a major risk factor for developmental orthopedic disease), fish oil supplements that contain high concentrations of vitamins A and D3, and numerous vitamin supplements containing excessive concentrations of vitamin D3. 

In general, all pets that are fed home-prepared diets (with the possible exception of those fed whole prey) will require a vitamin and mineral supplement to provide essential nutrients. As noted above, most supplements marketed for general use in pets will not contain adequate nutrient concentrations to balance a home-prepared diet, especially if the labeling suggests that they can be used “for pets of all ages, on all types of diets”. Special products are required for balancing home-prepared diets and these diets often require multiple supplements; for example, the author will commonly use perhaps five or even seven different human supplements to achieve the desired vitamin/mineral balance when designing a home-cooked recipe.

Q. Are carbohydrates bad for cats?

There are few controversies more heated in feline medicine than the role of carbohydrates in the diet of cats in health and disease. As the cat’s historic diet consists of small rodents, birds, and reptiles that are typically low in carbohydrate, it has been argued that cats should not be fed substantial amounts of carbohydrates in their diet. Despite the cat’s adaption to low carbohydrate intake (e.g., up-regulated gluconeogenic pathways, low activity of hepatic glucokinase), cats have maintained the ability to process carbohydrates and properly cooked carbohydrates can have a digestibility of 93% or above 5. Cats do have an upper threshold for dietary carbohydrate that is considerably lower than that of dogs, pigs and humans, but most cats do very well on the carbohydrate concentrations typically present in commercial cat foods (5-40% of calories on a metabolizable energy (ME) basis).

Proponents of limiting carbohydrate in cat diets often blame the carbohydrates for various problems including obesity and the development of diabetes (Figure 1). Low carbohydrate diets are often described as “magic bullets” for weight loss – because they are more “natural”, cats will supposedly limit their intake to appropriate amounts. Other sources suggest that high carbohydrate diets lead to direct fat deposition, regardless of the calories consumed. Interestingly, diets low in carbohydrates are typically higher in fat and thus caloric density, which is a risk factor for obesity.

In fact, many dry low-carbohydrate diets (carbohydrate < 20% ME) are extremely high in calories (495-592 kcal/cup or 3960-4366 kcal/kg, as fed), which can make them very difficult to feed in appropriate amounts to cats with lower energy needs. One notable study compared diets with variable concentrations of fat and carbohydrates (using simple carbohydrates) and found that dietary fat (and thus dietary calories) were much stronger predictors of weight gain than dietary carbohydrate; cats fed the high fat/low carbohydrate diets gained considerably more weight after spaying and neutering than cats fed the high carbohydrate diets 6. It has also been the author’s experience that caloric density is much more of a concern than carbohydrate content when it comes to feline obesity, as many cat owners are very bad at limiting the amount of food that their cats are allowed to consume (Figure 2).

While many people also believe that higher carbohydrate diets (particularly dry diets) cause diabetes mellitus in cats, there is no data at this time to support a link. A known risk factor for feline diabetes is obesity, so feeding a diet that will help maintain the cat at an appropriate weight before diabetes develops is paramount. Indeed, one study did not find a link between dry food consumption and the development of diabetes, but did find physical inactivity was associated with the development of diabetes when the analysis was controlled for body weight 7.

Once a cat develops diabetes, there is data to support a potential benefit of feeding lower carbohydrate diets in some cats 8 9 10. However, these studies have a number of limitations and should not be interpreted as meaning that all cats with diabetes require a low carbohydrate diet for best management. Additionally, there is little data available to address the “ideal” carbohydrate concentration (i.e., no well-designed dose-response studies) or source (e.g., simple versus complex carbohydrates) for cats with diabetes. The practice of focusing on individual carbohydrate-containing ingredients and their potential effect on blood glucose (typically by extrapolating human glycemic index data) may not appropriately estimate the effect of the ingredient in the complex pet food matrix when fed to an actual cat.

For normal or underweight diabetic cats, the author typically looks for the lowest carbohydrate diet that meets the cat’s other needs. However, for obese cats, it may not be possible to find a low carbohydrate diet with appropriate caloric and nutrient density for weight loss, particularly if the cat is unwilling to eat canned diets. In these cases, the emphasis is on weight loss as a method to mitigate diabetic signs.

Q. Are grain-free diets healthier for my pet?

In recent years, there has been an explosion of pet foods marketed as “grain-free” or “cereal-free”. These diets, available in both extruded and canned forms, use potato, tapioca, peas, or other legumes (pulses) as carbohydrate sources instead of corn, wheat, rice or other grains. These diets are frequently marketed to the pet owner as being healthier, less likely to cause allergies, etc. However, despite the widespread popularity of these diets, there are no known health benefits to substituting non-grain carbohydrates for grains in commercial pet foods. Contrarily to popular belief, often fed by internet rumor, grains are less likely to cause food allergies in pets than meats, so switching to a grain-free diet whilst maintaining the same meat proteins fed previously is unlikely to benefit the pet with a true food allergy.

Many people equate grain-free diets with low carbohydrate diets, but this correlation is frequently inaccurate. Many grain-free diets have similar carbohydrate levels to grain-containing diets and the carbohydrates used may be simpler carbohydrates compared to the whole grains. At this time, unless a pet has a specific known allergy to a specific grain (which is quite rare), there is no health benefit to feeding a grain-free diet. Likewise, gluten-free diets are unlikely to be of any benefit, even to pets with gastrointestinal disease. The only reported cases of gluten-sensitive enteropathy in dogs or cats have been in several closely related Irish Setters and there is no evidence that any other dog breeds or cats are affected 11.

Allergies to environmental antigens such as pollens, molds, dust mites, and fleas are the most common causes of allergic skin disease in dogs and cats. For gastrointestinal signs, food is often a factor; however, other properties of a food – e.g., its digestibility, fat, or fiber content – are more likely to cause digestive upsets than an immunogenic response to specific food ingredients.

Diagnosis of specific food allergies is difficult as it involves laborious dietary elimination trials followed by provocative testing of individual ingredients. As a result, food allergies are rarely definitively diagnosed in dogs and cats and it can be hard to find much information in the literature on the most common confirmed food allergens. However, it has been reported that beef, dairy, wheat, egg and chicken are the most common food allergens in dogs, while beef, dairy, and fish are the most common in cats 12. However it is likely that these lists merely reflect the most common ingredients in pet diets over the past two decades, rather than enhanced antigenicity of any specific pet food ingredients.

If a food allergy or intolerance is suspected, dietary elimination trials should be conducted using veterinary therapeutic diets containing limited, novel antigens (i.e., one protein plus one carbohydrate that the pet has never previously been fed). For pets with extensive ingredient exposure, particularly those with exposure to white potato (dog) or green pea (cat), it may not be possible to find an appropriate novel therapeutic diet. For these cases, a diet consisting of hydrolyzed protein plus a simple carbohydrate (i.e., no protein component) should be utilized.

Only when an animal has signs that do not improve on one or more strict trials with a novel or hydrolyzed therapeutic diet (but an allergic cause is still suspected) should a home-cooked diet be tried. While many resources recommend an unbalanced combination of one protein and one carbohydrate, it is the author’s experience that when these diets are successful, they tend to be fed long-term by the client, typically without regard for the fact that they are deficient in essential nutrients. For this reason, the author always ensures that recipes for trial diets include appropriate supplements to provide essential nutrients for long-term feeding, utilizing sources that do not introduce additional antigens.

Q : I always check the ingredient list when I am considering buying a new pet food so I can determine the quality of the food. What ingredients should I look for or avoid?

Unfortunately, the ingredient list of a pet food is not a good way to assess either the quality of individual ingredients or the diet as a whole. While in most countries ingredients are regulated in that they have very specific definitions which must be followed, these definitions typically do not contain any detail on the quality or nutritional composition of those ingredients. A company could use either top quality or poor quality chicken meal and the label will still say “chicken meal”.

Likewise, there is no requirement for companies to prove that certain ingredients improve health or are even bioavailable to the pet. Manufacturers may use exotic meats (e.g., bison, rabbit, salmon, venison, duck) or fruits, vegetables, or herbs in diets in amounts that are unlikely to have any nutritional benefit, solely for marketing purposes as the pet-owning population is increasingly looking for ingredient lists that are more similar to the foods that they themselves are eating or that reflect their perception of what their pets “should” eat. This explains the presence of expensive diets that contain foods such as smoked salmon and goji berries that may confer no additional nutrients or benefits than a traditional corn and chicken-based diet.

Many pet owners strive to avoid artificial colors and preservatives, and these compounds have largely been removed from commercial pet foods due to consumer pressure. It is important to keep in mind, however, that natural preservatives may actually have less safety and efficacy data available for them than the artificial preservatives that were traditionally used. Thus the use of natural preservatives puts a lot of onus on the manufacturer to assure that the amounts and types of preservatives used are appropriate to protect the nutritional quality of the food throughout its entire shelf life.

Q : My veterinarian has recommended that I feed an expensive veterinary therapeutic diet rather than a diet available at the pet store or discount store. Are these diets really any different from the diets I can buy without a veterinarian’s approval?

In most countries, pet foods sold to consumers for healthy pets must meet minimum nutrient concentrations that are thought to be appropriate for the specific life stage of the pet (adults, pregnant or lactating bitches or queens, or growing kittens or puppies). These diets have variable nutrient levels and are designed to provide adequate nutrition for healthy dogs, and they may not contain ideal nutrient composition and other properties (e.g., fiber, digestibility) needed for some pets with health problems. Some health issues that may necessitate a special diet include obesity, gastrointestinal disease, kidney disease, or pets suspected of having an adverse food reaction.

Many obese pets have low energy needs and their caloric intake must be heavily restricted compared to that of a lean animal to allow for weight loss. In order to ensure that they are not restricted in essential nutrients along with calories, diets that contain higher nutrient levels per calorie are required. While there are dozens of diets sold in pet stores and supermarkets that are marketed for overweight or obese-prone pets, there is a huge amount of variation in these diets in both calories and nutrient concentrations 13 and many of them may be reduced in calories without necessarily being higher in nutrients. For instance, many lower calorie dry maintenance diets for dogs are rather low in protein even before the calories are restricted, and maintaining adequate dietary protein intake is thought to be important for maintenance of lean body mass during weight loss. 

Veterinary therapeutic diets for weight loss are often lower in calories than maintenance diets while having higher concentrations of nutrients such as protein. These diets may also have higher concentrations of fiber or other compounds that may enhance weight loss, maintenance of lean body mass, or satiety. These products often result in healthier, more successful weight loss, particularly in pets with very low energy needs, compared with lower calorie maintenance diets (Figure 4).

For pets with moderate to severe kidney disease (as classified by the International Renal Interest Society, Stages 2-4), appropriate concentrations of protein, phosphorus and other nutrients will not be available in maintenance diets, as these concentrations are typically below regulatory minimums for healthy dogs. Several studies have shown improved clinical signs and longer survival times in dogs and cats fed specially designed veterinary therapeutic diets compared to typical maintenance diets 14 15. In these pets, an appropriate diet can potentially double lifespan and improve quality of life during the disease progression.

Pets with signs of gastrointestinal disease that do not improve on maintenance diets may respond better to the higher digestibility and fiber complexes available in veterinary therapeutic diets designed for pets with gastrointestinal disorders. There are also a number of diets designed for pets with potential food allergies or intolerances that contain very limited, less common ingredients as well as diets made with hydrolyzed proteins. While many over-the-counter diets market themselves as being for pets with “sensitive stomachs” or containing few ingredients, there are no regulations that guide these claims and these products may be no different in digestibility or ingredients than diets marketed for “normal” pets.

Veterinary therapeutic diets should be used exclusively when possible to rule in or out food allergy/intolerance. Although it often appears that similar diets are available in pet supply stores, many readily available “limited antigen” diets contain common ingredients in addition to potentially novel ones (e.g., a “venison” diet may also contain chicken or egg) or more ingredients than the name would suggest (e.g., “Venison and Potato” that also includes chicken, egg, peas, and barley). Additionally, at least one study suggests that these diets are frequently contaminated with other ingredients not listed on the label 16. Thus the results of a feeding trial are likely to be confused by the use of these diets, leading to incorrect diagnoses. When a veterinary therapeutic diet is important for a pet’s health, it is important that veterinarians and veterinary staff clearly explain to owners the differences between the therapeutic diet and the foods they may have easier access to, so the pet owner can better perceive the value of therapeutic diets.