Do Prescription Cat Food Diets Really Work?


Lots of special diets exist for cats. These diets may be recommended by veterinarians or pet store staff based upon life stage, lifestyle, breed, or known medical problems. How useful are they? How strict must one be when feeding the diets to cats?

This article will focus primarily on the diets designed to treat medical problems. These diets are known as prescription diets because they most often are used on the recommendation of a veterinarian. In the past these diets were only available through veterinarians; in the Internet age, it is now possible to purchase most of them online and often from pet stores.

But before we delve into the prescription diets, I want to touch briefly upon the life stage, lifestyle, and breed-based diets mentioned above. Special diets exist for kittens, elderly cats, indoor cats, outdoor cats, Persians, Siameses, and many other cats who can be shoehorned into some category or other. These diets have been tweaked to “fit” the age, lifestyle, or breed of the targeted individual (for instance, kitten diets contain more calories than adult maintenance diets). However, it is my opinion that most of them, and especially the breed-based ones, are more marketing ploys than good-faith efforts to better serve cats and their owners. I’m generally not a fan of them.

What about prescription diets? These diets are manufactured by a few different companies. Hills, Purina, and Royal Canin are the market leaders. These diets are produced to address issues such as obesity, pregnancy, diabetes, urinary problems, skin problems, kidney disease, thyroid disease, intestinal problems, and pregnancy. Some of them seem to work better than others. A discussion of all of them is beyond the scope of this article, so I will focus on four of the most commonly used classes of prescription diets: those used to treat kidney disease, diabetes, intestinal problems, and urinary problems.

The ordering of the four classes that I just listed is not a coincidence. I have listed the diets in order of, in my opinion, increasing importance and efficacy.

Prescription diets for kidney disease

It may surprise many people to learn that I am not terribly impressed with prescription diets designed to treat kidney disease. Since kidney disease is one of the most common medical conditions in cats, kidney formulas (such as Hills k/d, Purina NF, and Royal Canin Veterinary Renal LP) are very commonly used.

The logic behind their use goes like this: It is the job of the kidneys to remove waste products from the bloodstream. The primary waste products they remove are the result of protein metabolism. Protein contains significant quantities of nitrogen and phosphorus, and the waste products that build up in the body with kidney disease are high in nitrogen and phosphorus. These waste products cause cats to feel sick. Therefore, kidney formula diets are low in protein to reduce the levels of these products and also to reduce the “workload” of the kidneys.

That sounds good in theory, but it appears there may be a flaw in the logic. Namely, cats’ bodies have very little control over their protein metabolism. This means that their bodies may be prone to metabolizing a similar amount of protein no matter how much of it they eat. Some experts therefore believe that kidney formula diets do not significantly reduce the workload of the kidneys, nor do they delay the progression of kidney disease.

To make matters worse, cats find protein palatable. Cats with kidney disease often have poor appetites, and they may be picky about foods that are low in protein. Also, increased consumption of protein is linked to a higher lean body mass, which in turn is linked to greater longevity. Many experts believe that older cats — the type of cats that usually have kidney disease — generally need more protein than their younger counterparts. Since cats don’t have much control over their protein metabolism, cats fed low protein diets may still break down as much protein as those fed high protein diets. That protein will come from their muscles, reducing their lean body mass. And, since protein is palatable, cats may eat less of the low protein diets, leading to even great reductions in lean body mass and therefore shorter life expectancy.

However, owners should remember that kidney diets do tend to affect blood phosphorus levels in a good way, and phosphorus appears to be linked to clinical symptoms.

By now your head may be spinning, so let’s cut to the chase. Here are my recommendation for cats with kidney disease. The most important thing is to maintain lean body mass. Therefore, the most important thing is that your cat actually eat the food on offer, and that his weight be maintained at a healthy level to the geatest extent possible. If your cat is enthusiastic about a kidney diet, then so be it — the decreased phosphorus may make him feel better. However, if your cat won’t eat the kidney diet, then find something that he likes. The most important thing is to keep him eating, and to monitor his weight and kidney values; if these parameters are unsatisfactory then an adjustment may be necessary.

This also means that occasional deviances from kidney diets aren’t likely to cause harm. A treat of some Thanksgiving turkey will not significantly affect the outcome in a cat with kidney disease.

Prescription diets for diabetes

Diabetes is one of the more common health concerns in humans and cats. The type of diabetes that is most common in cats is physiologically similar to the most common type (adult onset, or type 2) of diabetes in people. Dietary modification is a mainstay of type 2 human diabetes treatment in people. Shouldn’t it therefore also be so in cats?

Yes and no. Diabetes is a disease characterized by high blood sugar. Current feline prescription diets designed to treat diabetes (such as Hills m/d, Purina DM, and Royal Canin Veterinary Diet Diabetic Formula) contain high protein levels and low carbohydrate levels because it is easier for the body to metabolize carbs into sugar. In my opinion the logic behind these diets is sound. But there is a rub.

Human diabetics often have blood sugars in the range of 200 mg/dL. Veterinarians don’t bat an eye, nor do we stat treatment (in 2013 — things may be different in 2025) when we see such a reading. Cats with diabetes often have blood sugar readings in the range of 400 to 600 mg/dL. (Seeing such a reading would probably cause a human physician’s eyes to pop.)

Dietary modification most likely will help cats with diabetes. But it rarely is enough on its own. The overwhelming majority of cats with diabetes also require insulin. I therefore recommend that diabetic cats be switched to a diet designed for diabetes, but I warn clients not to expect a miracle. It is best to try to stick to diabetic diets exclusively, although an occasional deviance is not likely to be catastrophic. Fortunately, most diabetic cats are not picky eaters, so these diets are accepted by most of them.

Prescription diets for intestinal problems

Cats with infiltrative/inflammatory bowel disease or food intolerances may exhibit chronic intermittent diarrhea, vomiting, weight loss, poor appetite, or any combination thereof. The most basic way to treat these problems is with dietary modification. Some cats may respond to over-the-counter sensitive stomach diets. For others, prescription diets (such as Hills i/d, Purina EN, or Royal Canin Veterinary Diet Gastrointestinal) that are easily digestible, or limited-ingredient diets (such as Hills z/d or d/d, Purina HA, or Royal Canin Veterinary Diet Limited Ingredient) may be effective.

It often is necessary to experiment with a few diets in order to find one that works for cats with IBD or food intolerance. But if you can find one that works, I recommend that you stick with it and avoid treats or other dalliances. These syndromes are often treated with medications such as prednisone; if possible it is better in my opinion to try to manage the problems with diet alone.

Prescription diets for urinary problems

Feline Idiopathic Cystitis (FIC), also known as Feline Lower Urinary Tract Disease (FLUTD) and Feline Urological Syndrome (FUS) is a common condition that causes bladder pain, house soiling, and in male cats a life-threatening catastrophe called urinary obstruction. Although the causes of the condition are controversial, most experts believe that diet plays a role. I am not an expert (I do not have advanced certification in veterinary internal medicine), but as an emergency vet I have seen more than my share of urinary obstruction, and I firmly believe that diet plays a role.

Diets designed to prevent FIC (such as Hills c/d, Purina UR, and Royal Canin Veterinary Diet Urinary SO) appear, in my experience, to be very effective at reducing crises associated with FIC. I recommend their exclusive use for cats with the condition.

What about raw?

Improperly prepared raw diets are nightmares full of Salmonella, E. coli, Campylobacter, and malnutrition. Those first three items on the list are bacteria that can spread to people in the house. Bad, bad, bad!

However, at the risk of being labeled a veterinary apostate, I am obliged to note that properly prepared raw diets may actually have some potential to treat each of the four syndromes addressed in this article. That said, I have yet to see sufficient evidence to enable me to recommend them.

As a side note, I am aware that many people believe that the major pet food companies are big and evil. I should point out that I have not seen any evidence to date that convinces me that commercial raw manufacturers are any less greedy (although they certainly are smaller). If said companies wish to provide me with evidence, I suggest that they fund (but not interfere with) some well-run studies on the effects of feeding raw diets to cats suffering from any of the above conditions.

Other stories by Dr. Eric Barchas:

Got a question for Dr. Barchas? Ask our vet in the comments below and your topic might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)

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