Why are prescription pet diets "prescription"?

Question

I have an older cat who, like most older cats, is starting to have some kidney issues. Not severe, but she's noticeably drinking/peeling more.

Our vet says that the "senior cat" mixtures from the better pet food brands are not bad, but that we should consider moving to one of the prescription diets specifically balanced to reduce load on the kidneys.

I'm not opposed... but I'm somewhat confused about why a script is required to purchase these foods. Presumably they are as nutritionally complete as other foods from the same company. It's not as if there were controlled substances in the mix, or as if the vet was getting any additional payment for writing the script, given that the critter gets a yearly checkup anyway. And the specialized foods are more expensive, which should be sufficient to deter dangerously inappropriate use.

So how did we wind up needing the vet's approval to order these diets? Is it purely a marketing scheme by the pet food companies to make the diet seem more curative so pet owners will take it more seriously, or is there actually a good reason?

Answer 1

The exact legalities may vary by location, but the simple reason prescription diets require a prescription is because they are appropriate only for certain pets with a certain medical condition.

That's not to say that it's necessarily harmful to give the prescription diet to another pet, in the same way that it's probably not harmful to give, say, a common antibiotic to a pet that didn't really need it.

Pet food nutrition is regulated (again, the details will depend somewhat on the location). For example, in the United States, for a pet food manufacturer to sell a food with "complete and balanced" on the label (AAFCO standard), there are certain nutrient minimums. Many prescription diets may fall outside the minimum range. For instance, for adult cat food, AAFCO requires phosphorus levels of at least 0.5% on dry matter basis. Kidney diets are low in phosphorus, as this is beneficial for the kidneys (e.g. Hill's k/d tuna canned has phosphorus 0.47%). Diets for growth require at least phosphorus of 0.8%. Obviously, for this reason a prescription kidney diet you be at very least a poor choice for a growing animal without kidney disease, and in worst case potentially quite harmful.

These prescription diets have to undergo rigorous testing to establish that they are appropriate for the medical condition they treat, in the same way a drug is tested before doctors can start prescribing it.

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Now, why feed a kidney diet? The two main benefits of a kidney diet are moderate protein restriction and phosphorus restriction. Opinions will vary among vets as to when best to start a kidney diet, but there is some good evidence that being proactive about starting a kidney diet may improve long term survival. As a clinician one concern with starting a kidney diet too early is the protein restriction; cats are carnivores, and consuming less protein may result in loss of muscle mass. For this reason a few of the pet food companies have developed 'early stage' renal diets (e.g. Hill's k/d Early Support or Royal Canin Renal Early Consult), the thought being that in these early stages those patients may not require quite as significant protein restriction.