Pet Food: Demystifying the Physiology, Definitions, Ingredients, Regulations

In regard to prescription diets, pet food ingredients, ingredient labels, laws, regulations, and fundamental physiology, there is an endless amount of misunderstanding and false misinformation currently littering the internet. Many websites claim this or that, but almost always have some type of underlying bias, utilize unreliable sources, or do not have the necessary medical knowledge to fully round out their evaluations. With this in mind, here is a breakdown of terms and topics — complete with all the acronyms you’ll ever need!

The Rundown:

To begin, pet food in the U.S. is regulated at two levels: federally by the FDA, then also at the state level, where most states follow the guidelines set forth by the Association of American Feed Control Officials (AAFCO):

“AAFCO is a voluntary membership association of local, state and federal agencies charged by law to regulate the sale and distribution of animal feeds and animal drug remedies.” [1]

AAFCO’s function is to develop and implement laws, regulations, standards, definitions, etc. for the animal feed industry — which covers not only the food itself, but the accompanying manufacturing, labeling, and sale.

Ingredients in pet food are constantly reviewed and evaluated by the FDA’s Center for Veterinary Medicine (CVM). CVM applies the mandates set forth by the Federal Food, Drug, and Cosmetic Act (FFDCA):

“FFDCA requires that all animal foods, like human foods, be safe to eat, produced under sanitary conditions, contain no harmful substances, and be truthfully labeled.” [2]

When it comes to pet diets, ingredients are almost always Battleground Zero for most people, regardless of where they may stand. This is where CVM comes in. Ingredients fall into one of two categories: “GRAS” (Generally Recognized As Safe) for their intended use, or approved food additive. Both categories are regulated and explained in various sections of the Title 21 Code of Federal Regulations. For an ingredient to be considered GRAS, qualified experts must determine that it is safe when added to food for that specific use. The FDA explains more on that definition, by stating that this determination can only be obtained by:

• “Experience based on common use in food. There must be information to prove that the substance has been commonly used in food for animals since before 1958, with a lengthy and known history of a significant number of animals consuming the food.”


• “Scientific procedures. These procedures require the same quality and quantity of scientific data needed for FDA to approve a food additive petition. Also, the data must be published in scientific literature or a similarly trusted source such as a textbook. Data in a company’s annual report do not meet this standard.” [3]

Companies are allowed to determine on their own if an ingredient is considered GRAS, then must submit information about that ingredient and its intended use to the FDA, who will then evaluate it using a scientific basis and ensure there are no questions or disagreement.

If the safety of an ingredient is ever questioned or outright found to be harmful to animals via factual evidence and scientific study/research, then it is either suspended from use and/or completely banned.

One aspect that many do not take into account is the safety or importance of one particular ingredient for one species vs. another. Propylene Glycol is a perfect example. In humans, dogs, and other mammals, propylene glycol is a safe, harmless food additive. However, it is banned from use in foods and drugs designed for cats, as studies have linked it to Heinz body anemia. [4]

Side Note:

Propylene glycol is an FDA-approved additive. It is NOT antifreeze. The antifreeze that poses a danger to animals utilizes Ethylene glycol — a “chemical cousin” for lack of a better term. Alpha propylene glycol is used in many foods and products; from ice cream to soda to medications to cosmetics to e-cigarette liquid, as well as various dog food brands. There are some antifreeze formulations which are deemed nontoxic and environmentally friendly. These formulations utilize Beta propylene glycol — another “chemical cousin”, and a prime example of the importance of high school chemistry!

One of the biggest arguments made by pet food critics is in regard to meat by-products, which are contained in most formulations. However, by-products are NOT inherently bad. The problem is that most people do not understand the true definitions used in pet food labeling.

Per AAFCO, the definition of meat is:

“The clean flesh derived from slaughtered mammals and is limited to that part of the striate muscle which is skeletal or that part which is found in the tongue, in the diaphragm, in the heart or in the esophagus; with or without the accompanying and overlying fat and portions of the skin, sinew, nerve, and blood vessels which normally accompany the flesh. It shall be suitable for animal food. If it bears a name descriptive of its kind, it must correspond thereto.” [5]

Thus meat is uncooked muscle tissue only — skeletal, heart, diaphragm, tongue, etc. These tend to be the most commonly accepted and consumed meats by humans.

AAFCO’s definition of meat by-products is:

“The non-rendered, clean parts, other than meat, derived from slaughtered mammals. It includes, but is not limited to, lungs, spleen, kidneys, brain, livers, blood, bone, partially defatted low temperature fatty tissue, and stomachs and intestines freed of their contents. It does not include hair, horns, teeth and hoofs. It shall be suitable for use in animal food. If it bears name descriptive of its kind, it must correspond thereto.” [6]

Thus by-products are the uncooked organs as listed above, plus necks, skulls, other bone, and fat. For poultry, it also includes heads and feet. The visceral organs cannot contain any undigested food or feces/urine, and are cleaned out and sanitized first.

Moving on, we have AAFCO’s definition of meal:

“The rendered product from mammal tissues, exclusive of any added blood, hair, hoof, horn, hide trimmings, manure, stomach and rumen contents except in such amounts as may occur unavoidably in good processing practices. It shall not contain extraneous materials not provided for by this definition. …. {the definition goes on to include the required mineral specifications and required nutrient guarantees}….. If the product bears a name descriptive of its kind, composition or origin it must correspond thereto.” [7]

This also applies to poultry (which can include undeveloped eggs, heads, and feet) in addition to mammals. So meal simply means that the meat or by-products have been rendered (cooked) so that fat and moisture have been reduced and evaporated. For example, “chicken meal” or “chicken by-product meal” is a dry, condensed, concentrated form of only chicken protein, vitamins, and minerals. If the species isn’t listed and the general term “meat meal” is used, then it can be cattle, pigs, sheep or goats, or any combination of those.

The FDA also adopts these same definitions and labeling requirements.

But! But by-products!

Unfortunately most pet owners have been wrongly convinced that by-products are bad. They aren’t!

In regard to by-products, the CVM states:

“Some people prefer to pass up animal by-products, which are proteins that have not been heat processed (unrendered) and may contain heads, feet, viscera [organs] and other animal parts not particularly appetizing. But protein quality of by-products sometimes is better than that from muscle meat.”

— William Burkholder, D.V.M., Ph.D.

People as a whole tend to anthropomorphize animals — which means to unnecessarily attribute human qualities and thought processes to them that they are not actually capable of having or understanding. Thus when owners think of feeding their pet a food that is made from bones, brain, lung, stomach, intestine, kidneys, liver, blood, etc, they shudder and think of this as a “lower quality” diet than one made with familiar muscle-based cuts of beef, pork, or chicken just like they would buy at the store — when in actuality it is superior.

We are also a bit sheltered here in America, and most of us have no experience with butchering carcasses. Most Americans (and Europeans/Brits) simply buy our neatly-packaged cuts of muscle meat from a refrigerated section of a supermarket. But by-products are a fantastic source of nutrition, and in most countries outside the U.S., humans utilize by-products (organs, viscera, tails, heads, necks, feet) in their regular every day recipes. These items are a large part of a staple diet for much of the world.

Furthermore, almost all predatory mammals tend to eat these cuts of meat FIRST before moving to muscle mass, as they recognize the densely packed nutrients they contain.

Wolves eat viscera before anything else:

“Immediately after killing an elk, wolves open the body cavity, using their canines and incisors, and remove and consume the internal organs such as the heart, lungs, liver, intestines, spleen, and kidneys. Consumed next are the large muscle masses of each leg.”

Foraging and Feeding Ecology of the Gray Wolf (Canis lupus): Lessons from Yellowstone National Park, Wyoming, USA

As do lions, leopards, and domestic cats. Tigers eat them immediately after the hindquarter muscles:

Now that you have the definitions, how do you apply them?

When reading an ingredient label, a pet owner needs to be educated and cautious. Labels are strictly regulated down to the exact wording, but if you don’t know what those words mean and how they are used, you can’t truly make an informed decision.

Per AAFCO definitions and upheld by the FDA:

“All ingredients are required to be listed in order of predominance by weight. The weights of ingredients are determined as they are added in the formulation, including their inherent water content. This latter fact is important when evaluating relative quantity claims, especially when ingredients of different moisture contents are compared.” [8]

They then give a fantastic example of why these definitions and regulations are so critically important when comparing and evaluating foods:

“One pet food may list “meat” as its first ingredient, and “corn” as the second ingredient. The manufacturer doesn’t hesitate to point out that its competitor lists “corn” first ( and “meat meal” is second), suggesting the competitor’s product has less animal-source protein than its own [implying lower quality].

However, meat is very high in moisture (approximately 75% water). On the other hand, water and fat are removed from meat meal, so it is only 10% moisture (what’s left is mostly protein and minerals). If we could compare both products on a dry matter basis (mathematically “remove” the water from both ingredients), one could see that the second product had more animal-source protein from meat meal than the first product had from meat, even though the ingredient list suggests otherwise.”

There are more definitions on the cited FDA page above, which I highly suggest reading for extremely detailed information on labeling and what specific terms like “dinner” and “chow” mean, and how they fit into the evaluation picture.

What should I feed my pet?

Clients always ask me for recommendations on what food to feed their pet or if they should change.

Unfortunately there is no “Best Food” or one-size-fits-all approach. This is definitely a discussion a pet owner needs to have with their Veterinarian, as that doctor knows the health and history of that specific pet. Some pets have kidney issues, heart issues, skin issues, etc. that will require specific dietary plans.

In general, for healthy animals with no organ function issues or sensitivities, I always tell clients to look for age-appropriate biologically-oriented moderate protein foods. Always look for foods which meet/exceed WSAVA (World Small Animal Veterinary Association) standards, meet/exceed AAFCO regulations, have undergone numerous food trials/studies, and are made by companies that employ full time ACVN board-certified veterinary nutritionists.

Here is WSAVA’s guidelines questionnaire for pet owners:

WSAVA Global Nutrition Committee: Recommendations on Selecting Pet Foods

For dogs, grains and vegetables are perfectly fine and are desirable in a healthy diet as they provide important sources of energy and essential vitamins and minerals. Dogs are omnivores, separated from wolves and other carnivorous canids by 12,000+ years of domestication and selective breeding by humans. They have evolved alongside humans, surviving on our foods and scraps/trash by scavenging, and have become oriented toward diets that are nowhere near the same as their wild cousins. There is significant evidence of this in the way our domestic companions exhibit dental changes and genetic changes, as well as the proneness toward severe dental disease in many breeds and the onset of acute pancreatitis in most dogs when they cannot handle the fat content of meats.

Here is a FANTASTIC article on the genetic changes and evolutionary diversion that domestic canines have undergone and how that affects their diet and digestion:

A key genetic innovation in dogs: diet

Here is another recent study that further explores the genetic changes behind increased production of amylase, which is an enzyme for digesting starch, that have accompanied the initial domestication of canines and subsequent selective breeding by humans:

“Over time, and via cohabitation, the canine diet has been transformed from the carnivorous diet of its ancestor, the wolf, to a diet more closely matching that of omnivorous humans…This transformation increased variation in the domesticated dog’s diet, potentiating impact on numerous biological pathways.”

Dietary Variation and Evolution of Gene Copy Number among Dog Breeds

Another study was conducted by evolutionary geneticist Erik Axelsson from Uppsala University in Sweden. He and his colleagues sequenced DNA from 12 wolves around the world and from 60 dogs belonging to 14 breeds. They found 10 key genes specifically giving dogs the ability to digest diets rich in starches:

“Ten genes with key roles in starch digestion and fat metabolism also show signals of selection. We identify candidate mutations in key genes and provide functional support for an increased starch digestion in dogs relative to wolves. Our results indicate that novel adaptations allowing the early ancestors of modern dogs to thrive on a diet rich in starch, relative to the carnivorous diet of wolves, constituted a crucial step in the early domestication of dogs.”

The genomic signature of dog domestication reveals adaptation to a starch-rich diet

More specifically, their sequencing determined that dogs had 30 copies of the gene responsible for amylase, a protein that starts the breakdown of starch in the intestine. Wolves have only two copies, one on each chromosome. As a result, that gene is 28-fold more active in dogs than in their wild relatives. More copies means more protein, and test-tube studies indicate that dogs should be FIVE times better than wolves at digesting starch (corn, wheat, rice, etc). a.

Dogs and wolves also showed that they have the same number of copies of another gene labeled MGAM, which codes for the production of maltase — another enzyme important in starch digestion. But there are four key differences between the sequence in dogs and in wolves. One difference causes dogs to produce longer chains of maltase. That longer protein is also seen in herbivores, such as cows and rabbits, and omnivores, such as lemurs and rats…but not in other mammals, suggesting length is important specifically to plant-eaters. These differences make the dog’s maltase much more efficient at breaking down plant material than their wild cousins.

Even with that said, wolves do consume significant amounts of plant material and digest it very effectively. In fact, a study on Yellowstone wolves that analyzed their feces (scat) determined that during summer months, the wolves kill 25% less prey and make up that portion of their diet with available small mammals, fruits, berries, plants, and primarily grasses:

“As most of our information on wolf kills comes from winter data, kill rates and prey selection are less known in summer. Current studies exploring this aspect of wolf predation are under way, but preliminary evidence indicates that wolf kill rates decrease as much as 25% in the summer (D. Smith and D. Stahler, Yellowstone Wolf Project, unpublished data). One indication of the seasonal differences in wolf foraging patterns is through an analysis of summer wolf scats. Scat analysis shows that summer diets are more diverse and include smaller prey species such as rodents, birds, and invertebrates, as well as ungulates, otherwise absent in the winter. Analyses of summer scats in 2003 show that mule deer was present in 133 (25%) of 530 scats analyzed. In addition, plant matter is prevalent in wolves’ summer diet, with 392 (74%) of 530 scats analyzed containing some type of plant material, largely grass (Graminae). This is consistent with summer observations of wolves consuming grass and other plant material.

Foraging and Feeding Ecology of the Gray Wolf (Canis lupus): Lessons from Yellowstone National Park, Wyoming, USA

This data observation is interesting, as it suggests that wolves only hunt prey at the rates observed during winter because other food sources are not available. During the summer months, they prefer to utilize more plant material in their diet and intentionally choose those plant sources over hunting prey items. They are choosing to eat vegetation over meat.

For a far more in-depth post on dogs, wolves, grain, genetics, the whole works, please check out this post:

Wolves Living Among Us: Should Dogs Eat Corn and Grains?

Grain-Free Diets and Dilated Cardiomyopathy:

There is also a current issue regarding Boutique diets (including homemade raw), Exotic protein sources, and Grain-free diets — together called “BEG” diets. These diets are currently being linked to thousands of cases of dilated cardiomyopathy (DCM) in MANY dog breeds, with golden retrievers being one of the most common by far. This issue is now also being seen in cats as well by practitioners at cat-only hospitals. Many young dogs are suddenly dying with no forewarning or illness. Others are becoming ill out of nowhere, only to be diagnosed with congestive heart failure caused by DCM.

Researchers are theorizing that a common ingredient in these diets is somehow causing an interruption in the absorption and utilization of Taurine at the cellular level. Over 90% of the affected animals show normal Taurine levels in their blood, which means they are taking in enough of the amino acid, but something may be preventing it from being utilized in the body. In the past, legumes have been implicated in this issue — specifically Beet Pulp. Unfortunately, almost all grain-free diets include legumes, lentils, and/or pulses.

The top brands implicated by the FDA and in the studies at UC Davis are Acana, Zignature, Taste of the Wild, Fromm, 4Health, and Blue Buffalo. Over 30 BEG diets are currently of concern.

Studies are ongoing, but for now, universities and veterinarians are advising pet owners to avoid these diets, and if they’ve been fed a BEG diet, to have their pets examined in order to catch it early. Switching to a grain-inclusive food that meets WSAVA guidelines as mentioned previously in this article is showing promise in reversing DCM in affected dogs.

Important links regarding this issue:

Latest FDA Report on nm-DCM

Canine Dilated Cardiomyopathy

Tufts First Statement on BEG-Diets and DCM

Tufts Follow-Up on BEG Diets, Taurine Deficiency and DCM

Morris Animal Foundation Article on Diet-related DCM

Stay tuned for a more in-depth and up to date post on this topic coming soon!

Quick Note on Allergies:

The overwhelming majority of allergies in cats and dogs are environmentally caused — dust, pollen, grass, mites, human dander, etc. — not by food ingredients. Yes, food allergies do exist, but they are less common. The overwhelming majority of true food allergies are to protein sources — beef and chicken are the most common. Animals with food allergies are also commonly reactive to soy, egg, and dairy as well. Legitimate grain allergies are almost unheard of.

Currently the American College of Veterinary Dermatology AND the American College of Veterinary Nutrition both advise limited ingredient food elimination trials to determine if a legitimate food allergy is occurring. Skin and blood tests are NOT accurate nor reliable for food allergy determination and are never recommended:

ACVN page on diagnosing food allergies

Tufts University article regarding food allergies

DVM360 article on food trials

Rx or not to Rx…THAT is the question.

Another point of contention that often comes up in the food debate is the validity of prescription diets. While Rx diets do not contain medications, the FDA does indeed define and regulate them as drugs. This is due to the fact that the specific formulation of ingredients and/or nutrients in the food is designed to alter the body’s function or affect a disease process:

When is a food a drug?

“Some food products may affect the structure or function of the body. For example, a food containing calcium may affect bone structure in puppies, or a food containing the amino acid taurine may help with heart health in cats. FDA considers these products to be food, not drugs, because they exert their effect by providing nutrition, taste, or aroma.

If a food affects the structure or function of the body apart from its nutritive value, such as by making a cat’s urine more acidic or improving joint function in an arthritic dog, FDA may consider it a drug. Similarly, a food marketed with a claim that it improves or increases production and performance may be a drug. Another example of when a food may be a drug is if it’s intended to treat or prevent a disease.” [9]

This specific definition is why Rx diets require a valid prescription from a licensed DVM in order to obtain them, whether it be from the veterinary clinic itself or from a pet store or online supplier. Only a DVM is qualified to determine whether an Rx diet is appropriate for a specific patient, given knowledge of that patient’s health and current physiological function.

The laws governing veterinary medical practice in each state (backed by AVMA regulations) require that in order to have a prescription, a patient must have a valid Veterinarian-Client-Patient Relationship (VCPR). The AVMA regulations stipulate that in order for a VCPR to be valid:

  • The veterinarian assumes all responsibility for medical decisions, and the client agrees to follow all instructions.
  • The veterinarian has personally examined the patient within a reasonable timeframe, has sufficient knowledge of the patient’s care and health to determine a diagnosis (or preliminary one), and maintains capability for follow-up examination and continued care.
  • The veterinarian oversees all treatment, compliance, and outcome for the patient, and maintains accurate medical records. [10]

Many states have laws that clarify the examination timeframe, and most clinics also set their own policies to state that a patient must have been examined within the past 6 to 12 months in order for the VCPR to be valid (and thus write a prescription and/or dispense medication/Rx foods).

Inevitably the ingredient critics usually make their way to a discussion regarding the “quality” of prescription food vs. the benefit it offers to the animal with the disease. The #1 thing I always tell clients and anyone who asks is this:

The ingredients in Rx food are not the priority.

When you get to the point of having an ill pet, the carefully researched and scientifically balanced levels of vitamins, minerals, proteins, fats, pH, etc. designed for the specific health issue they are prescribed to treat (i.e. GI problems, renal disease, urinary crystals, metabolic issues) are the primary focus and the reason Rx diets are successful. They also have ingredients that are altered at the molecular level for better absorption and utilization without allergic reaction, such as hydrolyzed proteins (chicken, beef, pork, etc).

When you get down to the basics, a little bit of corn gluten pales in comparison to what an imbalance of phosphates and protein does to malfunctioning diseased kidneys.


Yes, veterinary professionals receive extensive training and education in Nutrition.

Don’t believe me? Click Here.

Now I need to address the proverbial elephant in the room.


NO, veterinary professionals do NOT get “kickbacks” for selling prescription diets. There is no such thing. It has been a persistent myth for decades with zero foundation or proof, and happens to be a favorite claim among the anti-Rx-diet and anti-vet crowds. Most veterinary practices hardly make any profit from Rx food whatsoever, so it isn’t a “moneymaker” and we don’t push Rx diets to rack up the bill. In fact, most distributors set a firm cap on how much the products can be marked up for retail sale in practices, in order to avoid competition and price gouging. Some companies can buy in such bulk quantities that the discount they receive allows them to price lower than others (i.e., but the cap is still in place. The difference between that cap and what most practices pay wholesale to buy the products is minuscule. We’re lucky if we make $5-$10 on that $40 bag of Rx food. Usually we break even.

The entire purpose of a practice keeping Rx food in stock and prescribing/recommending it is owner convenience and effectiveness. The overwhelming majority of pet owners want a quick solution. They have no interest in spending hours upon hours researching how to read labels to find a commercial food that could work for their pet’s issue, nor spending hours upon hours formulating their own diets to manage chronic illnesses, nor spending the money or time to cook/prepare said diets. Owners want to leave the appointment with a solution in hand, and one that has a proven track record of effectiveness. This is why we have Rx diets — to help owners manage their pet’s illness.

How much do veterinarians and veterinary technicians/nurses REALLY understand nutrition?

Many people believe veterinary professionals have little to no knowledge or education when it comes to nutrition, and while that was perhaps the case in years past, recent decades have seen drastic shifts in the goals of education and modernization. When I was in school as a technician, we had a semester-long course specifically on nutrition and nursing, taught by a DVM who specialized as a nutritionist. I’m sure clients wish we had even more than that, but in the grand scheme of things, learning extensive info on nutrition/food is less vital than focusing on pharmacology, anatomy, physiology, anesthesia, surgery, parasitology, hematology, radiology, microbiology, pathology, etc.

Clients also need to understand that our education doesn’t simply end when we graduate, and that we don’t only work with what we learned in those 2-8 years. We have to complete a set amount of continuing education hours every year to maintain our licenses. An LVT has to complete 12-15 hours each year — more than a human RN in most states. A DVM has to complete 28-32 hours. Each state has different exact amounts but they’re all in that range, and most professionals exceed those basic requirements each year. Most of us attend huge conferences such as VMX, CVC, WVC, the ACVS Summit, etc, where we have hundreds of lectures to choose from that are presented by the best of the best in our profession. Conferences last 4-5 days, and go from 7am to 6pm almost every day, with 1-hour lecture blocks scheduled throughout that day. Nutrition-related lectures are almost always an available section in the course schedule at these conferences, and they tend to be very popular as most veterinary professionals want to keep updated on the changing knowledge, and stay in line with current client concern trends.

In the end…

The bottom line is that the internet is full of misinformation. Sometimes we feel like we’re fighting a futile battle. DogFoodAdvisor was started and run by a human dentist with literally ZERO education or training in animal nutrition or physiology.

Please be selective, check your sources for reputable organizations/authors and citations, look for empirical evidence and peer-reviewed studies, and always remember that animals are not the same as people — what is good for us is not always good for them. Trust your veterinary team! We’re the animal lovers who went to school for 2-8+ years to become licensed veterinary doctors/nurses (and who continue to add endless more hours of education every single year). We don’t do what we do just for the adorable kittens and the cool medical toys — although those are definitely a bonus…🤓

[featured image source]

4 thoughts on “Pet Food: Demystifying the Physiology, Definitions, Ingredients, Regulations

  1. Lindsay says:

    I have a 5mo old neopolitan mastiff. He was on Merrick, but recently taken off. Currently have him on petco brand. I noticed he started shedding recently. Possibly from the change. What brand would you recommend for him? Averaging 60lbs.


    • pinkladylvt says:

      Hi Lindsay — At present, most veterinary professionals are only recommending brands that are WSAVA-compliant. Hill’s, Royal Canin, Science Diet, Purina One, and Eukanuba. I personally feed Royal Canin and prefer that to others.


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