What Are "Fillers" in Pet Food?

You want your pet’s food to provide good nutrition. So, a bag that promises “no fillers” can sound like a better choice. You want every ingredient to have a purpose. But what does that "no filler" promise actually tell you?

In this Behind the Mix conversation, we asked Dr. Casey Bradley what people mean by “fillers,” why some overlooked ingredients deserve a closer look, and what to ask when choosing your pet’s food. 

What are "fillers" in pet food? 

Dr. Casey Bradley: “Filler” has no standard definition in pet food. Yet people often use it to describe an ingredient that takes up space with no perceived benefit to their pet. But the ingredients given that label can have very different jobs. [1]

Corn, rice, soy, fiber ingredients, and animal by-products are sometimes grouped together as "fillers". Depending on the ingredient, they can provide energy, protein, fiber, or other nutrients. The useful question is: What does this particular ingredient contribute?

As a nutritionist, I start with the nutrients an animal needs. I select ingredients that can deliver those nutrients in the right amounts while considering safety, cost, and available resources. An inexpensive ingredient can be a useful part of that plan.

Think about venison. It may sound appealing, but could we supply enough to feed every dog? We need practical ways to provide good nutrition at different prices. Pet parents should feel comfortable choosing an appropriate food that fits their budget. 

Are chicken by-products nutritious? 

Dr. Casey Bradley: Something people do not choose for their own plates can still provide nourishment. By-products are ingredients that are produced while making another product. For example, chicken parts that are less popular on our dinner plates, can become useful pet-food ingredients. Human food preferences help determine which chicken parts are popular and which have less demand. That does not make the less popular parts nutritionally worthless.

Chicken by-product meal is made from suitable chicken parts that are cooked, dried, and ground. These can include parts such as necks, feet, and organs, and can supply protein and minerals. Poultry by-product meal excludes feathers except for amounts unavoidably present during good processing; processed feather meal is a separate ingredient. [3]

I have valued chicken by-product meal in diets for young pigs transitioning off their mother’s milk to solid feed. That experience reminds me to ask what an ingredient contributes before judging it by its name.

Quality and processing matters. Neither “by-product” nor “meat” guarantees better nutrition or easier digestion. Research in dogs shows that foods made with different protein ingredients can perform well while differing in how easily their nutrients are digested. We need to evaluate the actual ingredients and finished food. [4]

Using suitable by-products also helps us use more of the animals already raised for food. That is a responsible use of resources. [2] 

What does a carrier do in pet food and supplements? 

Dr. Casey Bradley: A carrier helps spread small amounts of vitamins or minerals evenly through a larger batch of food. It has a practical purpose, even if someone looking at the ingredient list might call it a filler.

One of my first jobs after graduate school was making premixes. A premix blends small amounts of vitamins and minerals so they can be added to a much larger batch of feed. I quickly learned that the mixing equipment made a big difference.

At the large mill where I worked, we could use a small amount of concentrated premix. A farmer using different equipment might need those same nutrients spread throughout more carrier so they would mix evenly.

Customers sometimes asked for less carrier because they thought they were paying for something unnecessary. But the carrier had a job, helping make sure those small amounts of important nutrients reached every part of the feed.

Pet foods and supplements face similar practical challenges. Some ingredients help powders flow through equipment or help a food hold its shape. A company should be able to explain that purpose and why the amount used makes sense. 

What about corn, rice, soy, and ingredients we cannot pronounce? Are they just fillers? 

Dr. Casey Bradley: A familiar-sounding name is not a measure of nutritional value. Properly prepared grains can provide useful energy. Soybean meal and corn gluten meal supply protein. Fiber ingredients such as beet pulp and cellulose can help change stool consistency, even though they are not meat. [5–7]

Different forms of an ingredient also have different jobs. Corn and corn gluten meal are not the same thing. Neither are whole peas, pea protein, and pea fiber. We need to understand the particular ingredient rather than judge everything from the same plant as one group.

Names can be misleading, too. Sodium chloride is simply salt. Potassium chloride is another salt that can supply potassium, a mineral animals need. An unfamiliar ingredient name can still describe an ingredient with a purpose.

I ask: What does this ingredient do? Is it suitable for my pet? Is it used in an appropriate amount? I use those same questions whether an ingredient is familiar, trendy, or inexpensive. 

Does more expensive pet food mean better nutrition? 

Dr. Casey Bradley: No, price alone does not tell me whether a food is right for my dogs. I choose food that suits them and works for our family. I have a senior dog and two younger dogs. Feeding one suitable food makes our routine easier, although each dog still needs the right portion and attention to individual health needs.

My dogs did well for a time on a salmon-and-potato food. Later, we noticed allergy-like signs and switched to a different salmon-based food. They improved. We did not establish the cause, so I would not call it a confirmed food allergy. The change simply worked better for my dogs.

I am also a “poop watcher.” I look for stools that are formed and easy to pick up. I also watch skin, itching, appetite, and activity. I recommend keeping track of weight and learning to check body condition with your hands, especially under a long coat.

Changes deserve attention, and persistent problems need veterinary advice. A pet’s response gives us useful information, but it does not prove that a particular ingredient is what caused the problem. 

What should pet parents look for beyond a no fillers claim? 

Dr. Casey Bradley: Ask who develops the food and what experience they have. Look for qualified nutrition expertise, such as a PhD in animal nutrition or a board-certified veterinary nutritionist, working with people who understand how to make the finished food. [9]

Then ask how the company checks that its food contains the nutrients promised and how it keeps the food safe. A recipe on paper needs to become a consistent product in your pet’s bowl. [9]

Check that a food intended as the main diet is labeled “complete and balanced” for your pet’s species and life stage. Treats and supplements are not necessarily complete diets, and AAFCO does not individually approve pet foods. [10,11]

I would rather hear clear answers about formulation, testing, and safety than rely on a “no fillers” statement. Pet parents deserve to understand why an ingredient is there and how the whole food supports their animal.