Fillers in Dog Food: What They Are and How to Spot Them

"Filler" has no legal definition in pet food. It's shorthand for ingredients that add bulk, calories, or kibble structure without adding much nutrition per bite. The ingredient matters less than how much of it there is and whether its source is named. Check the first five ingredients for splitting, compare foods on a dry matter basis, and calculate the carbohydrates yourself, since no label is required to list them.

There is no legal definition of "filler" anywhere in pet food regulation. AAFCO, the organization that writes the model label rules state regulators use, defines hundreds of feed ingredients and never uses the word. It's a term pet parents and marketers made up, and it gets pinned on everything from corn to beet pulp to chicken liver.

That doesn't make the concern wrong. Some ingredients really are in the bag mainly to add bulk, hold kibble together, or hit a protein number cheaply. The useful question isn't "does this food contain fillers?" It's "how much of this ingredient is here, why, and what is it replacing?" This guide walks through the ingredients that get called fillers, which ones deserve the name, and how to read a bag well enough to decide for yourself.

What Are Fillers in Dog Food?

A filler, the way pet parents use the word, is an ingredient that adds bulk, calories, or texture. What makes it a filler is that it delivers less protein, fat, vitamins, or minerals per gram than the ingredient it displaces. Cheap starches, indigestible hulls, and vaguely named animal ingredients are the usual suspects.

Two things complicate the picture. Plenty of ingredients called fillers have a real nutritional job (beet pulp feeds gut bacteria, and corn gluten meal is about 60% protein). And the same ingredient can be reasonable at 3% of a formula and a problem at 30%. Where an ingredient sits on the label tells you more than its name.

Why Dog Food Companies Use Fillers

Cost is the obvious reason. A pound of rice or corn costs a fraction of a pound of fresh meat, and starch is one of the cheapest ways to deliver calories.

Manufacturing is the less obvious one. Dry kibble is made by extrusion: dough is pushed through a die under heat and pressure, and it needs starch to hold its shape. A dry food with no starch at all wouldn't form a kibble. So every dry food carries some starch. What changes between foods is where that starch comes from and how much of the formula it takes up.

Palatability and shelf life round out the list. Animal digest and brewer's yeast make kibble smell better to a dog. Sugars and glycerin keep semi-moist foods and treats soft.

Common Dog Food Fillers, Ingredient by Ingredient

The table covers the ingredients that show up on filler lists most often. The last column is the one that matters.

Corn and Corn Gluten Meal

Corn gets called a filler more than any other ingredient, and it's also one of the most misunderstood. Cooked, ground corn is digestible for dogs, and it brings starch, linoleic acid (an essential fatty acid), and the antioxidants lutein and zeaxanthin along with it. Corn gluten meal is a different product: the protein-rich fraction left after the starch is removed, at roughly 60% protein. In feeding trials dogs digest it well (Smith and Aldrich, Translational Animal Science).

The concern with corn isn't safety. It's displacement. Corn gluten meal raises the crude protein number on the label, but it runs low in lysine, an essential amino acid dogs have to get from food. A bag can show 26% protein and get a meaningful share of it from corn rather than meat. If corn and corn gluten meal both appear in the first five ingredients, you're looking at a formula built around corn. Our guide to amino acids for dogs explains why the source of protein matters as much as the total.

Beet Pulp vs. Cellulose: Two "Fillers" That Behave Differently

Beet pulp and powdered cellulose both land on filler lists, and both are fiber. They do very different things once they reach the colon.

Beet pulp is what's left after sugar is pressed out of sugar beets, and it's a moderately fermentable fiber. Gut bacteria break it down into short-chain fatty acids, which the cells lining the colon use as fuel. Cellulose is close to non-fermentable. It passes through mostly intact, adding bulk to stool without feeding much along the way.

A 2019 feeding trial in Beagles compared the two directly. Dogs fed beet pulp digested more of their total diet and produced less wet stool per day than dogs fed cellulose (Donadelli and Aldrich, Journal of Animal Science). Beet pulp earns its place. Powdered cellulose has a legitimate job in weight-control formulas, where the point is fullness with fewer calories. Outside that context, a high position on the list is bulk. For more on how fermentable fiber supports the gut, see what beta glucan does for dogs.

Are By-Products Fillers?

This is where filler lists get the facts wrong most often, including an earlier version of this post. By-products have a legal definition, and it excludes the parts people usually picture. Meat by-products are the clean, non-rendered parts of slaughtered mammals other than muscle meat: lungs, spleen, kidneys, liver, blood, and bone. The definition specifically excludes hair, horns, teeth, and hooves. Poultry by-products are heads, feet, and viscera (AAFCO). Feathers aren't part of that definition. They have their own ingredient name, hydrolyzed poultry feathers, and have to be listed as such.

Organ meat is some of the most nutrient-dense tissue an animal has. Liver carries far more vitamin A, copper, and B12 than muscle meat. Veterinary nutritionists at Tufts point out that by-products often provide more nutrients per gram than the muscle cuts people prefer (Tufts Petfoodology). A named by-product can be a better ingredient than its reputation.

The real issue is naming. "Chicken by-product meal" tells you the species. "Animal by-product meal" or "meat and bone meal" doesn't, and an unnamed source can change from batch to batch. Named versus unnamed is the line worth drawing.

Hulls, Sugars, and Two Ingredients That Aren't Bulk

Peanut hulls, oat hulls, and soybean hulls are the closest thing to pure filler on a pet food label. They're cheap insoluble fiber with almost no digestible nutrition. A little is fine. High on the list of a regular adult food, they're there to fill the bag.

Corn syrup and added sugar show up mostly in semi-moist foods and treats, where they add sweetness and keep the texture soft. Dogs have no nutritional need for added sugar.

Brewer's yeast and animal digest get lumped in with fillers, but neither one is bulk. Brewer's yeast is a concentrated source of B vitamins and protein. Animal digest is a flavor coating, used in small amounts. The only thing to check is whether the animal is named.

How to Spot Fillers on a Dog Food Label

Pet food labels follow rules, and once you know them the bag tells you a lot. Five checks cover it.

1. Read the First Five Ingredients, Then Look for Splitting

Ingredients are listed in descending order by weight as they go into the recipe (AAFCO). Fresh meat is roughly 70% water, so "chicken" listed first carries a lot of water weight that cooks off. A named meat meal (already dried) in the top three is a steadier sign of animal protein.

Ingredient splitting is the pattern to watch. A formula lists "peas, pea protein, pea fiber" or "ground rice, brewer's rice, rice bran" as separate entries. Each piece lands lower on the list than the meat. Add them back together and that one crop can outweigh the meat. When the same plant appears two or three times in the top eight, combine them in your head.

2. Check the Product Name Against AAFCO's Naming Rules

The name on the front of the bag is regulated, and small words change the math:

• "Beef Dog Food": beef must be at least 95% of the product, not counting added water.
• "Beef Dinner," "Beef Entrée," or "Beef Formula": beef must be at least 25%, not counting added water.
• "With Beef": beef only has to be 3%.
• "Beef Flavor": no minimum percentage. The food only needs an ingredient that gives it a detectable beef flavor.

A "Chicken Dinner" at 25% leaves the other 75% open to everything else on the list.

3. Compare Foods on a Dry Matter Basis

The guaranteed analysis lists protein and fat minimums and fiber and moisture maximums "as fed," with the water still in. A canned food at 78% moisture and a kibble at 10% moisture can't be compared until the water is taken out.

The math: divide the nutrient percentage by the dry portion of the food (100 minus the moisture percentage), then multiply by 100.

The canned food that looked like 8% protein carries more protein than the kibble once the water is out.

4. Calculate the Carbohydrates Yourself

No pet food label is required to list carbohydrates, and starch is where filler usually hides. You can estimate it by subtraction, a method nutritionists call nitrogen-free extract, or NFE (Tufts Petfoodology):

Carbohydrates % = 100 − (protein % + fat % + fiber % + moisture % + ash %)

Ash is the mineral content of the food, not an added ingredient (Tufts Petfoodology). If the label doesn't list it, use about 7% for dry food and 2% for canned as a working estimate.

Take the kibble above, with 14% fat and 4% fiber: 100 − (24 + 14 + 4 + 10 + 7) = 41% carbohydrate as fed. On a dry matter basis that's 41 ÷ 90 × 100, or about 46%. Close to half of what that dog eats is carbohydrate. The canned food, with 5% fat, 1% fiber, and 2% ash, works out to about 27% on a dry matter basis.

Two caveats keep this honest. The guaranteed analysis shows minimums and maximums rather than exact values, so the result is an estimate. And NFE counts some soluble fiber as carbohydrate, so it runs a little high. It still works well for comparing two foods side by side. Starch isn't harmful by definition, and research on how dogs handle high-starch versus high-fat diets is still developing (Lyu et al., Frontiers in Veterinary Science). What the number tells you is how much of the bowl is doing structural work rather than delivering protein and fat.

5. Look for Named Sources

"Chicken fat" over "animal fat." "Salmon meal" over "fish meal." "Chicken by-product meal" over "poultry by-product meal," which can come from any bird. A named source tells you the species and makes batch-to-batch consistency more likely. Preservatives and artificial colors are a separate question, covered in our post on dog food ingredients worth avoiding.

Is Cheaper Dog Food Always Full of Fillers?

Not always, and the bag price is the wrong number to compare. A food built on more digestible ingredients usually has a smaller daily feeding amount, so the bag lasts longer. Check how many days a bag lasts at your dog's feeding amount, then divide the price by those days. Price per day, not price per bag, is the real comparison, and it closes a lot of the gap between a budget food and a mid-priced one.

Stool is the other free signal. The less digestible the food, the more of what goes in comes back out. Larger or more frequent stools after a switch usually point to lower digestibility. Our guide to how long it takes a dog to digest food covers what normal looks like.

Do Dog Treats and Supplements Have Fillers?

They can, and they're easy to overlook because they're small. Treats should stay at or under about 10% of daily calories, but a dog getting several a day takes in a real share of their diet from them. Soft treats often rely on wheat flour, glycerin, and sugar to hold their shape. Limited ingredient dog treats are the simplest way around that, since a short list is easy to read.

Our Calming Chews for Dogs - Beef & Pumpkin - 10mg list real beef as the first ingredient and are grain-free. The Calming Chews for Dogs - Duck & Apple - 5mg start with duck and skip sugary binders entirely.

Supplements have their own version of the problem, and mushroom supplements are the clearest case. When mushroom mycelium is grown on grain and the whole mass is ground into powder, the result can test at 35 to 40% grain starch. That's filler by any definition. Earth Buddy extracts instead of grinding, which concentrates the mushroom compounds and leaves the starch dilution behind. Every batch of Focus+Immune Mushrooms for Dogs - 5mg is third-party tested, and a QR code on the label links to the lab results for that batch. Our medicinal mushrooms page covers the extraction process in full.

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Better Alternatives to Fillers in Dog Food

When you compare foods, look for three things. A named animal protein or named meat meal should hold one of the first two positions. The carbohydrate should come from whole grains or whole vegetables rather than stacked fractions. And at least one fermentable fiber, such as beet pulp, pumpkin, or chicory root, should be on the list.

Adding fresh food on top of a complete diet is the other route. Eggs, lean meat, carrots, and pumpkin raise the nutrient density of the bowl without replacing the base diet. The quantity is what matters here, and our guide to the best whole foods for dogs covers how much to add without unbalancing things. For protein, see our breakdown of protein sources for dogs.

Homemade and Raw Diets: No Fillers, Different Risks

Cooking at home or feeding raw removes fillers by default, because you choose every ingredient. It also removes the formulation work a complete commercial food has already done.

Veterinary nutritionists at UC Davis analyzed 200 home-prepared dog food recipes. 95% came up short on at least one essential nutrient, and more than 83% were short on several (UC Davis). Raw feeding carries a similar gap. A 2019 survey of owners feeding raw found that few had involved a veterinarian in building the diet (Morelli et al., BMC Veterinary Research).

If you want to go homemade or raw, the recipe is the part to get right. A qualified pet nutritionist can build one that's complete for your dog's age and size.

How to Switch Your Dog Off a Filler-Heavy Food

A new food changes what reaches the gut, and the bacteria there need time to adjust. A gradual switch over 7 to 10 days keeps the change manageable.

For dogs with sensitive stomachs, stretch each step to four or five days. Watch the stool. Some softening in the first week is common, and it should firm back up as the gut settles. If loose stool lasts more than a couple of days, or your dog stops eating, slow down and check in with your vet. We also have a guide to supporting gut health naturally through a change like this.

The cells lining the gut turn over every few days, so a food switch is a good time to back that process up. Bovine colostrum carries growth factors, including IGF-1 and TGF-beta, that take part in that normal renewal. It comes as a powder, so it mixes straight into the new food.

Fillers in Dog Food: Frequently Asked Questions

What are fillers in dog food?

Fillers are ingredients that add bulk, calories, or kibble structure while contributing less nutrition per bite than the ingredients they replace. "Filler" has no legal definition, so the term gets applied loosely. Indigestible hulls, stacked starches, and unnamed animal ingredients fit it best. Beet pulp, brewer's yeast, and named organ meats usually don't.

Is corn a filler in dog food?

Not by itself. Cooked corn is digestible and supplies starch, linoleic acid, and antioxidants. Corn becomes a concern when it appears high on the list more than once, as corn and corn gluten meal. At that point the formula is leaning on corn for both calories and protein instead of meat.

Are by-products bad for dogs?

Named by-products aren't bad by definition. AAFCO defines meat by-products as clean parts other than muscle meat, such as liver, kidneys, and spleen, and excludes hair, horns, teeth, and hooves. Organ meats are nutrient-dense. The concern is unnamed sources like "animal by-products," where the species and consistency are unknown.

Is beet pulp a filler in dog food?

No. Beet pulp is a moderately fermentable fiber. Gut bacteria turn it into short-chain fatty acids that fuel the cells lining the colon. In a 2019 feeding trial, dogs on beet pulp digested more of their diet than dogs on cellulose. It's one of the more useful fibers in dry dog food.

Is rice a filler in dog food?

White rice is a digestible starch and binder, and it's gentle on digestion, so it isn't a problem in moderation. Brewer's rice, the broken fragments left from milling, is a cheaper starch with fewer nutrients. Watch for several rice ingredients stacked in the first five.

How can I tell if my dog's food has too many fillers?

Check three things. Look for the same crop listed two or three ways in the first eight ingredients. Calculate the carbohydrate percentage from the guaranteed analysis. And watch the stool, since large, frequent stools often signal lower digestibility. A named meat or meat meal should lead the list.

How do you calculate carbohydrates in dog food?

Subtract protein, fat, fiber, moisture, and ash from 100. If ash isn't listed, estimate 7% for dry food. A kibble with 24% protein, 14% fat, 4% fiber, and 10% moisture works out to about 41% carbohydrate as fed, or about 46% on a dry matter basis.

Does grain-free dog food have fewer fillers?

Not necessarily. Grain-free kibble still needs starch to hold its shape, so it swaps grains for potatoes, tapioca, peas, or lentils. Pea protein, pea fiber, and pea starch listed separately can hide a formula built mostly on peas. For the full picture, see Is Grain-Free Good for Your Dog?

For further reading:

• What's in the Ingredients List? (AAFCO)
• Reading Labels (AAFCO)
• Carb Confusion, Part 2: Measuring and Comparing Carbohydrate in Pet Foods (Tufts Petfoodology)
• All About Ash (Tufts Petfoodology)
• Don't Be Bothered by By-Products (Tufts Petfoodology)
• The effects on nutrient utilization and stool quality of Beagle dogs fed diets with beet pulp, cellulose, and Miscanthus grass (Journal of Animal Science, 2019)
• Evaluation of corn-fermented protein as a dietary ingredient in extruded dog and cat diets (Translational Animal Science, 2023)
• Differences in Metabolic Profiles of Healthy Dogs Fed a High-Fat vs. a High-Starch Diet (Frontiers in Veterinary Science, 2022)
• Homemade dog food recipes can be risky business, study finds (UC Davis)
• Raw meat-based diets for dogs: survey of owners' motivations, attitudes and practices (BMC Veterinary Research, 2019)

Research on pet food ingredients is still developing, and no single study predicts how an individual dog will respond to a specific food.