The Link Between Dry Food, Tumors and Disease

Analysis by Dr. Karen Shaw Becker

dry pet food extrusion

Story at-a-glance -

  • Extrusion is the manufacturing method used to make 95% of dry pet food and many other ultra-processed foods on the market
  • Extrusion alters the molecular structure of ingredients — especially protein — through a process called protein denaturation; it causes vitamin loss, especially vitamins A, E and the B-group vitamins; it also results in starch gelatinization and inactivation of nutritionally active factors
  • Kibble processing creates toxic reactions including advanced glycation end products and heterocyclic amines
  • Reducing the amount of ultra-processed food your entire family consumes and increasing the amount of minimally processed or fresh foods is a step towards health
  • Nutritionally balanced fresh food is ideal for pets, however, if you can’t avoid feeding your dog or cat ultra-processed food, take steps to include as much fresh food as possible

About 95% of dry pet food is manufactured using the extrusion process, which turns ingredient mixes into kibble. It also causes significant damage to those ingredients.

Here’s how the process generally works: Batches of dog or cat food ingredients are mixed, sheared and heated under high pressure, forced through a spiral shaped screw (either a single screw or a twin-screw) and then through the die of the extruder machine. The result is called extrudate, which is a ribbon-like product that is subsequently knife-cut and dried.

The extrusion process involves extremely high temperatures. Research shows that drying pet food at 160°C (320°F) to 180°C (356°F) can significantly reduce its nutritional value.1 In small-sized kibble (4 mm or about .16 inch), a drying temperature of 200°C (392°F) lowered concentrations of the amino acids proline, total lysine, and reactive lysine.

It also markedly decreased concentrations of the linolenic (omega-3) and linoleic (omega-6) essential fatty acids, and increased the concentration of oleic acid (omega-9 monounsaturated). The increase in oleic acid may point to lipid oxidation of the smaller kibbles during the drying process.

Lipid oxidation can create off-flavors and aromas, as well as potentially rancid, toxic compounds. This is the main reason food formulator Steve Brown recommends fats be added to pet food at the time of feeding and not prior to processing.

The high temperature used in extrusion and the short timeframe to process (under five minutes) creates continuous chemical and physical alterations to the ingredient mixture. These changes include vitamin loss, protein denaturation (i.e., changing the protein’s molecular structure), starch gelatinization, and inactivation of nutritionally active factors.

Vitamin Loss

According to a 2008 report by the Animal Nutrition Group at Wageningen University in the Netherlands, the extrusion process primarily destroys vitamin A, vitamin E and the B-group vitamins in dry food ingredient mixtures.2 No data on vitamins D or K was available for the report.

The percentage of vitamin loss during extrusion varies widely, from a low of 4% loss of thiamin to a high of 65% loss of vitamin A. It’s important to note that B-group vitamins are water soluble, meaning your pet’s body can’t store them — they must be provided daily through diet.

Protein Denaturation

The protein sources used in dry pet food formulas are often a combination of animal and plant. Less costly plant proteins don’t contain amino acids sufficient for the nutritional needs of carnivorous dogs and cats, which is why there must be some type of animal protein in any processed pet food labeled “complete and balanced” per AAFCO standards. Amino acids don’t fare well during extrusion. A study cited in the Animal Nutrition Group report,

“… observed a large overestimation of the available lysine content such that the amino acid pattern relative to lysine in these diets may not be optimal to promote health. In addition to lysine, other amino acids such as arginine, tryptophan, cysteine and histidine can also be affected by the extrusion process.

Of particular importance may be the sulphur amino acids (cysteine and methionine) which are often limiting in diets for cats as these amino acids are susceptible to oxidation.”

With regard to protein denaturation, according to the report, “Mild denaturation of proteins can make them more susceptible to digestive enzymes and, therefore, improve the digestibility of these proteins (Hendriks and Sritharan, 2002).”

Denaturation takes place during the extrusion process, and often prior in the case of animal proteins, which are heated after grinding to a target temperature before being added to the ingredient mixture.

Denaturation modifies the structure of protein. In the case of plant-based proteins like soy and corn, denaturation makes these biologically inappropriate foods easier for pets to digest. However, denaturation is only beneficial to meat-based proteins if the protein sources are substandard, which is the case in the vast majority of popular dry dog and cat food diets made up of the meat industry’s indigestible leftovers.

Rendered meat by-products are a common protein source in dry pet food, and they are indeed difficult for dogs and cats to digest and assimilate.

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Denaturation Occurs Even With High Quality Animal Meat

Unfortunately, denaturation of high quality, lean, whole cuts of meat used in superior quality dry pet foods also occurs. Denaturation of biologically appropriate protein has the opposite effect of what is achieved with grain-based and low-grade animal meat. Denaturation makes these once healthy proteins more difficult for your dog or cat to digest and assimilate.

In a study published in 2017, researchers at the Norwegian University of Life Sciences set out to see if adding fresh chicken meat to processed dog food made with hydrolyzed salmon protein and poultry meal increased protein digestibility.3 Not surprisingly, the chicken meat did not increase digestibility, because according to the researchers, “extrusion seemed to negate the higher digestibility” of the chicken.

The researchers posited that “heat during extrusion may be the culprit in reducing raw chicken’s nutritional value.” They observed that single amino acid digestibilities, especially for aspartic acid and cysteine, known to be heat sensitive, “revealed a disproportionate reduction when tested in the extruded food compared to when tested alone.”

Again, this is not surprising. Raw food contains all its original nutrients — nutrients are lost during cooking of any kind, and especially during the high and prolonged heat cooking, plus extrusion, used to manufacture processed pet food.

The change in the structure of healthy protein that occurs during exposure to high heat is a possible trigger for food allergies. Research shows the immune system may not recognize the altered protein structure and treats it as a foreign invader. Additionally, the profound amino acid loss during high heat processing begs the question of whether extrusion has played a part in the nutrition-related DCM crisis.

Starch Gelatinization

It’s important to realize that all dry pet food, including grain-free kibble, contains starch, because starch is essential to the formation of the end product. When the raw ingredients are exposed to heat and moisture during extrusion, the starch in the mixture gelatinizes (melts). This helps bind the kibble and also causes expansion of the product after it travels through the die.

A high starch content of 30% to 40% of the ingredient mixture decreases the density and therefore the weight of the end product, which is a benefit to the manufacturer. Dry cat and puppy foods normally contain around 30% starch, and 40% is about average in dog foods.

However, some dry pet food formulas can contain twice that amount, which is derived primarily from cereal grains, which like all grains are biologically inappropriate for dogs and cats. Interestingly, the extrusion process is thought to lessen the biological inappropriateness of the grain content in dry pet food formulas. According to the Animal Nutrition Group report:

“One of the challenges when using cereals in canine diets is the presence of anti-nutritional factors that are harmful for dogs. Study on NAF [nutritionally active factors] in canine diets show that extrusion cooking inactivates NAF activity especially those of a proteinaceous structure (Purushotham et al., 2007).”

Another starch-related fact mentioned in the report is that extrusion conditions and the type of starch used can affect glucose and insulin response in dogs after a meal of dry food. Extruded rice, which many pet parents mistakenly assume is a healthier starch, causes a higher glucose and insulin response than other extruded starches like barley, corn, wheat and sorghum.

Inactivation of Nutritionally Active Factors

The ingredients used in dry pet food mixtures, in particular grain legumes, contain undesirable nutritionally active factors (NAFs) that interfere with digestion and absorption of nutrients.

An example of nutritionally active factors is trypsin inhibitors, also called protease inhibitors. These toxins are most often associated with soy products. Trypsin inhibitors are chemicals that reduce the availability of trypsin, an enzyme crucial for digestion.

The extrusion process used in the manufacture of dry pet food inactivates undesirable NAFs in the ingredient mixture. It also reduces the activity of naturally occurring toxins like allergens, glycoalkaloids and mycotoxins present in grains prior to extrusion.

However, the extrusion process doesn’t entirely eliminate the activity of these toxic substances. Mycotoxins are still a big risk in dry pet foods even after the manufacturing process is complete and are regularly a cause of pet food recalls.

The inactivation of antioxidants and polyphenols naturally found in raw pet food ingredients has not been evaluated, but human nutrition studies demonstrate these and other beneficial bioactive compounds are reduced or completely inactivated during high heat processing.

Fresh Is Best

As you can imagine, this is especially troublesome for animals fighting cancer, liver and kidney disease, as well as the myriad of other significant health challenges that gave rise to extruded “prescription” or specialized kibble diets. Consuming these substances while concurrently trying to recover from or improve a major medical problem can be fruitless.

The above is just a sampling of the problems with one type of ultra-processed pet food. When kibble exits the extruder, enough nutrients have been destroyed that manufacturers must add them back (using synthetic replacements) in to meet minimal nutritional requirements.

There’s also the potential for cancer-causing chemicals in kibble, as well as all that deleteriously comes about with the Maillard reaction, including advanced glycation end products (AGEs), which in humans have been shown to exacerbate diabetes and interfere with kidney function, and are linked to aging, Alzheimer’s disease, neurologic disease, and cancer.

Of course, pet food companies producing kibble will never conduct research evaluating the levels of these compounds in their products, since the results would be catastrophic for sales. But the non-profit organization CANWI is.

The results of the first study comparing AGEs in dry, canned and fresh food are in and the information is being prepared for veterinary and nutrition journals so that doctors, scientists, researchers, pet parents and manufacturers will all have the information they need to make better food processing choices (backed by research).

If you've watched my pet food rankings video, you know I advocate feeding your dog or cat the highest quality diet you can afford. The top five types of pet food I recommend are a variety of nutritionally balanced, unprocessed (living) or minimally processed (frozen, air dried or freeze dried), whole food diets.

That's because the goal in feeding pets food they can truly thrive on is to mimic their ancestral diet (unprocessed, and the bulk of calories coming from protein and healthy fat) as closely as possible without breaking the bank. This is especially important if your pet is sick, isn’t thriving, or you are obsessed with healthspan or intentionally creating vibrant health.

My essential recommendation is to feed your pet (and your entire family) as much unprocessed, fresh food as you can afford. If you can't afford to feed an entirely fresh, living, raw or gently cooked diet, offer fresh food snacks instead. Research shows that providing any amount of healthy foods to dogs and cats is better than no healthy food at all.

Other options to consider: Feed, for example, two to four fresh food meals out of 14 in a week, or do a 50/50 split, meaning one meal a day is a processed pet food, and the other is a fresh food meal. Take small steps toward providing the best diet you can afford for your dog or cat, and keep in mind that any amount of specie-specific fresh food snacks and meals is better than none.

Every bite of food your pet swallows is either healing or harmful; all foods impact the body in some way. The more minimally processed foods your dog or cat consumes, the better.