The Taboo Protein Hiding in up to 20% of Pet Foods

Analysis by Dr. Karen Shaw Becker


Story at-a-glance -

  • Soy is found in fewer and fewer pet food brands these days in response to pet parent concerns about its appropriateness for dogs and cats
  • Most soy grown in the U.S is genetically modified, so in addition to potentially toxic levels of glyphosate, the plants also contain naturally high levels of anti-nutrients and phytoestrogens
  • Raw, mature soybeans contain phytates that prevent mineral absorption and substances that block the enzymes needed to digest protein
  • In dogs and cats, soy has been linked to gas and bloat, bladder stones, blood sugar fluctuations, thyroid damage, and seizures
  • All in all, the potential risks associated with feeding soy to furry family members are unacceptably high

Once a principal ingredient in many brands of pet food, today soy is found in in less than 13% of dog foods and 20% of cat foods.1 Soy is a very popular ingredient with producers of ultra-processed pet foods because it's easy to find and cheap.

The constant supply is due in part to decades of genetic engineering to improve yields of soybeans. The vast majority of soybeans harvested in the U.S. have been sprayed with the weed killer Roundup (glyphosate), which is passed up the food chain to your pets.

In addition, soy is higher in protein than many other plants used in commercial pet foods, and even though it's a biologically inappropriate type of protein for dogs and cats, pet food manufacturers are allowed to include it in their crude protein percentages on the guaranteed analysis printed on pet food labels.

Factory Processed vs. Traditionally Fermented Soy

Plant estrogens, also called phytoestrogens, produce biological effects in humans. In soy protein, the most common of these compounds are isoflavones. Processing methods affect the level of phytoestrogens in soy. Traditional fermentation reduces the levels of isoflavones dramatically, however, factory processing does not.

U.S. varieties of soy are manipulated to be pest resistant (soybeans have some of the highest concentrations of pesticides of any crop), with the result that they contain higher levels of isoflavones than soy grown in Japan or China.

Raw, mature soybeans contain not only phytoestrogens, but also phytates that prevent mineral absorption and substances that block the enzymes needed to digest protein. Soy also contains other anti-nutrients (natural toxins), including:

  • Antigens in the form of non-denatured proteins that can create serious allergic reactions in both animals and people
  • Trypsin inhibitors that hinder the action of proteolytic enzymes in the gastrointestinal (GI) tract, reducing the digestibility of proteins
  • Oligosaccharides — indigestible sugars that cause gassiness and diarrhea
  • Phytic acid, which can interfere with the body's use of vital minerals like calcium, magnesium, iron and zinc

The soy in traditional oriental diets has been fermented for long periods (18 months on average) using molds, cultures or other substances that radically alter its biochemistry. The transformation that occurs through fermentation lessens the impact of anti-nutrients while making the amino acids in soybeans available for use by the body.

In contrast to fermented soy, factory processed soy starts with defatted soy protein meal rather than the whole bean. The meal is produced in a crushing process. Raw beans are crushed into thin flakes. The flakes are mixed with a petroleum-based hexane solvent to extract the soy oil. Flake waste is toasted and ground down to soy meal or soy flour, both of which wind up in animal feed. The soy oil is then cleaned, bleached, degummed and deodorized.

"Naturally brewed" soy sauce means the processed soy protein meal has been mixed with mold spores and "aged" at high temperatures for 3 to 6 months. Regular, non-brewed soy sauce takes only 2 days to produce. Soy flour is blended with hydrochloric acid at high temps, under pressure, and the result is hydrolyzed vegetable protein.

Various preservatives and additives are used to improve color and taste. This method employs the use of the enzyme glutaminase, which in turn produces large quantities of the "g" (glutamate) in MSG.

Click here to learn moreClick here to learn more

Finding the Hidden Soy in Pet Food

Soybeans and soybean-related products can be found in a variety of ultra-processed pet food formulas, dry, semi-moist and wet, as well as veterinary formulas and prescription diets. Because plant proteins are less expensive than meat proteins, pet food manufacturers use them to increase profit margins.

The ingredient label might not even say soy, as it is commonly listed as vegeta­ble broth, textured vegetable protein or TVP, and perhaps other aliases. Pet nutrition experts agree soy isn't good nutrition for cats or dogs. It's considered a low-quality, incomplete protein well known to create food allergies and potentially much worse in pets. According to Sally Fallon and Mary G. Enig, PhD, authors of Cinderella's Dark Side:

"The soybean contains large quantities of natural toxins or "antinutrients." First among them are potent enzyme inhibitors that block the action of trypsin and other enzymes needed for protein digestion. These inhibitors are large, tightly folded proteins that are not completely deactivated during ordinary cooking. They can produce serious gastric distress, reduced protein digestion and chronic deficiencies in amino acid uptake.

In test animals, diets high in trypsin inhibitors cause enlargement and pathological conditions of the pancreas, including cancer. Soybeans also contain haemagglutinin, a clot-promoting substance that causes red blood cells to clump together. Trypsin inhibitors and haemagglutinin are growth inhibitors."2

Soy's Effect on the Health of Dogs and Cats

A 2004 study looked at the number of phytoestrogens in two dozen commercial dog foods. Results revealed all the foods containing soy ingredients had concentrations of phytoestrogens in large enough quantities to have a biological effect on the pet.3

Soy has been linked to gas and deadly bloat in dogs. It's high in purines, making it a completely inappropriate protein source for urate-forming dogs (including all Dalmatians). It's also high in silicates and promotes the formation of silica stones. The carbohydrate action of soy can cause a rise in blood sugar in cats. Soy is also linked to thyroid damage, and since hyperthyroidism is common in kitties, this is yet another reason it should not be part of a feline's diet.

The ingestion of high-glutamate beans, including soybean products, is also linked to seizures in both dogs and cats.

Soy and Excess Estrogen in Other Species

Approximately 35 years ago, captive breeding of North American cheetahs was undertaken to reverse a population crisis within the species. But in 1985, 29 cheetahs in American zoos died, many from liver disease. Only 18 were born, and 7 of those died before reaching adulthood.

As few as 10% of adult female cheetahs living in captivity in North America produced live cubs in the mid-1980s. Yet in other countries, 60 to 70% was the norm. The difference? Cheetahs living and breeding successfully in other parts of the world were fed whole animal carcasses. North American cheetahs were fed a commercial feline diet of horsemeat and soy.

Researchers in Ohio studied the food the North American cheetahs were eating. They found the soy portion of the diet contained plant estrogens similar to the hormones found in female mammals.4 Four cheetahs in a U.S. zoo were switched to a diet of chicken meat and no soy. Liver function improved, however, whether the cats would ever be able to breed successfully remained a question mark.

The researchers theorized the cheetahs were probably extra-sensitive to the effects of plant estrogens due to inbreeding (the result of a previous population crisis). However, the amount of soy in their diets was relatively small, leading the scientists to conclude all felines probably have difficulty ridding their bodies of excess estrogens.

In the early 1990s, a couple in New Zealand was raising parrots and decided to feed them a new "wonder food," soya feed. Parrots in the wild don't eat soya beans, but the couple assumed they were feeding their exotic birds the best diet available. Sadly, the experiment was a disaster. Some of the birds became infertile. Many died. Young males hit puberty years early and aged prematurely.

In case it isn't obvious, I recommend avoiding pet foods containing soy products. The potential risks associated with feeding soy are unacceptably high, especially when you consider your carnivorous cat or dog receives a much higher level of nutrition from animal protein sources.