It's used not only in pet food, but also in hundreds of other products, including beer, ice cream, jelly, diet soda, yogurt, toothpaste, shampoo, and gel air fresheners. Carrageenan is often used in vegetarian and vegan food products as a substitute for gelatin. It's even permitted as an ingredient in organic foods. However, in the European Union, its use in infant formulas is prohibited. On the other hand, poligeenan, or "degraded" carrageenan, consists of smaller fragments, and for decades this form has been used to intentionally induce inflammation in animal models for research purposes. It's also carcinogenic. Poligeenan is not permitted in food.
Veterinary nutritionists, pet food manufacturers, and, of course, producers of carrageenan assert that food-grade carrageenan is completely safe for pets to eat. However, even food-grade carrageenan is not perfectly pure; it contains a "low percentage" of the smaller, more damaging fragments. This may explain why even food-grade carrageenan can cause problems.
Research has shown that carrageenan induces the body to produce a cytokine (messenger molecule) called tumor necrosis factor alpha (TNF-⍺). This molecule stimulates inflammation and promotes apoptosis (cell death). These counterbalancing functions help maintain the equilibrium of the immune system, and play an important role in defending an organism's system from invading pathogenic organisms such as bacteria. However, TNF-⍺ is thought to be a causal factor in many chronic inflammatory diseases, such as asthma and inflammatory bowel disease (IBD), autoimmune diseases, and—despite its hopeful-sounding name—cancer. All types of carrageenan stimulate the production of TNF-⍺.
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Research on carrageenan in animals has provided mixed results—often depending on who funded the study. One leading researcher, Joanne Tobacman, MD, has studied the effects of carrageenan on intestinal epithelium and the substance's role in inflammation and carcinogenesis for more than 20 years.
While her research has been criticized by the carrageenan industry, she remains convinced that the harmful effects of carrageenan occur with both native (food-grade) carrageenan and degraded (poligeenan) forms. Both have been shown to increase free radicals, directly cause intestinal inflammation, and disrupt insulin metabolism (potentially leading to diabetes), and there is increasing evidence for carrageenan's role in cancer development.
Heat, digestive enzymes, acid, and bacteria can convert high-weight carrageenans to dangerous poligeenans in the human (and presumably animal) gut. The feline stomach environment is extremely acidic; could this make carrageenan especially dangerous for the animals? Could carrageenan be a factor in IBD, food intolerance, and the skyrocketing rates of cancer and diabetes in cats? The FDA has expressed no interest in all the newer research showing that carrageenan is indeed problematic, and the big-money players (including the pet food industry) are determined to keep it that way. Once again, it's up to consumers and pet parents to decide—and to vote with their dollars.
It seems clear enough that carrageenan could be behind many of the health problems experienced by animals and people, so avoiding it (as difficult as that may be, given its widespread use in food) may be well worth the effort.