It’s important to avoid feeding rancid fats to our dogs and cats. When fatty acids become oxidized, or “turn rancid”, they are profoundly changed. Rancid fats in the diet reduce the nutritive value of the protein, degrade vitamins and antioxidants, and can cause diarrhea, liver, and heart problems, macular degeneration in the eyes, cell damage, cancer, arthritis and death.
Some fats are much more fragile and go rancid faster than others. Keep in mind that the fats we get from fish—EPA and DHA—are among the fats that turn rancid the fastest. Think fish left out at room temperature.
Some dry foods, most often premium puppy foods, do include fish oils, a primary source of omega-3 DHA. Even unopened pet food bags let some oxygen in, causing some oxidization of the oils in the food. And once the bags are opened, air rushes in and accelerates the oxidation of the fats.
Furthermore, the original DHA content of the food, listed on the bag, is not necessarily the amount of DHA that is still in the food when you feed it to your dog. Food that is cooked quickly under high pressure, the way most dog and cat foods are processed, and long-term storage before sale make oxidation of the DHA likely (see the National Research Council’s Nutrient Requirements of Dogs and Cats, 2006.)
All the studies and nutrient analyses tests I’ve seen on the DHA content of dog foods were conducted very close to the time of manufacturing, when the foods were fresh. These tests do not reflect when the dog eats the food.
How much DHA is left under typical conditions, for example, four months after manufacturing, including three weeks in a hot Houston warehouse and then in the dog owner’s sometimes humid kitchen or garage 20 days after the bag has been opened? The data suggest little DHA is left and many of the other fats have turned rancid. This is especially common when large bags of dry food are opened but not completely consumed for several weeks.
Here’s what you can do:
Dry foods: Check the processing date and buy recently produced foods, then use it up within 14 days after opening. If the food contains fish, fish oils or DHA, use up within 5 days after opening. Store in dry, cool location (avoid garages in the summer).
Raw foods: Feed within 90 days of manufacturing date. (According to the USDA, ground meats, to be considered quality, have 3 to 4 month shelf lives. Fats turn rancid even in the freezer). (Darwin’s meals are in customers’ hands typically 2 to 5 weeks after manufacturing.)
(More on the importance of fats in a pet’s healthy diet can be found in my new book, Unlocking the Canine Ancestral Diet, Healthier Dog Food the ABC Way, Dogwise Publishing, 2009, www.seespotlivelonger.com)
Further information:
Enig M. Know Your Fats: The Complete Primer for Understanding the Nutrition of Fats, Oils and Cholesterol. 2000; 230
Banwart G. Basic Food Microbiology, 2nd ed. 1989: 7.
Hermann Esterbauer, Cytotoxicity and genotoxicity of lipid-oxidation products, Am J Clin Nutr 1193;57: 779S-86S.
Halliwell, B. Gutteridge, J. Free Radicals in Biology and Medicine, 2nd ed. 1989: 208.
National Research Council, Nutrient Requirements of Dogs and Cats, 2006.