We think of the production of pet food as a fairly scientific affair, but the truth is, all pet food manufacturers make mistakes in formulation or production that result in nutrient excesses or deficiencies. No matter how good, bad or indifferent the manufacturer's reputation may be----at one time or another they fail one or more tests for protein, calcium, magnesium, or other nutrients.
Here are some things to consider:
- The standards by which pet food is made aren't perfect. Animal nutrition is an evolving science and we don't yet know all there is to know.
- The exact amounts of nutrients in a given ingredient may not be known, or may be inaccurately assessed. A vitamin/mineral premix added to the food may guarantee minimum levels of each item, but if the quality control on that premix was poor the finished pet food will compound the error.
- Also, because of the nutritional standards themselves, which specify minimums for most nutrients but not maximums, pet food makers may not test for the probability that their finished product is too high in some nutrient. Excesses of certain minerals, like zinc for example, can wreak havoc on dogs' health. Even a big difference between the stated and actual amount of something like protein or fat can cause problems for some dogs. Cats have been the victims of nutrient deficiencies more frequently. Cats on some foods for long periods of time have turned up with taurine, copper, vitamin E, and potassium deficiencies.
- Some manufacturers develop a formula and/or recipe for making their food, and they will stick with that recipe no matter what. If that recipe results in a food that is typically at the very low or high end of acceptability for some nutrients, you can imagine that after years of eating the same food, your dog's body will eventually exhibit the effects of that chronic over- or undersupply of those nutrients.
A dietary intolerance is a reaction to something in the food, but this reaction does not involve an immune response. Signs of gastrointestinal disturbances (especially vomiting and diarrhea) are far more likely to be caused by food intolerance than by a food allergy. A food tolerance can develop at any time in the pet's life.
True food allergies----immediate immune responses triggered by exposure to a certain food----are thought to be fairly rare. Food allergy usually causes skin reactions, such as papules, rashes, and ear infections. (What actually happens is the inflammation sets up the scene, and then bacteria and yeast that are normally present cause secondary infections.) Keep in mind, however, most skin reactions are due to inhalant allergies; only a small portion turn out to be caused by food. It usually takes months to years of exposure to a food to develop a food allergy, and food allergies are usually caused by proteins. (Proteins are found in animal products and also to some degree in cereal grains.)
Another good reason to periodically change foods is to prevent finicky eating. Many pet food makers use palatability enhancers, and pets fed a single food can sometimes become "addicted" to one particular flavor.
For all these reasons it's a good idea to change foods occasionally----as often as every three to four months. There is some evidence in people that avoiding a particular "problem food" for four months may resolve the issue and the body will again tolerate it. Also, that's a short enough period that allergies are unlikely to develop.
Although feeding a mixture of commercial foods is not recommended, it's good to vary the pet's diet by switching brands and varieties including the source of protein (i.e. from chicken to lamb or turkey, etc.).
When you get ready to change from one food to another, be sure to plan ahead. A sudden switch could cause tummy upset. For most dogs and cats, a four-day or eight-day changeover works best. Younger pets usually adjust quickly; older pets may take longer. For the first day (or two), feed 75 percent of his/her old food mixed with 25 percent of the new food. After a couple of days, feed 50/50, then 25 percent of the old food and 75 percent new food, and then finally all new food.
For the first two weeks on a new food, monitor your pet's appetite, stool quality, and energy level. Watch for any unusual symptoms----itchiness, runny eyes, diarrhea----that could indicate the food is not right for him. Eventually, you'll be able to settle on three or four different foods which you can rotate.
Variety is the spice of life...so change your pet's food occasionally. Feeding the same kind, year after year, can cause health problems. Not to mention it gets boring to your pet. Would you like to eat the same thing for every meal, day after day, month after month, year after year? If it was up to them, they wouldn't!
Resource: Handbook of Dog & Puppy Care and Training by The Whole Dog Journal, 2008
Update: (Here's a perfect example!) On May 21, 2009, Nutro Products recalled some of it's dry cat foods due to excessive levels of zinc and under-supplemented potassium. This was caused by a production errror. Read more about it at The U.S. Food & Drug Administration.