First of all, why do people feel they should feed a grain-free diet? One theory is that cats and dogs evolved primarily as carnivores, and when we feed them too many carbohydrates this contributes to medical problems such as diabetes, obesity, and arthritis. Other people feed grain-free because they believe that their pets, especially those with skin disease, may be allergic to corn, wheat or gluten in the diet.
Are grain or gluten allergies common in cats or dogs? According to most veterinary allergists, the most common cause of food allergy in dogs is beef, followed by dairy and wheat. In cats, beef, dairy and fish top the list. Of course, allergy can develop to any protein in the diet. The only reliable test for food allergy is a strict hypoallergenic or novel protein diet for 6-8 weeks, then a challenge with the suspected allergen.
Often people assume that a grain-free diet is also a low-carbohydrate diet, but this is not necessarily the case. Any dry kibble diet must have a certain amount of carbohydrate (usually at least 20%, though more commonly 50%) so the source in grain-free diets will be potatoes, vegetable starch, peas or tapioca. If your main concern is to limit carbohydrates, you need to look at the nutritional information for the amount of carbohydrates, not just the source.
You can roughly estimate the carbohydrate content of a dry food by adding the percent protein, fat, ash, and moisture content, and subtracting from 100. This will give you an estimate of the carbohydrate percent. (You can ignore the fiber content for this calculation, as it will count as carbohydrate.) Ash content is not usually on the bag, so you will have to contact the manufacturer if you want an accurate amount, but for most calculations you can estimate 7% and be fairly accurate. Water content in most dry foods is 10%. For example, Blue Freedom Beef Grain-free for dogs is approximately 50-55% carbohydrate. Purina SmartBlend True Instinct is approximately 35% carbohydrate. (Whether you need to limit carbohydrates in your pet’s diet will depend on multiple factors.)
So does your pet need to go grain free? Whole grains are not evil unless your pet happens to be sensitive to them, so a diet that contains wheat, rice, oats or corn may work well for your pet. If your pet is eating a particular diet and doing well, there is no need to change it. If you do decide to switch diets, the WSAVA (World Small Animal Veterinary Association) has good articles on how to research and choose a diet.
And it’s a good idea to check with your veterinarian as well, to discuss dietary considerations that might be unique to your pet.