Although grain-free diets are not harmful to most dogs, they are not necessarily healthier for the average pet than a diet that contains grains. Dogs who have allergies to grains do benefit from grain-free diets.
Many advocates of grain-free diets claim that dogs cannot digest grains, but this is untrue. Dogs evolved alongside humans and subsisted on table scraps for thousands of years, so healthy dogs are well-suited to eating grains. Some dogs do develop allergies to wheat or other grains, and switching to a grain-free diet may alleviate that. However, allergies to more than one type of grain are rare, so most dogs would also do well simply by switching to another type of grain.
Grain-free diets often replace the grains with other types of carbohydrate-heavy foods, including potatoes. This can cause increased weight gain in older or more sedentary pets, contributing to health problems such as arthritis. Dogs prone to obesity may do better on a weight-control diet that includes grain.
The most important consideration when choosing a dog food for the average healthy pet is its full nutritional profile. Owners should look for foods that are approved by the Association of American Feed Control Officials. AAFCO develops veterinary-approved nutrient profiles and health standards for commercial dog foods, and its testing helps ensure the food is safe and healthy. It approves foods that contain grain and that are grain-free.