If a pet is overweight, a grain-free food will not help it lose weight faster than another food. Animal nutritionists and veterinarians agree that there is no data to support the idea that grain-free diets are better for pets. “Claims like no gluten, no grains and no soy generally mean no science.”
Source: Washington Post, August 15, 2017 (paywall).
Some pet owners have a false impression that grains are more likely to cause an allergic reaction, but “it’s much more common for dogs to have allergies to meat than to grain,” Heinze said. Chicken, beef, eggs, dairy and wheat are the most common allergies in dogs. And it’s not that there’s anything particularly allergenic about these foods, she said, they’re just the most frequently used ingredients.