Vegecat™ for Adult Cats

...studies found that cats can adapt to different protein and carbohydrate intakes so long as their minimal requirements for protein is met.(5) For example, it was found that "when properly processed starches and carbohydrates from whole grains are provided as a major component of balanced diets, cats are able to digest and use them, with an average apparent digestibility exceeding 90 percent. Thus, while cats may metabolize carbohydrates differently than dogs and other species, healthy cats are easily able to digest and metabolize dietary carbohydrate."(6)

...cats cannot use plant foods to obtain enough vitamin A (as preformed retinol), taurine, arachidonic acid, and vitamin B12. All of these, as well as choline, methionine, calcium, and other nutrients are typically added to commercial foods to ensure a complete and balanced food...Similarly, home-prepared diets with little or no meat require careful attention to these nutrients. Vegecat is designed to meet all these needs to official standards...

Ultimately, the proof is in the pudding. If cats could not utilize grains and other carbohydrates for much of their energy needs, they would not be able to live on typical supermarket pet foods, which are largely just that.