A colleague lamented to me that she had recommended a homemade diet containing vegetables, and the pet owner's regular veterinarian said that vegetables didn't contribute anything and weren't necessary. The owner apparently found that the undigested chunks of carrots in her dog's stool supported the regular veterinarian's opinion.
Here was my answer:
It is true that vegetables contain much cellulose and that if the cell
walls are not broken down by prolonged mastication, steaming/cooking,
or pulping them, they will pass out unidgested. Dogs are gulpers, and
descend from animals that got any benefits from vegetable matter
secondarily from their prey, who predigested it for them. So veggies
cannot just be given as chunks, unless you are trying to give the dog
something to chew on just for fun.
The benefits of veggies to dogs are mostly unknown, though one study
in Scottish terriers showed that dogs eating vegetables in their diets
had lower cancer incidence. In the absence of studies, we kind of
assume that the benefits are similar to the benefits for humans,
including not just some of the essential vitamins and minerals, but
also functional ingredients that may prevent cancer, like flavonoids.
In addition, vegetables are great for diluting the calories in a diet,
and with so many overweight dogs, they can be an essential part of
I recommend that pet owners feeding complete and balanced of ANY type - dry, canned or homemade, include veggies as part of that dog's regular fare. If you start from the time they're puppies, they won't object to the taste, which helps later in life if veggies are used therapeutically in the diet.
If your vet tells you not to feed veggies or fruits, smile sweetly and reply that you appreciate the reminder and that you would never feed onions or grapes to your dog!