Vegetables, rich in phytonutrients and high in fiber, make a fine addition to a dog's diet. However, a dog's digestive system is different from that of a human being. It does not digest sugars as well, and it cannot break down plant cells. Some vegetables are high in sugars and should be fed to a dog in low amounts; others are actually harmful for a dog. Cook or puree any vegetable before adding it to a canine meal.
Cruciferous and Bulb Vegetables
Broccoli, cabbage, Brussels sprouts, bok choy and cauliflower are low in sugar and high in the phytonutrients carotenoids, flavonoids, indoles, sulforaphane and sterols. Onions, scallions, shallots and garlic can cause gastrointestinal problems for a dog and in large amounts can cause red blood cell damage. However, minute amounts for flavoring are not likely to cause a problem.
Spinach, kale, Swiss chard and other dark leafy greens are low in sugars, high in fiber and an excellent source of carotenoids. Greens can be fed to dogs in small amounts. Steam greens to cook them to retain most of their nutrients. A dog may benefit from low-sugar green beans and celery, but avoid green peas, which are high in sugar.
Root Vegetables and Squashes
Cucumbers and summer squashes, such as zucchini and yellow squash, are relatively low in sugars and easy for a dog to digest. Winter squashes, including pumpkins and other hard-rind squashes, are high in carotenoids and sterols. Carrots, sweet potatoes, beets, parsnips and rutabagas are high in sugar and should be given to a dog in moderation. Carrots contain carotenoids, while sweet potatoes contain carotenoids, flavonoids and sterols. White potatoes are very high in sugar, so it's best to keep them out of a dog's diet.
Other Plant Foods
Tomatoes are high in phytonutrients like lycopene, sterols and flavonoids. Bell peppers are a good source of carotenoids. Eggplant is a good source of sterols. However, in a dog with arthritis tomatoes, peppers, potatoes and eggplant may aggravate the condition. All parts of the avocado -- flesh, pit, skin and even the leaves and bark of the tree -- contain persin, a chemical that causes vomiting and diarrhea in dogs.