In a large stockpot, combine the chicken broth, potatoes, onions, carrots, and celery.
Bring to a boil, reduce heat to medium-high, and simmer until the potatoes are tender, about 30 minutes.
Add the zucchini, tomato sauce, garlic, parsley, and cilantro. Reduce heat to medium-low and cook for 10 to15 minutes more, or until the zucchini is just tender. Season to taste with black pepper and serve.
If desired, add protein powder just before serving.