1] As the person will have a very diminished appetite because of taking HCG they will not be able to eat very much.
2] Vegetables when cooked in vegetable stock or raw (no oil or fat)have extremely low calories.
3] Also have lots of fiber so are very good for detoxification which aids in weight loss.
4] They are loaded with nutrients so help build the bodies strength, general health and well being.
All in all the more vegetables the better -remembering the person using HCG will only be able to eat a small amount anyway.
But consider – a salad of lettuce, capsicum, carrot, tomato, asparagus with a boiled potatoe would only be 100 calories.