I made this cabbage soup about 2 days ago and it was good. I don't know if its a diet soup though.
but here is the recipe because it doesn't have much in it.

4 thick slices bacon,diced
1 onion diced
2 carrots,diced
2 potatoes, cubed
1 head green cabbage, coursely chopped
6 cups water
2 chicken bouillon cubes
parsley (optional)
garlic salt to taste (optional)
salt and pepper to taste
2 green onions sliced
2 sticks celery sliced

in a 6-quart pot combine all ingredients except salt, pepper, half onion and bacon. simmer partially covered for 1 1/2 to 2 hours.meanwhile fry onion and bacon until brown or crisp, then add to soup with or without drippings (drippings add flavor). In mine when it was almost down i added Kielbasa(hillshire farms). salt and pepper to taste and serve. I don't know how healthy this dish is but it is really good. my 15 year old daughter had 3 bowls the first night and then finished off the second.