Lettuce
Calories: 5 per cup
You can literally eat pounds of any variety of lettuce and never gain an ounce. Romaine lettuce alone is a great source of B vitamins, folic acid, and manganese, which helps regulate blood sugar and is essential for proper immune system function.
Choose other dark green or purple varieties such as green or red leaf for the most nutrients, and toss with a zesty homemade vinaigrette.