Zero-calorie foods include cucumbers and celery, which are both high in water and create a feeling of fullness. Oranges and grapefruits fall into the zero-calorie category as well.
Fitbie explains that foods which are classified as "zero calorie" are those that contain less calories that the amount of calories it takes to burn off the food. Cucumbers are rich in potassium and vitamin C, as well as silica, a compound that helps to maintain and build the muscles.
Celery has very little calories as well, and is packed with nutrients. Celery is a significant source of fiber and contains vitamin A. Phthalides are abundant in celery. These compounds relax the muscle tissue in the walls of the arteries to increase blood flow and lower blood pressure.
Citrus fruits such as grapefruits and oranges are high in vitamin C as well, which helps to lower fat in the midsection. Research from the University of Arizona discovered that people with high vitamin C levels were able to oxidize 25% more fat during treadmill exercise than those with low vitamin C levels.
Studies from the University of Iowa state that apples contain ursolic acid, a compound that prevents weight gain. Ursolic acid is abundant in the peel of this low-calorie fruit. However, it is not a "zero-calorie" food.