While this concept is popular in dieting guides, there is no scientific evidence that any of the foods claimed as negative calorie foods are such. Foods that are claimed to be negative in calories are mostly low-calorie fruits and vegetables such as grapefruit, lemon, lime, apple, lettuce, celery, broccoli and cabbage.Celery, a commonly cited negative calorie food, actually requires only about 10% of its food energy content to be digested (due to the thermic effect).Zero-calorie beverages, such as water, do take more than zero calories to process within the body, however they do not offer the dietary sustenance that other so-called negative calorie foods do in the form of fiber, carbohydrates, vitamins, etc. and as such, are not generally regarded as being negative calorie.