There is no such thing as a truly calorie-free food, but there are a number of items that contain fewer calories than the body uses in breaking them down. Some of these foods include celery, cucumbers, kelp, asparagus and tomatoes.
The term calorie-free food is generally used to refer to items that contain less than 25 calories per serving, according to WebMD. This term is used because the body requires more than 25 calories to chew, swallow and digest a serving. There are also many "almost free" foods, which contain between 25 and 60 calories per serving. This list includes carrots, grapefruit, strawberries, oranges and peaches.
Celery is one of the lowest calorie foods, and it typically contains approximately seven calories per stalk. Cucumbers are also very low in calories, as one half of a normal sized cucumber generally contains around 20 calories, which is about the same amount as is in a half cup of cooked asparagus.
The term calorie is the most commonly used measure of food energy. In nutrition, this term actually refers to the kilocalorie or large calorie, which is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius.