I used to be intrigued by a story I was told as a kid, that hard boiled eggs use more energy to digest than they contain and so you loose weight if you eat them.
Sounded dodgy to me, even then. It implied that if you were locked in a room with nothing but water and hard boiled eggs, you would starve to death.
But apparently that is the case with celery.
The calories in food are a measure of energy content. For something we eat to be a source of "negative calories," it must provide fewer of these units of energy than we expend in consuming it. Yet everything contains calories, so at first this concept appears impossible.
Therefore, the hunt is on for ingestibles whose energy content is not released into our bodies because we humans lack the ability to break them down — it doesn't matter how many calories these goodies have, provided we can't extract them.
Cellulose in plants is one such substance: although it contains a goodly amount of carbohydrates, they are packaged in a form we cannot digest, so we fail to absorb their calories.
Celery has about 6 calories per 8-inch stalk, making it a dieter's staple.
Its ingestion can result in negative calories, but it is a fallacy to believe that effect has to do with energy expended in chewing. Though chewing might feel like a somewhat strenuous activity, it burns about the same amount of energy as watching paint dry. It is the bodily energy devoted to the digestion of the green stalks that exhausts calories.