Many diet memes come and go, but some seem to build up a history in our consciousness, and the celery's negative calorie meme is no exception. A commonly believed thought by many dieters is that celery has negative calories--eating it, as it is said, will actually end up burning calories for you.
Separating Fact From Fiction
Whether celery has negative calories seems to be beside the point of most dieters. When one chooses to modify their body image not through natural exercise and sensible eating, but rather through unverified memes and unhealthy diets, there's no real way to explain the truth to that person without it getting distorted in the process.
The real truth is that celery both does and does not have negative calories. As a plant, celery stores its calories throughout its body, just as every other plant does. There are plenty of food calories in celery; the difference is that humans cannot adequately process those calories.
The majority of energy that celery has locked within it passes through our digestive system without being processed. The cellulose calories celery has is irrelevant; all that counts is the six or so food calories we can get from each eight-inch stalk. Since it takes more than six food calories of energy to digest the celery, the net result is that digesting celery does, in fact, exhaust calories. In a very real sense, celery is a "negative calorie" food.
Truth About Celery
However, none of this really matters. If all you're looking for is a negative calorie food, then cold water fits the bill even better. This is more because bringing the cold water up to our internal body temperature takes energy than it does because digesting water takes any energy at all. Regardless, it doesn't matter. It takes around 3,500 food calories to work off a pound of fat. Eating celery all day long would make about as much difference as sleeping all day would. Both activities burn about as many calories, and sleeping is much more fun. Eating celery to lose weight is like trying to build a mountain by transferring specks of sand one by one. Sure, it has an effect, but it is so minuscule that it is hardly worth the effort.
Pros of Celery
Still, this doesn't mean that celery is a pointless food, even for fully informed dieters. If you're serious about not letting calories in (a diet mantra that isn't healthy, but does give raw results in the short term), sometimes it helps to have foods around that you can eat instead of whatever else you're trying to stay away from. In this way, celery can become a kind of chew toy: it doesn't help dieters because it has "negative calories" (even though it does), but it helps by taking one's mind off of whatever it is you're trying to avoid eating.
To Eat or Not to Eat Celery
Diets that lean on restricting calories, however good their intention may be, have been shown again and again to be unhealthy short term answers. Yes, you will lose weight if you deny yourself calories. However, the next time you eat, your body will learn to conserve energy in preparation for the next phase of restrictive calories it thinks you will start. The body conserves energy in this way by creating more fat than it would otherwise have done, and thus makes you gain weight you otherwise wouldn't. Most serious scientific studies have shown that restrictive calorie diets cause more weight gain in the long term through period of yo-yo dieting; those who instead eat sensibly without restricting calories gain less weight over time.
In the end, if you care about your health, you should eat celery because it tastes good, and not for any other reason. That way you get the added benefit of having dip with your celery, which is, after all, what celery is best for in the end.