Snake oil salesmen all over the web would have you believe that, for a few easy payments, they'll reveal to you a diet focused on “negative calorie” foods. You’ll be able to eat all you want and lose weight while doing it, because the amount of energy used to chew and digest these foods is supposedly greater than the amount of energy they provide. But is that really true?
Some of the energy in every piece of food you eat is considered a wash because an equal amount of energy is expended to eat and digest it, but the thermic effect of food—or diet-induced thermogenesis, as it's technically known—usually only accounts for around 10 percent of your caloric intake (there are some outliers, of course, and high-protein foods may lose as much as 30% of their energy to digestion, while certain easily-processed fats have a thermic effect of as little as 2 or 3 percent).
Celery is one of the most touted negative-calorie foods, because much of its caloric content is bound up in cellulose, a fiber that humans can’t digest. The amount of energy we can extract from celery is negligible, a measly 6 calories in a medium-sized stalk. But even then, it only takes a little more than one half of a calorie’s worth of energy to digest what we can of that piece (and maybe even less: the thermic effect has been shown to be lower after high-fiber meals).
The bottom line is that any kind negative-calorie snacking, celery or otherwise, is purely wishful thinking. But celery stalks are still worth a chew: They're obviously better for your caloric balance sheet than, say, a candy bar or a Slim Jim. Just don’t expect them to be green, fibrous magic bullets for your diet.