I was in high school when I first heard (by way of starving wrestlers, not a pack of mean girls) that celery has negative calories—that the actual energy expended in chewing and metabolizing negates the low caloric content of a stalk.
While there is scant evidence (no formal studies) to support this theory, many biologists and gastroenterologists acknowledge that it's simple math and may be true.
As they point out, however, no man can subsist on nutrient-low celery and lettuce alone (seriously, manorexics, don't try it). But if you're trying to shed a few pounds, there may be a way to choose nutrient-rich foods that require so much energy to burn that your body will ultimately retain fewer calories than it would following other diets.
Whole (unprocessed, unrefined) foods require much more energy to metabolize than processed foods, according to this Food & Nutrition Research study. The researchers fed men a sandwich made of either processed white bread and processed cheese or whole-grain bread and raw cheese. Both sandwiches contained the same calorie count, but those who ate the whole-food sandwiches spent twice the amount of energy digesting and metabolizing than those who ate the processed meals.
So if your goal is to make food choices that will help you lose weight, opt for whole and natural foods, which require more work to digest. Eat whole grains, scarf down your colorful vegetables raw (as well as certain fruits like grapefruit and apples), and have your steak rare (cooking makes food easier to metabolize). You'll be burning more calories without even knowing it. And that's a healthy negative experience.
—Mike Dawson is a magazine writer and editor, and a regular contributor to Details.