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A negative-calorie food is food that requires more food energy to be digested than the food provides. Its thermic effect or specific dynamic action—the caloric "cost" of digesting the food—would be greater than its food energy content. Despite its recurring popularity in dieting guides, there is no scientific evidence supporting the idea that any food is calorically negative. While some chilled beverages are calorically negative, the effect is minimal and drinking large amounts of water can be dangerous.
Foods that are claimed to be negative in calories are mostly low-calorie fruits and vegetables such as celery, grapefruit, lemon, lime, apple, lettuce, broccoli, and cabbage. These foods are not negative-calorie foods. There is no scientific evidence to show that any of these foods have a negative calorific impact. Celery has a thermic effect of around 8%, much less than the 100% or more required for a food to have "negative calories". A stalk of celery provides 6 kcal to the body, but the body expends only half of a single calorie digesting it. Even proteins, which require the most energy to digest, have a thermic energy of only 20%–30%. Diets based on negative-calorie food do not work as advertised but can lead to weight loss because they satisfy hunger by filling the stomach with food that is not calorically dense.
Water has zero calories and cold water is "negative-calorie" since the body must warm it to body temperature. Some infusions like plain tea and coffee are also effectively zero calorie and their caffeine can also increase one's metabolic rate. The weight loss occasioned from heating chilled beverages, however, is minimal: five or six ice-cold glasses of water burn about 10 extra calories a day and would require about a year to eliminate a pound of fat. (Excessive water consumption, however, can be dangerous.)
Urban legends that diet soda manufacturers bribe the American FDA to defraud the public are untrue, but the FDA does permit any food or drink with less than 5 calories per serving to be labelled as containing 0 calories. Replacement of standard sodas with diet ones has been linked to "significant weight loss" by some studies but others have found no benefit at all, as participants simply consumed more calories from other sources. Claims by Coca Cola and Nestlé that their Enviga line of soft drinks had "negative calories" occasioned a false advertising suit against them by the Center for Science in the Public Interest in 2007. The companies settled with the Connecticut Attorney General in 2009, paying hundreds of thousands of dollars and agreeing that any weight-loss-related marketing of the product must be accompanied by disclaimers that weight is only reduced by diet and exercise.
What are these magic foods? Just the low-calorie, high-nutrient-density fruits and vegetables that you might expect to be recommended to someone who is dieting: celery, grapefruit, lemon, lime, apple, lettuce, broccoli, cabbage, and other such items.