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A negative-calorie food is a food that is incorrectly claimed to require more food energy to be digested than it provides. That is, its thermic effect – the caloric "cost" of digesting the food – is greater than its food energy content. The thermic effect is scientifically called specific dynamic action. While this concept is popular in dieting guides, there is no scientific evidence to support that any of the foods claimed as negative-calorie foods are such.
Some soft drinks are erroneously advertised as having "negative calories". The only truly negative-calorie beverage is ice water, which has no calories but requires the body to expend a very small amount of energy to raise the liquid to body temperature.
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. 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 calories 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 has a lower calorie count per volume.
Soft drinks advertised as being "zero-calorie" actually contain some calories, and take fewer calories to process within the body than the amount they provide. Moreover, such drinks do not offer the dietary sustenance that other so-called negative-calorie foods do in the form of fiber, carbohydrates, or vitamins, and as such are, in general, not regarded as being negative-calorie.:84 Ice-cold water is the only beverage that could be called a "negative-calorie" beverage.:84 Cold water will expend a greater number of calories because the body has to warm the liquid to body temperature, although a single glass of ice water at 0°C would burn only 8.8 kcal. Drinking one such glass a day, it would take a person over a year to lose a single pound of weight.
In February 2007, the watchdog group Center for Science in the Public Interest sued The Coca-Cola Company and Nestlé S.A. for falsely advertising that their diet soda Enviga had "negative calories". The settlement required that any announcement of "negative" calories, burning of calories, etc. had to be accompanied by a disclosure that weight loss happens only if the consumer also diets and exercises. See more details in Enviga#Lawsuits over health claims.