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 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 is not calorically dense. A 2005 study based on a low-fat plant-based diet found that the average participant lost 13 pounds (5.9 kg) over fourteen weeks, and attributed the weight loss to the reduced energy density of the foods resulting from their low fat content and high fiber content, and the increased thermic effect. A study on chewing gum reports mastication burns roughly 11 kcal (46 kJ) per hour.
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. 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. (Additionally, excessive water consumption can be dangerous.)
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