Because of the remarkable efficiency of the human digestive system, there are very few foods that provide fewer calories than they require for digestion. Some skeptics, including Dr. Nancy Snyderman of Time magazine, actually state that there are no negative-calorie foods, and that the calories burned during chewing and digestion are negligible.
However, some evidence supports the use of negative-calorie or calorie-free foods in very low calorie diets (VLCD). A study published in the International Journal of Obesity found that morbidly obese people eating extremely low-calorie foods lose about two kilograms per week-- a rate that is nearly identical to total starvation. These findings would suggest that low-calorie foods are capable of enabling catabolic activity. The following foods are considered to be negative calorie because of their high fiber content, enzymatic action and/or extremely low caloric load.
Grapefruit
A core component of the once-common grapefruit diet, the grapefruit is known for its high content of catabolic enzymes. In theory, the compounds in grapefruit can break down surrounding foods in the stomach, resulting in increased weight loss.
Vegetable Broth
Broths made using only vegetables, salt and seasonings contain few or no calories per serving. The body also expends energy to cool piping hot liquids to body-temperature.
Leafy Greens
Spinach, kale, collard greens, turnip greens and mustard greens are all extremely high in fiber, which can speed digestion and expend energy during the digestive process. They contain very few calories in each serving.
Cucumbers
Pickles and cucumbers are composed largely of water and they are a very low calorie source of bulk fiber.
Celery
Perhaps the most famous of negative-calorie foods, celery contains only fifteen calories per serving and it is loaded with laxative fiber and catabolic enzymes.
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