The concept of negative calorie foods, or the idea that you burn more calories digesting and metabolizing certain foods than the foods themselves contain, is more of a diet myth than a reality. Even though some foods may give your metabolism more of a boost than others, and some contribute so few calories to begin with that they do require a couple of extra calories for digestion, the extra calories burned won't be enough to affect your weight.
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Food supplies the calories you need to keep your heart pumping, your lungs breathing and all other body systems functioning, including digestion. It is an ongoing process, so what you eat for breakfast may end up giving you the energy you need to digest your dinner. Basically, you are burning calories all the time, at different rates, depending on what those calories are being used to do. If you stopped burning calories, your body functions would all stop, too. Because you are burning calories even when you're not eating, it is impossible to account for every calorie or to pinpoint which calories from which food are being used to perform any particular function in your body.
The types of foods often credited as negative calorie foods include celery, apples, broccoli, asparagus, grapefruit, cucumbers, cabbage, radishes and other spicy foods and foods that contain caffeine. Cold foods, including cold water, are sometimes called negative calorie foods because your body burns extra calories warming them up during the digestion process.
To understand why there is no real validity to the theory that negative calorie foods can be a weight loss aid, it helps to understand what a calorie is. By definition, a calorie is actually a measure of the amount of energy required to produce heat. In terms of food, a calorie is used to define the amount of stored energy in a particular food. If a food contains 50 calories, that is the potential amount of energy stored in that food. High calorie foods have more potential energy than low-calorie foods. Water has zero calories, and foods that contain a lot of water are very low in calories, but no food has a negative energy value.
When your body uses calories to digest food, it is called the thermic effect of food. In essence, 5 percent to 10 percent of the calories from any food is used to digest that food. For example, if an apple contributes 75 calories to your diet, the thermic effect of that apple is anywhere from about 3.5 to 7.5 calories. That's how many calories your body will burn to digest the apple. Those calories, however, don't necessarily come from the apple itself. They come from foods that supplied your body with energy well before you ever bit into the apple.
Complete weight loss plans have been marketed around the idea of negative calorie foods, and a search of the Internet will yield many sites that list supposed negative calorie foods. The real value of these foods as diet foods is not their negative calorie status, however -- it is their low-calorie status. Most foods credited with negative calories are very low in calories and, for that reason, can be eaten freely as part of any weight control plan.