Foods That Burn Calories During Digestion
It's not true, as some believe, that there are "negative calorie" foods. Certain foods, as the story goes, burn more calories during digestion than they provide. Supposedly, the net result is that it's possible to lose weight by eating large amounts of these foods. While it is possible to lose weight with low-calorie foods, and all foods burn some calories in digestion, there are no true negative-calorie foods.
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Identification
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Many foods appear frequently on negative-calorie food lists. Grapefruit, oranges and celery are among the most common. Other foods cited as burning more calories than they provide are broccoli, cabbage, carrots, garlic, lettuce, apples, pineapples, lemons and mangos. While all these foods are healhtful, none of them burns more calories in digestion than it ultimately provides.
Function
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A medium-size piece of celery has about six calories. However, it takes much less than even those six calories to digest that piece of celery. Calories are units of heat energy. The amount of increase in body temperature due to digestion of celery, a way of measuring expenditure of energy called the thermic effect, is only about half a calorie. The relatively low level of energy needed to digest food is part of the body's natural efficiency, and is essential to healthy function.
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Effects
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Of the three major types of macro-nutrients--proteins, carbohydrates and fats--proteins require the greatest energy to digest. This is why lean meats like salmon occasionally appear on lists of negative-calorie food. Unfortunately, even in the case of protein, the amount of energy required to digest the food is only about 30 percent of the total calories released from the food.
Potential
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Perhaps the persistence of the negative-calorie-food myth is because it is feasible hypothetically. It is possible that a food could require more energy to pass through the digestive tract than it contains if it were made entirely of indigestible matter like cellulose. Cardboard, for example, fits this description. The problem is that all foods, even the fiber-rich ones that contain large amounts of cellulose, also contain more calories, even if negligible, than it takes to digest them.
Considerations
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Another reason the myth might be appealing is that eating the foods that appear on the negative-calorie-food lists can actually result in weight loss. Many of them, especially the fruits and vegetables, are relatively low-calorie and nutrient-dense. If you were to eat mostly celery, for example, you would undoubtedly experience weight loss. But this would be because celery contains fewer calories than the foods you normally eat, and a mostly celery diet would therefore contain fewer calories than your normal diet. Weight loss would not occur because of any negative-calorie effect of the celery itself.
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