Foods with negative calories are those whose caloric content is lower than the calories burned by the body to digest them. But are they in reality or is it all just a nutritional myth?
A little anatomy of digestion
When we eat, only a portion of the calories ingested are available to the body to be burned for energy, or to be stored. That's because some of the calories are lost in the digestion process. For example, let's say you eat something that contains 100 calories from carbohydrates, fats and protein. During intestinal absorption, 80% of the calories are absorbed by the body and 20% are excreted (this can happen by eliminating partially digested fiber or food). Moreover, the processes by which the body processes food, digestes it and absorbs it, consume energy (the thermal effect of food, which varies depending on what macronutrients it contains). Let's say you burn 10 calories for this. So 70 calories remain for the body to use as energy.
But are there foods that bring in less calories than needed to be digested?
What is a negative calorie food?
The hypothesis behind foods with negative calories suggests that certain foods have fewer calories than the body needs to digest and metabolize them. Theoretically, the result is a food that creates a caloric deficit, thus helping to lose weight!
There is not much evidence that such foods exist. A small study investigated the effects of celery - which is said to be the highest calorie-negative food due to its very low net caloric content, of only 16 calories per 100 g. The study measured the basal metabolic rate of 15 young women (mean age 23 years), before and after the consumption of celery. Of the 16 calories of celery, only 2.24 calories were absorbed.
Celery is not a food with negative calories, but being very rich in water and dietary fiber, it is a perfect weight loss food.
Another study went ahead and analyzed the so-called negative calorie diet, with a traditional low calorie diet. The negative calorie diet claims that it is made up of foods that create a calorie deficit. The study lasted 3 months and included 37 obese or overweight men. The two groups consumed the same number of calories per day. In the end, there were no notable differences between the two groups. This has led to the conclusion that foods with negative calories have no external applications or meanings.
Negative Calorie Supplements?
Some foods may increase your metabolism; some examples are synephrine and naringenin from grapefruit or hygenamine from nandina fruit. Is it possible that these bioactive compounds increase the rate of metabolism so much that it creates a deficit?
The reality is that these supplements increase metabolism, like caffeine, but growth is usually negligible. A supplement that helps you burn 10 extra calories will not have a significant impact on your attempt to lose weight.