Negative-calorie or zero-calorie foods are controversial despite their popularity on diet websites. The idea is that our bodies use as much (or more) energy to digest a chosen fruit or vegetable as the energy it provides.
But there's no scientific evidence for this food fad, with even the lowest calorie foods, such as celery and watercress, containing more energy than the body needs to process them.
In 2019, researchers from the University of Alabama ran a study on celery-munching bearded dragons. The choice of animal may seem unusual, but bearded dragons have an omnivorous diet and the same mechanisms for digesting food as humans.
The giant lizards ate five per cent of their body weight in celery. The researchers measured their metabolic rates, and collected faeces and urine to calculate the energy lost to excretion.
They found that the lizards only used about three-quarters of the celery energy, debunking the idea of negative-calorie foods.
In any case, some nutrition experts argue that it’s not sensible to focus on calories for maintaining or losing weight, because our bodies use different biological mechanisms to digest different types of food.
We use far more energy to digest proteins than fats or carbohydrates, which have highly accessible energy. So, the calorie content is less important than the types of foods you’re consuming.
Ultra-processed foods also tend to be high in fats and carbohydrates, offering instant satisfaction, but they also deliver a big dose of energy, much of which we store in fat cells.
Whole foods, especially those packed with protein and fibre, generally require more energy to digest than processed foods.
Fresh fruit and vegetables touted as negative- or zero-calorie foods make a valuable contribution to a balanced diet.
But if you tried to survive on these alone, you’d risk not only depriving yourself of nutrients, but also becoming very hungry and irritable.
This article is an answer to the question (asked by Dani Sherman, via email) 'Are negative-calorie foods real?'
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