There is an enduring myth that certain type of foods have "negative calories", meaning you use up more calories eating and digesting them than the number of calories they actually contain. Many have argued against this notion, and now a new study – albeit in lizards, and currently under review – has provided some important empirical evidence that negative calories are not a thing. That said, the research does have some good news.
Foods that are low in calories and high in water content and fibers, like celery, lettuce, grapefruit, cucumber, and broccoli are often purported to have "negative calories". This is why they are often suggested in short term, sometimes worryingly extreme, weight-loss diets. For this new study, available to read on the biology pre-print site BioRxiv, the researchers looked at celery in particular, as it is the most cited example of this alleged property.
Researchers from the University of Alabama fed an all-celery diet to an animal model, in this case, bearded dragons (more on this later), and measured how many calories were lost to either digestion or excretion after each meal. On average, the lizards retained a quarter of the calories consumed from each celery meal. One-third of the calories were required for digestion, 43 percent were excreted in either feces or urate, and the remaining 24 percent was absorbed by the dragons. Thus calories were consumed, and celery is not calorie deficit.
Why bearded dragons? Using lizards might seem an unconventional choice, but they are naturally omnivorous, like us, and actually have a similar digestive system – the way they digest, absorb and assimilate food – to humans. On top of that, they are docile and seem to like celery, which makes them pretty good for this study.
While the study concluded that negative calories are not a thing, celery remains nutrient poor, which is the "good news" from the study. Despite still gaining calories from it, the amount is so small that it doesn’t cover the body's metabolic budget, and you will eventually end up in "negative budget", meaning eating these types of food could result in loss of body mass.
Although the study just looked at celery and lizards, they used their findings to make some assumptions about whether other oft-cited negative calories foods exist, including watermelon, tomatoes, carrots, apples, and blueberries. They estimated that humans would retain between 19 and 50 percent of the calories from these foods, but that these foods are also negative budget.
Still, just to cover one's basic metabolism (ignoring moving about), an adult woman would have to eat 12.6 kilograms of raw celery, 9 kilograms of raw tomatoes, or 4.3 kilograms of raw carrots.
“Rather than labeling such foods as “negative calorie” it would be more accurate to pitch these foods as “negative budget”, the consumption of which will favor a daily negative energy budget, and hence weight loss via the catabolism of body fat.” the researchers write in the paper, which has been submitted to the Journal of Experimental Biology.