It's long been claimed that some foods take more calories to digest than what they actually contain.
But research has now suggested 'negative calorie' foods, as they are branded, are nothing more than a myth.
Lizards chowed down on celery-based meals in what is thought to be one of the first studies to test the theory.
The reptiles held on to around a quarter of the foods' calories after digestion and excretion - busting the idea that so called 'negative-calories' exist.
Celery is just one food considered to be negative calorie, alongside cucumber, lettuce and broccoli among others.
Dieters are a fan foods like celery as they are 'negative calorie' foods. But researchers at the University of Alabama showed that lizards retained around a quarter of the calories in celery
The study, led by Katherine Buddemeyer from the University of Alabama, used a group of bearded dragon lizards.
Their metabolic rate - how much energy is required for the body to carry out basic functions - was calculated.
The lizards were fed meals of diced celery equal to five per cent of their body mass - something they would willingly eat as part of their omnivore diet.
Faeces and urine samples were collected after they ate to determine how much energy was lost.
The findings showed that the lizards used about 33 per cent of the calories in the meal for digestion and about 43 per cent were excreted.
The basic amount of calories an average adult needs per day is 2,000kcal for women or 2,500kcal for men.
This is based on the amount of energy the body needs to carry out basic functions and to walk and work throughout the day.
People who exercise a lot need to eat more calories to fuel their efforts, and young people and children burn more energy, too.
If you eat more calories than you burn in a day, you will get fatter.
Eating fewer calories than you burn will make you lose weight.
Foods which are processed and have high levels of carbohydrate, sugar and salt have higher calorie numbers than fresh fruit and vegetables.
Example calorie counts include:
A McDonald's Big Mac contains 508kcal A two-finger KitKat contains 106kcal A banana contains 95kcal An apple contains 47kcalSource:
