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.
HOW MANY CALORIES DO I NEED EACH DAY?
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 47kcal
That meant that the animals retained about 24 per cent of the calories from the celery – however, it was still a minute amount.
The authors wrote: ‘By evaluating these energy tradeoffs, we determined bearded dragons to experience a net gain in energy from their celery meals. However, this gain is rapidly abolished by the lizard’s resting metabolism.’
Although the study only looked at one animal, the researchers made a few assumptions about how this could be translated to humans.
‘The same is undoubtedly true for humans,’ they said in the report of their findings published on bioRxiv.
‘Those foods touted as negative calorie do generate a net energy gain; however this gain is quickly abolished by the body’s own basal rate of metabolism.’
The researchers used the example of a 132lb (60kg) woman to suggest that humans would retain around 19 to 50 per cent of the calories from these foods.
That’s assuming 25 per cent of the energy is used for digestion and absorption of food, five per cent is lost in urine and 30 per cent in faeces.
A meal of five per cent of a woman’s body mass – the same that was given to the lizards – would equate to 6.6lbs (3kg) of celery, which would only sustain the woman for six hours of doing no activity.
She would need to consume 28lbs (12.6kg) of raw celery to fuel her day, which the researchers said unlikely that anyone could achieve.
But, they said, it does make an ideal food to aid weight loss, and said: ‘The central aim of the majority of weight loss programs is to achieve a negative energy balance.
‘Rather than labelling such foods as “negative calorie” it would be more accurate to pitch these foods as “negative budget”, the consumption of which will favour a daily negative energy budget, and hence weight loss.’
Essentially, low calorie foods will not take more energy to digest than they do contain, but they can easily be burned off quickly.
The study has been submitted to the Journal of Experimental Biology and is under review, according to Live Science.
Dr Sarah Brewer, a nutritionist and medical director at Healthspan, accepted there is ‘nonsense’ talked about ‘negative-calorie’ foods.
However, she said this study doesn’t confirm that a person may use more calories to burn a food than it contains.
She said: ‘It would be great to disprove this theory but, this study doesn’t do that, for it was done in lizards.
‘Lizards are cold-blooded and have to obtain their heat for the sun and their metabolism is very different to that of humans.
‘Humans are warm-blooded and generate their own heat, and therefore would use a significant amount of calories to do so. Sadly, this study doesn’t really stack up for me.’