The idea of negative calorie foods has been around for a while now. With one of the most commonly cited examples being celery.
These So called Negative calorie foods take advantage of two scientific principles.
See when we eat food, the total energy contained within the food is measured by calories.
Digestion though isn’t perfect, so eating a certain number of calories, doesnt always mean you get them….
some foods are tough for our bodies to break down, an other way of saying this is, the calories are less bioavailable.
When you eat a food which contains 100 calories, a portion of those calories will end up being excreted. Foods high in fibre are often cited as an example of something particularly tough for our bodies to digest.
So we thought we were getting 100 calories, but in our example now we might be down to 80 calories after we factor in excretion.
And that isn’t even the entire story. Even more calories are lost through something called the Thermic Effect of Food.
Simply put, eating food also requires energy for your body to digest it.
The energy needed to access the calories in the food, needs to come from food which was previously eaten, or stored energy aka fat. While this amount of energy required to digest varies it is typically around 1% to 3% for carbohydrates and fats, but up to 20% for proteins.
So to answer this negative calorie food question, we can build our equation.
We start with the calories we consume, and after we subtract the calories excreted and the thermic effect of the food (the energy cost to digest it), we are left with this, the net calories available.
So the essence of the negative calorie food question, is it possible for these two numbers to ever exceed the initial calories eaten, leaving you after all is said and done, in an energy deficit.
This is exactly what a team of researchers at the University of Cambridge set out to answer.
One morning they invited 15 female students into their lab, the women had been instructed to fast overnight.
They then ate 100g of celery, which is the equivalent of 16 calories of celery. Celary is perhaps the most famous example of a so called, negative calorie food. During the following 3 hours the researchers used a special ventilated hood to measure the increase in energy expenditure.
Amazingly, the researchers watched, as across the subjects Energy Expendeture and fat oxidation rose in the participants. But would it exceed the 16 calories of energy in the celery.
No, although it got quite close, on average 86% of the calories in the celary were burned by the body from simply digesting it.
They didn’t however measure how much if any of the calories were excreted. Assuming none we would be left with just 2.24 calories of celery absorbed. It is possible that they may have exceeded a few calories, and gone into the negatives, but until more research is done we wont know for certain.
So negative calories, are they real?
Well so far no study or test has proven it. And even if! all the celery’s calories were excreted
(which is highly unlikely), that’s only a deficit of less than 20 calories, which to be honest wouldn’t make a big enough difference to be a stand alone weight loss solution. If anything I would say celery is most likely close to calorie neutral. While it may be a great low calorie option to help you feel full while dieting, eating it wont burn through your fat.
The fact still remains, if you want to burn more calories, whether its cardio, weight lifting, or walking the dog moving your body is key.
Thermic Effect: https://www.ncbi.nlm.nih.gov/pubmed/9449148