The world is obsessed with fad diets and weight loss, yet few of us know how fat actually vanishes off the scales.
Even the 150 doctors, dietitians and personal trainers we surveyed shared this surprising gap in their health literacy.
The most common misconception, by far, was that fat is converted to energy.
The problem with this theory is that it violates the law of conservation of matter, which all chemical reactions obey.
Some respondents thought fat turns into muscle, which is impossible, and others assumed it escapes via the colon.
Only three of our respondents gave the right answer, which means 98 per cent of the health professionals in our survey could not explain how weight loss works.
So if not energy, muscles or the loo, where does fat go?
Here, writing in a piece for The Conversation, two scientists from the University of New South Wales explain.
The enlightening facts about fat metabolism
The correct answer is that fat is converted to carbon dioxide and water. You exhale the carbon dioxide and the water mixes into your circulation until it’s lost as urine or sweat.
If you lose 10kg of fat, precisely 8.4kg comes out through your lungs and the remaining 1.6kg turns into water. In other words, nearly all the weight we lose is exhaled.
This surprises just about everyone, but actually, almost everything we eat comes back out via the lungs.
Every carbohydrate you digest and nearly all the fats are converted to carbon dioxide and water. The same goes for alcohol.
Protein shares the same fate, except for the small part that turns into urea and other solids, which you excrete as urine.
The only thing in food that makes it to your colon undigested and intact is dietary fibre (think corn).
Everything else you swallow is absorbed into your bloodstream and organs and, after that, it’s not going anywhere until you’ve vaporised it.
Weight in versus weight out
We all learn that 'energy in equals energy out' in high school. But energy is a notoriously confusing concept, even among health professionals and scientists who study obesity.
The reason we gain or lose weight is much less mysterious if we keep track of all the kilograms, too, not just those enigmatic kilojoules or calories.
According to the latest government figures, Australians consume 3.5kg of food and beverages every day. Of that, 415g is solid macronutrients, 23g is fibre and the remaining 3kg (6.6lbs) is water.
What’s not reported is that we inhale more than 600g worth of oxygen, too, and this figure is equally important for your waistline.
If you put 3.5kg of food and water into your body, plus 600g of oxygen, then 4.1kg of stuff needs to come back out, or you’ll gain weight.
If you’re hoping to shed some weight, more than 4.1kg will have to go. So how do you make this happen?
The 415g of carbohydrates, fats, protein and alcohol most Australians eat every day will produce exactly 740g of carbon dioxide plus 280g of water (about one cup) and about 35g of urea and other solids excreted as urine.
An average 75kg person’s resting metabolic rate (the rate at which the body uses energy when the person isn’t moving) produces about 590g of carbon dioxide per day.
No pill or potion you can buy will increase that figure, despite the bold claims you might have heard.
The good news is that you exhale 200g of carbon dioxide while you’re fast asleep every night, so you’ve already breathed out a quarter of your daily target before you even step out of bed.
Eat less, exhale more
So if fat turns into carbon dioxide, could simply breathing more make you lose weight? Unfortunately not.
Huffing and puffing more than you need to is called hyperventilation and will only make you dizzy, or possibly faint.
The only way you can consciously increase the amount of carbon dioxide your body is producing is by moving your muscles.
But here’s some more good news. Simply standing up and getting dressed more than doubles your metabolic rate.
In other words, if you simply tried on all your outfits for 24 hours, you’d exhale more than 1 200g of carbon dioxide.
More realistically, going for a walk triples your metabolic rate, and so will cooking, vacuuming and sweeping.
Metabolising 100g of fat consumes 290g of oxygen and produces 280g of carbon dioxide plus 110g of water. The food you eat can’t change these figures.
Therefore, to lose 100g of fat, you have to exhale 280g of carbon dioxide on top of what you’ll produce by vaporising all your food, no matter what it is.
Any diet that supplies less 'fuel' than you burn will do the trick, but with so many misconceptions about how weight loss works, few of us know why.
WHAT IS THE BEST TIME OF DAY TO EAT CARBOHYDRATES?
TV doctor Dr Michael Mosley has suggested the best time of day to eat carbohydrates.
He found eating pasta and bread at dinner is better for people's waistlines than toast in the morning.
Experts previously thought carbohydrates should largely be eaten at the start of the day as the body has longer to burn the glucose they release.
Failing to do this causes the release of insulin to bring blood sugar levels back to normal, which is done by storing excess sugar from carbs as fat, causing people to put on weight.
Yet, the new study, broadcast on the BBC's Trust Me I'm a Doctor, found eating carbohydrates in the evening causes less dramatic blood sugar spikes than carb-loading at breakfast, providing the rest of a person's food intake for that day has not been too starch-heavy.
Dr Mosley advises people are consistent with their carb-eating habits and avoid overindulging with every meal.
He carried out the research with the University of Surrey by asking healthy volunteers to eat either the majority of their daily carbohydrate intake in the morning or evening.
All of the study's participants ate the same amount of carbs every day, which included bread, pasta and vegetables.
For the first five days, they ate most of these foods for breakfast, followed by five days of eating a normal diet before finally switching to a low-carb breakfast, high-carb dinner for the last five days.
The researchers analysed the participants' blood sugar levels throughout the study.