When we lose weight, where does it go?

March 14, 2018 by Ruben Meerman, University of New South Wales
Don’t worry, 98% of health professionals surveyed didn’t know either. Credit: www.shutterstock.com

The world is obsessed with fad diets and weight loss, yet few of us know how a kilogram of 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% 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?

The enlightening facts about fat metabolism

The correct answer is that fat is converted to carbon dioxide and . 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.

Kilograms in versus kilograms out

We all learn that "energy in equals energy out" in high school. But energy is a notoriously confusing concept, even among 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.

The metabolic fate of an average Australian’s daily intake of food, water and oxygen. Credit: Nutrient intake data: Australian Bureau of Statistics, Australian Health Survey: Nutrition First Results - Foods and Nutrients

According to the latest government figures, Australians consume 3.5kg of food and beverages every day. Of that, 415 grams is solid macronutrients, 23 grams is fibre and the remaining 3kg is water.

What's not reported is that we inhale more than 600 grams 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 600 grams 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 415 grams of carbohydrates, fats, protein and alcohol most Australians eat every day will produce exactly 740 grams of carbon dioxide plus 280 grams of water (about one cup) and about 35 grams of urea and other solids excreted as urine.

An average 75kg person's resting metabolic rate (the rate at which the body uses when the person isn't moving) produces about 590 grams 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 200 grams 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 ? 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 . 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 1,200 grams of carbon dioxide.

More realistically, going for a walk triples your , and so will cooking, vacuuming and sweeping.

Metabolising 100 grams of fat consumes 290 grams of oxygen and produces 280 grams of dioxide plus 110 grams of water. The food you eat can't change these figures.

Therefore, to lose 100 grams of fat, you have to exhale 280 grams of on top of what you'll produce by vaporising all your , 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 works, few of us know why.

Explore further: When you lose weight, where does the fat go?

Related Stories

When you lose weight, where does the fat go?

December 16, 2014
Despite a worldwide obsession with diets and fitness regimes, many health professionals cannot correctly answer the question of where body fat goes when people lose weight, a UNSW Australia study shows.

Shed post-Christmas pounds just by breathing

December 16, 2014
Ever wondered where the fat goes when somebody loses weight? Most of it is breathed out as carbon dioxide, making the lungs the primary excretory organ for weight loss, explain Australian researchers in the Christmas issue ...

Do ketogenic diets help you lose weight?

September 20, 2017
Is a ketogenic diet effective for weight loss? The answer depends on whether it achieves a reduction in total kilojoule intake or not.

When you lose weight, where does the fat actually go?

March 1, 2016
The answer to this question may surprise you. In fact, according to a recent British Medical Journal article discussing this issue, few health professionals, including doctors, dieticians, and personal trainers know the correct ...

Be sure to read labels for portion, calorie control

January 11, 2018
(HealthDay)—"Nutrition Facts" labels mandated by the U.S. Food and Drug Administration have made it easier to know more about what's inside packaged foods. But the information isn't always easy to decipher, especially since ...

Recommended for you

Study finds behavioral changes insufficient at preventing early childhood obesity

August 7, 2018
Young children and their families in poor communities were able to make some achievable and sustainable behavioral changes during the longest and largest obesity prevention intervention, but, in the end, the results were ...

Responsive parenting intervention results in lower BMIs through age three

August 7, 2018
An intervention designed to promote healthy growth, which taught first-time moms how to respond with age-appropriate responses to their babies' needs, resulted in children having lower body mass indexes (BMIs) when they were ...

Measure of belly fat in older adults is linked with cognitive impairment

August 1, 2018
A new study using data from over 5,000 individuals has found that a measure of belly fat (waist:hip ratio) was associated with reduced cognitive function in older Irish adults (>60 years of age). These findings have significant ...

How can you avoid regaining those lost kilos?

July 23, 2018
The hardest part about losing weight is maintaining it. Many people manage to lose weight, but in time the kilos can slowly come back.

Food for thought: How the brain reacts to food may be linked to overeating

July 19, 2018
The reason why some people find it so hard to resist finishing an entire bag of chips or bowl of candy may lie with how their brain responds to food rewards, leaving them more vulnerable to overeating.

Children are less likely to be obese if mothers stick to a healthy lifestyle

July 4, 2018
Children of mothers who follow a healthy lifestyle have a substantially lower risk of developing obesity than children of mothers who don't make healthy lifestyle choices, finds a study published in The BMJ.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.