Most health professionals lack a clear understanding of how body fat is lost, often subscribing to misconceptions like fat converting to energy or muscle.
The truth is, fat is actually broken down into carbon dioxide and water, with the majority of the lost fat being exhaled as carbon dioxide. This insight is crucial for understanding the real mechanics behind weight loss and dispelling common myths.
Unveiling Weight Loss Misconceptions
The world is obsessed with weight loss and fad diets, yet few people actually understand where fat goes when we lose weight.
Even among 150 doctors, dietitians, and personal trainers we surveyed, this knowledge gap was surprisingly common. The most widespread myth was that fat is converted into energy. However, this idea contradicts the law of conservation of matter, which all chemical reactions obey.
Some respondents believed fat transforms into muscle, which is impossible, while others assumed it exits through the digestive system. In reality, only three people in our survey got the correct answer. That means 98% of these health professionals couldn’t accurately explain how weight loss works.
So if fat isn’t turned into energy, muscle, or waste, where does it actually go?
The Surprising Science of Fat Loss
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 10 pounds of fat, precisely 8.4lb comes out through your lungs, and the remaining 1.6lb 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 fiber (think corn). Everything else you swallow is absorbed into your bloodstream and organs and, after that, it’s not going anywhere until you’ve vaporized it.
Understanding Energy and Weight Dynamics
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, 415 grams is solid macronutrients, 23 grams is fiber 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 energy 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.
Active Solutions to Weight Loss
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 1,200 grams of carbon dioxide.
More realistically, going for a walk triples your metabolic rate, and so will cooking, vacuuming, and sweeping.
Metabolizing 100 grams of fat consumes 290 grams of oxygen and produces 280 grams of carbon 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 carbon dioxide on top of what you’ll produce by vaporizing 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.
Written by:
- Ruben Meerman, Assistant scientist, UNSW Sydney
- Andrew Brown, Professor and Head, School of Biotechnology and Biomolecular Sciences, UNSW Sydney
Adapted from an article originally published in The Conversation.
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33 Comments
thank you for this
When do we charge start making overweight fat people pay their major % on Carbon Tax? it’s only fair since their constant overeating and fad diets cause them to input more co2, then they are causing more problems with global warming.
No one needs your input on life you are just a rude individual .
Thanks for the information.
That was very informational and I like earning new things:) Thank you!!
When do ppl get to start taxing ppl like you? You just breathing should be taxed.
If only we all really cared about pollution and air quality maybe the planet wouldn’t be dying out to all the the chemical, plastic and petroleum plants not to mention the exhaust on vehicles. Only to mention a few factors influencing the issue.
I’m always amazed at just how stupid people can be!
Thanks for the information.
That’s why it’s good to do breathing exercises.
“The truth is, fat is actually broken down into carbon dioxide and water, with the majority of the lost fat being exhaled as carbon dioxide”.
Okay. But we must get some sort of energy or other benefit from storing fat, otherwise why would the body go to all that trouble in the first place rather than just excrete the excess?
Or is storing fat merely an inconvenient set of conversions the body is compelled to perform because we are ingesting too much energy?
Exactly what I was thinking! Why do our bodies store fat if it’s not used for converting to energy at some point later on when food is scarce. And water and CO2 are the result of that fat conversion to energy! I’m just not buying what this article is selling. It’s essentially click bait and very disappointing.
The article is simply focusing on the fact that energy has no weight, and asking what, specifically, accounts for the lost weight when we expend more energy than we take in. The author seems to have assumed that readers understand that the energy stored in fat is manifested primarily in the covalent chemical bonds between these carbon atoms. There is a complex biochemical cycle mediated by many enzymes which manages to harness much of the energy released when the chemical bonds in fat molecules are systematically broken down to carbon dioxide and water. Of course our bodies store fat for the purpose of storing energy for later use. The article’s point is that, when our bodies use that energy, the building blocks which compose the fat will remain and must be disposed of if they are not reused. There is nothing controversial or surprising here– these are well-established facts. Note that camels store water in their humps only in the form of fat as well– with that water being the end-product most needed in the case of camels.
Oh no! You just asked for explanation of the kreb cycle. Nightmares are made learning about it. Seriously though, it’s during the chemical processing of nutrients that energy is released, ATP.
Yes, but we risk confusing some folks even more by jumping into the methods the body uses for short-term storage of energy, don’t we? I’m amazed to see how many readers want to argue with the facts instead of learning something.
For anyone still confused: energy has no weight, and fat is composed of energy plus its building blocks. No weight change occurs when that energy is released. The scale will move only after those building blocks have also left the body, which can occur more gradually.
If a plant-based meal plan has left the blood slightly more alkaline than one’s usual diet, then the body will retain CO2 to keep the blood pH balanced. Breathing exercises won’t eliminate that CO2 if our body now needs it for that purpose. The same effect will be seen if the diet food is more salty than one’s usual maintenance-level diet– H20 may be retained. This is why our weight loss often does not seem to parallel our reduced calorie intake when we diet.
Right on 👍 .
cellular respiration is an exothermic reaction
C₆H₁₂O₆ + 6O₂ => 6H₂O + 6CO₂
so, energy is produced
we get all our energy from glucose oxidation
Is this why the food industry and climate change want to keep us fat? If we all lost weight it would increase the carbon levels in the world.
Fat isn’t turned into energy but the breakdown process that allows the excretion though breath and fluid is the process that releases stored energy from fats. Just like burning wood releases heat but leaves behind smoke and ash, fat releases energy and exits as breath and fluid.
Whoever wrote those has never heard of autophagy, which is the real answer is of where fat goes
Inserting the concept of autophagy here merely begs the question. The author is describing the end-stage result of the break-down of fat, after the energy is harvested, whether autophagy happens along the way or not.
So well put! 👏
Sorry, they are just playing with words and concepts. Your body breaks fats down until the end stage where energy is released. Yes carbon dioxide and water are also end results after the energy is released for your cells to use. Weight loss is simple; calories in vs calories out, period. Calories in can be controlled by you and what you eat and learning as much as you can about the calorie content you devour each day. Calories out involves your baseline calorie use for daily metabolic functions, which can be also be impacted by your level of exercise. Get a Fitbit and start counting calories, or join NOOM. You’ll become a master.
We do not eat for energy. Energy from food is a myth. Regardless, energy in energy out cannot explain weight loss and weight gain because energy has no weight. How it is stored determines weight. In the energy in/out paradigm fat is not the only place of stored energy. Muscle is stored energy too. Different types of tissue hold different amounts of energy by weight. In this paradigm the body can store and release energy from different types of tissues independent of each other. Therefore it would be possible to gain weight in a deficit and loss weight in a surplus, depending on how the body shifts stored energy. Its even possible to gain or lose weight in a state of energy balance. What the body does with the energy determines weight loss or gain. Building muscle is also a way for the body to store energy. When you lose muscle you also release energy. Burning fat is not the only way for the body to release needed energy. That point is often times missed by people. What the body does with energy and where it stores it is determined by what you eat and what you do, not per se the number of calories. Again weight can be gained in a deficit and lost in a surplus. This can be proven mathematically. That is if we eat for energy, which like I said we don’t. The energy in/out paradigm falls apart on its own terms with regard to weight loss and weight gain. Weight alone cannot on its own be used to determine the state of energy balance or imbalance in the energy in/out paradigm. In order to know the body’s energy state you would need to know the change in body composition not merely weight loss or gain.
A person loses a lb of fat and gains a lb of muscle. Fat has more energy by weight then muscle. More energy is lost by the fat then gained by the muscle, therefore person was in a deficit while maintaining weight. Reverse that, fat gained, muscle lost. Makes for a surplus. In both cases no change in weight. Example of gain in deficit and loss in surplus.
Oops, last line above should say: example of no weight change in surplus and no weight change in deficit.
This article is misleading and filled with semantics. The metalobism of fat releases stored energy. For the common person this equals fat turns into energy. All this article will achieve is create further confusion because now people are going to think “I thought fat turned into energy but it turns out it turns into carbon dioxide” and they will have no idea why.
Why describe an accurate article as “misleading” merely because you doubt the intellect of their readership? Surely the effort to teach others how things work is always worthwhile. If “the common person” believes that “fat turns into energy” then they are simply wrong, and that serious misapprehension will prevent them from comprehending how more complex aspects of our bodies work. Greater understanding can be achieved only after one realizes that fat breaks down into its physical building blocks PLUS the energy released by the breaking of those covalent bonds. Anything else is mere vague handwaving that should only be taught to small children.
WTF is going on with the people making articles on this site?
Is there zero care about spewing misleading and downright false information?
When you do an extended fast say 84 hours or more, if your body does not turn fat into energy how do you stay functioning?
By this articles reasoning you should be passed out or dead.
You are saying all that fat loss is due to you breathing it out?
What does your body run on?
Idiocy
No, the author is aboslutely correct, but they have failed to communicate these well-established facts clearly. Of course our bodies depend on fat for the storage of energy, but the point is that our bodies do not literally “turn fat into energy.” That is just the vague handwaving explanation that we give small children, but which many people apparently have not had reason to give much critical thought to as adults. Fat is composed of carbon, oxygen and hydrogen, all assembled with the input of energy into the formation of covalent bonds. That carbon, oxygen and hydrogen do not disappear or “convert” when the covalent bonds in fat are broken down. Those atoms remain unchanged but are instead reassembled into smaller, lower-energy molecules such as CO2 and H2O. When we try to lose a pound of fat, the scale will only register a change in weight after we have eliminated those now surplus molecules from our bodies. Perhaps it will help to understand that camels do not literally store water in their humps. Camels store fat in their humps, but when that fat is broken down the by-product (for us) of H20 is the end-product the camels most need. If we cut our calorie intake by 3500 but have also taken in more salt and eaten more alkaline foods, then we may be disappointed by the immediate results if our bodies retain some of that liberated water and CO2 to keep our systems balanced. That retention is generally temporary, so we may only see weight loss on the scale later, after a delay, perhaps after we’ve resumed our usual diets, which can be confusing. (“I ate nothing but low-calorie salads all last week and lost little or no weight, but this week after I gave up and ate my usual steak and potatoes with more calories for several days in a row, I finally lost a pound. Go figure.” Of course, if we binge in frustration on high-calorie foods that second week, taking in more calories than usual for weight maintenance, we may never see that weight loss on the scale at all.
Thank you for your explanations and responses. So how does the body eliminate those extra molecules ?
This is so interesting… and this makes a lot of sense because when we do exercise, we do breathe a lot harder and faster!🏃♀️
The body contouring treatment worked wonders for me. I feel lighter, healthier, and more energetic.