Q: How does fat leave your body?
A: You see, human fat cells store triglyceride—a type of fat in the blood—which is made up of carbon, hydrogen and oxygen. These atoms are released through oxidation and when that happens, fat is burned off. ... When 22 pounds of fat is oxidized, 18.5 pounds leave the body through the lungs as CO2.
In figure 2 of this Christmas Feature by Ruben Meerman and Andrew Brown (BMJ 2014;349:g7257, doi:10.1136/bmj.g7257) the number in pale blue above H20 should be 9.4 kg, not “8.4 kg” as was published. This error in figure 2 was corrected online on 3 February 2015.
- Ruben Meerman, researcher,
- Andrew J Brown, professor
- Author affiliations:
1School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney, 2052, Australia
- Correspondence to: R Meerman rubenmeerman@me.com
- Accepted 14 November 2014
Abstract
Ruben Meerman and Andrew Brown explain why the answer might not be what you expect.
Considering the soaring overweight and obesity rates and strong interest in this topic, there is surprising ignorance and confusion about the metabolic process of weight loss among the general public and health professionals alike. We encountered widespread misconceptions about how humans lose weight among general practitioners, dietitians, and personal trainers (fig 1⇓). Most people believed that fat is converted to energy or heat, which violates the law of conservation of mass. We suspect this misconception is caused by the “energy in/energy out” mantra and the focus on energy production in university biochemistry courses. Other misconceptions were that the metabolites of fat are excreted in the faeces or converted to muscle. We present a novel calculation to show how we “lose weight.”
Fig 1 Responses of a sample of doctors, dieticians, and personal trainers to the question “When somebody loses weight, where does it go?” (Correct answer CO2)
Weight we want to “lose”
Excess carbohydrate or protein in the diet is converted to triglyceride and stored in the lipid droplets of adipocytes. Excess dietary fat needs no conversion other than lipolysis and re-esterification. People who wish to lose weight while maintaining their fat-free mass are, biochemically speaking, attempting to metabolise the triglycerides stored in their adipocytes.
The chemical formula for an average triglyceride molecule can be deduced from fatty acid composition studies. In 1960, Hirsch and colleagues published data that yield an “average fatty acid” with the formula C17.4H33.1O2.1 This 50 year old result is in remarkable agreement with more recent data.2 Three “average fatty acids” esterified to the glycerol backbone (+3C, +6H) give an “average triglyceride” with the formula C54.8H104.4O6. The three most common fatty acids stored in human adipose tissues are oleate (C18H34O2), palmitate (C16H32O2), and linoleate (C18H32O2),1 2 which all esterify to form C55H104O6.
The complete oxidation of a single triglyceride molecule involves many enzymes and biochemical steps, but the entire process can be summarised as:
C55H104O6+78O2→55CO2+52H2O+energy
Stoichiometry shows that complete oxidation of 10 kg of human fat requires 29 kg of inhaled oxygen producing 28 kg of CO2 and 11 kg of H2O. This tells us the metabolic fate of fat but remains silent about the proportions of the mass stored in those 10 kg of fat that depart as carbon dioxide or water during weight loss.
To calculate these values, we traced every atom’s pathway out of the body. The carbon and hydrogen atoms obviously depart as CO2 and H2O, respectively. The fate of a triglyceride molecule’s six oxygen atoms is a conundrum solved in 1949 by Lifson and colleagues.3 They used labelled heavy oxygen (O18) to show that the oxygen atoms of body water and respiratory carbon dioxide are rapidly exchanged through the formation of carbonic acid (H2CO3). A triglyceride’s six oxygen atoms will therefore be shared by CO2 and H2O in the same 2:1 ratio in which oxygen exists in each substance. In other words, four will be exhaled and two will form water.
Novel calculation
The proportion of a triglyceride molecule’s mass exhaled in CO2 is the proportion of its molecular weight (daltons) contributed by its 55 carbon atoms plus four of its oxygen atoms:
(661 Da (C55)+64 Da (O4))/(861 Da (C55H104O6))×100=84%
The proportion of mass that becomes water is:
(105 Da (H104)+32 Da (O2))/(861 Da (C55H104O6))×100=16%
These results show that the lungs are the primary excretory organ for weight loss (fig 2⇓). The water formed may be excreted in the urine, faeces, sweat, breath, tears, or other bodily fluids.
Fig 2 When somebody loses 10 kg of fat (triglyceride), 8.4 kg is exhaled as CO2. The remainder of the 28 kg total of CO2 produced is contributed by inhaled oxygen. Lungs are therefore the primary excretory organ for weight loss. (This calculation ignores fat that may be excreted as ketone bodies under particular (patho)physiological conditions or minor amounts of lean body mass, the nitrogen in which may be excreted as urea)
Lifting the veil on weight loss
At rest, an average 70 kg person consuming a mixed diet (respiratory quotient 0.8) exhales about 200 ml of CO2 in 12 breaths per minute.4 Each of those breaths therefore excretes 33 mg of CO2, of which 8.9 mg is carbon. In a day spent asleep, at rest, and performing light activities that double the resting metabolic rate, each for 8 hours, this person exhales 0.74 kg of CO2 so that 203 g of carbon are lost from the body. For comparison, 500 g of sucrose (C12H22O11) provides 8400 kJ (2000 kcal) and contains 210 g of carbon. Replacing one hour of rest with exercise that raises the metabolic rate to seven times that of resting by, for example, jogging, removes an additional 39 g of carbon from the body, raising the total by about 20% to 240 g. For comparison, a single 100 g muffin represents about 20% of an average person’s total daily energy requirement. Physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food.
Our calculations show that the lungs are the primary excretory organ for fat. Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of “eat less, move more.” We recommend these concepts be included in secondary school science curriculums and university biochemistry courses to correct widespread misconceptions about weight loss.
Notes
Cite this as: BMJ 2014;349:g7257
Footnotes
- Competing interests: I have read and understood BMJ policy on declaration of interests and have no relevant interests to declare
- Provenance and peer review: Not commissioned; externally peer reviewed.
This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.
How does fat turn into muscle?
Here's the cold, hard truth: It's physiologically impossible to turn fat into muscle because they're two entirely different types of body tissue with different types of cells (fat is made up of adipose tissue, while muscle is made of proteins). So fat is fat and muscle is muscle.
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 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.
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 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 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 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.
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 1,200 grams of carbon dioxide.
More realistically, going for a walk triples your metabolic rate, and so will cooking, vacuuming and sweeping.
Metabolising 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 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.
When you dig into your meal or grab a quick bite on the way to work, the food you eat goes toward fueling your body. As your favorite (or not so favorite) foods pass through your digestive system, your body absorbs nutrients and uses them to power you through your daily routine. The remaining waste heads out through your bladder or intestines. It’s a good system.
But what happens if you get busy and skip a meal, or have to exert more energy than you devoured at your last snack session? Thanks to fat—which you’ve saved for just this occasion without even thinking about it—your body is ready. Fat acts like a battery, ready to provide energy for you when you need it the most. But what actually happens when you dip into those strategic reserves?
What is a fat cell?
Humans have two kinds of fat cells, brown fat cells and white fat cells, the latter of which are far more common. (For more information about brown fat, check out our article about the difference between the two types of cells.)Fat cells in humans develop primarily during childhood and puberty. After that, the number of fat cells in your body remains relatively stable. These cells make up adipose tissue, and they can send out hormones into the body to help regulate everything from metabolism to body weight.
Because we generally have a set number of these cells, gaining weight doesn't typically involve making more. Instead, individual cells expand as the digestive system breaks down and stores food for later use, then they shrink when the body turns to the fat cell in a moment of need—whether you're running a marathon, hiking a mountain, or wandering aimlessly around your neighborhood.
What happens when you burn fat?
So what happens during that shrinking process, when you actually lose some fat? Contrary to popular beliefs, all of the fat used by the body doesn’t get turned directly into energy, it doesn’t magically transform into muscle, and it doesn’t exit with your other solid waste.But it has to go somewhere. That’s the beauty of the conservation of mass. Much as some people might want it to, fat can’t just disappear.
The fat inside your adipose cells is stored and then sent back out into your body as triglycerides. These go through a series of chemical reactions to convert into energy, but the process isn’t 100-percent efficient. Our bodies are good, but they’re not that good.
Along with the energy to lift your leg or hoist your bag onto your shoulder, the process of using up fat stores creates byproducts: water and carbon dioxide.
Where does it go?
Some of the water produced as the body consumes fat exits in the usual way, through your urine and sweat. But a 2014 study in the British Medical Journal found that most of the byproducts of fat (including all that carbon dioxide) leaves the body through the respiratory system.Yep, you read that right. You breathe out the byproducts of most of the fat that you burn, the CO2 from that process mingling with the CO2 produced by your lungs as they process oxygen. It’s something to think about while you hit the gym or walk on a track: getting rid of fat can be exhausting, but it's as natural as breathing.
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