When you lose weight, where does the fat go?

When you lose weight, most of the lost fat leaves your body in your breath, as carbon dioxide. Your cells combine the fat with oxygen and break it down into carbon dioxide and water. Of every 10 kg of fat you lose, about 8.4 kg is breathed out and about 1.6 kg leaves as water. 1

Those numbers come from a 2014 paper in the BMJ by Ruben Meerman and Andrew Brown, who worked out where each atom of a fat molecule ends up. 1 This page starts with a campfire, then follows one fat molecule from a fat cell to your lungs, puts the whole reaction on a scale, and ends with the question everyone asks next: why not just breathe faster?

Start with a campfire

Picture a big log on a campfire. By morning most of it is gone. The grey heap left behind is small: firewood leaves only about 1% of its dry weight as ash. 21

So where did the rest of the wood go? Into the air. Wood is made mostly of carbon, hydrogen and oxygen, and burning it produces mainly carbon dioxide and water, which drift away as gases. 21 The fire also kept you warm, and that warmth is energy. But the wood itself did not turn into heat. Its atoms were rearranged into new molecules, and those molecules left. Mass doesn't disappear in a chemical reaction. 1

Now swap the log for your body. Fat is your stored fuel: it is kept in fat tissue until your body needs energy. 6 To use it, your body needs oxygen, which you breathe in, just as a fire needs air. 1 The products are the same two simple things the fire made, carbon dioxide and water. 1

The water leaves the way water always does: in urine, sweat, breath and tears, and some in faeces. 1 The carbon dioxide travels in your blood to your lungs, and you breathe it out. 8 That is the whole simple answer. Your lungs are the main exit for lost fat. 1

The campfire picture is good, but it breaks in a few places that matter. We'll come back to them once the chemistry is on the table.

What most people get wrong

Meerman and Brown asked general practitioners, dietitians and personal trainers where lost fat goes, and found widespread misconceptions. 1 The most common answer was that fat is converted to energy or heat. Other answers were that it leaves in faeces or turns into muscle. 1

The energy answer is an easy mistake, because using fat really does release energy. The authors suspect the "energy in, energy out" mantra, and the focus on energy production in university biochemistry courses, are behind it. 1

Why fat can't simply become energy

Fat turning into energy would break the law of conservation of mass. 1 In a chemical reaction no atoms are destroyed; they only swap partners. So every atom in the fat you lose is still there afterwards and has to leave your body somehow. 1

Physics adds one footnote. Any release of energy comes with a tiny loss of mass, by E = mc². Food-energy tables give fat about 37 kJ per gram 14, so 10 kg of fat holds about 370 million joules. Divide that by the speed of light squared 20 and you get roughly 4 micrograms: four millionths of a gram, out of ten thousand grams. 14 20 Everything else leaves as matter.

Why fat can't simply become muscle

Muscle is built from proteins, proteins are built from amino acids, and every amino acid carries an amino group containing nitrogen. 11 The average fat molecule, C55H104O6, has no nitrogen at all. 1 So fat can't simply turn into muscle protein. These sources settle the composition question only; they don't track whether any of fat's carbon ends up in other molecules along the way.

The faeces answer, half right

Some of the water made from fat can leave in faeces. 1 The fat itself does not; Meerman and Brown list "excreted in the faeces" among the misconceptions. 1

Follow one fat molecule out of your body

Stored fat sits inside fat cells as triglycerides, the most efficient form for storing and transporting fatty acids. 5 A triglyceride is a glycerol backbone holding three fatty acid chains. 5 6 An average one in human fat has 55 carbon atoms, 104 hydrogens and just 6 oxygens. 1

Step 1: the fat cell lets go

When your body needs fuel, signals from nerves and hormones switch on the breakdown. 5 22 Among hormones, the catecholamines, the family adrenaline belongs to, are the most important activators of one of the key enzymes. 5

Three enzymes then take the triglyceride apart in sequence. ATGL makes the first cut, HSL mainly handles the second, and MGL finishes the job, leaving free fatty acids and glycerol. 5 This splitting is called lipolysis. 5 6 The glycerol enters glycolysis, a separate pathway. 6

Step 2: into the blood

The fatty acids leave the fat cell into the bloodstream, ride on a blood protein called albumin, and eventually enter tissues around the body. 5 Not every tissue uses them equally: the brain uses fatty acids only poorly as fuel. 23

Step 3: chopped up in the mitochondria

Inside a cell, a fatty acid is carried across the membrane of a mitochondrion, where all of its oxidation takes place. 6 13 A process called beta-oxidation chops it into two-carbon acetyl pieces, carried as acetyl-CoA. 6 9

Those pieces feed the citric acid cycle in the inner space of the mitochondrion, the matrix. Every turn of the cycle releases two molecules of carbon dioxide. 9 Turn after turn, this is where the fat's carbon becomes the gas you will breathe out. 1 9

The hydrogens take another route. Their electrons are passed along the electron transport chain to oxygen, the final electron acceptor, and that oxygen picks up hydrogen ions to form water. 10

Step 4: out through the lungs

Carbon dioxide is a waste product of cellular respiration. 8 In the blood, about 70% of it travels as bicarbonate, about 20% bound to hemoglobin, and 7 to 10% dissolved. 8 At the lungs it is released and breathed out. 8

Put 10 kg of fat on a scale

Meerman and Brown wrote the whole reaction for an average human triglyceride 1:

C55H104O6 + 78 O2 → 55 CO2 + 52 H2O + energy

Scaled up to 10 kg of fat, both sides weigh the same 1:

Goes in	Mass	Comes out	Mass
Fat	10 kg	Carbon dioxide	28 kg
Oxygen you breathe in	29 kg	Water	11 kg
Total	39 kg	Total	39 kg

The oxygen outweighs the fat almost three to one. 1

Now follow only the atoms that started in the fat. All 55 carbons end up in carbon dioxide, and all 104 hydrogens end up in water. The fat's 6 oxygens split in a 2:1 ratio: four are exhaled in carbon dioxide and two form water. 1 Add up the masses and 84% of the fat leaves as carbon dioxide and 16% as water, which is 8.4 kg and 1.6 kg out of 10 kg. 1

The rest of those 28 kg of carbon dioxide comes from the oxygen you breathed in. 1 So inhaled oxygen ends up in both products: in the carbon dioxide and in the water. 1

By weight, not by atom count

The 84% is a share of the mass. Count atoms instead and the picture flips: 106 of the molecule's 165 atoms, all 104 hydrogens plus 2 oxygens, end up in water. 1 Hydrogen atoms are so light that those 106 atoms add up to only 16% of the weight.

One more caveat from the paper: the calculation ignores fat that can leave as ketone bodies under particular physiological or disease conditions. 1

Where the energy goes

Fat is a dense fuel. Food-energy tables use 37 kJ (9 kcal) per gram of fat, against 17 kJ (4 kcal) per gram of carbohydrate. 14

When fat is oxidized, your cells catch part of the released energy in carrier molecules, most often ATP and NADH. 13 Around 60% of the energy produced while making ATP comes out as heat, which keeps your body at its working temperature. 7 So the energy and the atoms leave by separate routes: the energy as heat and in carrier molecules, the atoms as carbon dioxide and water. 1 7 13

Where the campfire analogy breaks

There is no flame inside you. In cells, enzymes run the oxidation as a series of small steps, so the reaction can happen at body temperature. 13

A log leaves ash. Fat that is used up completely leaves nothing behind but carbon dioxide and water. 1 21

The biggest difference is what's left of the container. A burned log is gone, but your fat cells stay and shrink. A 2008 study in Nature found that adults keep a constant number of fat cells, whether lean or obese, even after marked weight loss, and that the number is set during childhood and adolescence. 3 The cells still turn over: about 10% of fat cells are renewed every year, at all adult ages and body sizes. 3

A 2014 study followed 62 women with obesity for two years after gastric bypass surgery. Their body weight fell by 33%, and their fat cells got smaller but not fewer. 4

Can you burn fat from one spot?

Fat leaves its cells into the blood, and the blood carries it to tissues all around the body. 5 Exercise trials find what that route suggests.

In one trial with 24 healthy, inactive adults, six weeks of abdominal exercise alone did not reduce the fat under the skin of the belly. 17 In another, 11 people trained one leg for 12 weeks. They lost fat, but from their arms and trunk, not from the trained leg. 18 A 2022 meta-analysis pooled 13 studies with 1,158 participants aged 14 to 71 and found no spot reduction. 19

Why breathing faster won't burn more fat

Think of carbon dioxide as the trash in your kitchen. Cooking fills the bin; taking the bag out more often doesn't cook anything. In your body, the cooking is your cells using fuel, which makes carbon dioxide as a waste product. 8 Breathing out only carries the bag outside.

Breathe faster than your body needs and you mostly just empty the bin early, lowering the carbon dioxide in your blood. 16 This is where the trash picture breaks: an empty bin is harmless, but your body notices when blood carbon dioxide drops. Rapid, deep breathing, called hyperventilation, can leave you lightheaded, dizzy, weak, numb or tingly. 16 Asked whether simply breathing more can make you lose fat, the study authors' university answers: no. 2

The numbers for a resting 70 kg person

At rest, an average 70 kg person on a mixed diet breathes out about 200 ml of carbon dioxide a minute, in about 12 breaths. 1 Each breath carries about 33 mg of carbon dioxide, and 8.9 mg of that is carbon. 1

Over a day of 8 hours' sleep, 8 hours' rest and 8 hours of light activity that doubles the resting rate, that person breathes out 0.74 kg of carbon dioxide, removing 203 g of carbon. 1 For comparison, 500 g of table sugar contains 210 g of carbon and 2,000 kcal of energy. 1

To breathe out more carbon, your cells have to use more fuel. Replacing one hour of rest with jogging, at about seven times the resting rate, removes an extra 39 g of carbon and lifts the day's total by about 20%, to 240 g. 1

Food puts the carbon back

Every meal brings carbon back in. A single 100 g muffin covers about 20% of an average person's daily energy needs. 1 Meerman and Brown conclude that physical activity as a weight-loss strategy is easily undone by relatively small amounts of extra food, and their paper lands on the old line "eat less, move more". 1

Underneath the carbon bookkeeping sits energy balance. Body weight change is tied to a gap between the energy in the food you eat and the energy you spend, and human physiology obeys the first law of thermodynamics. 15 Resting energy expenditure makes up about two thirds of the energy you spend, and a change in one part of the balance tends to cause compensating changes in the others. 15

This page explains the chemistry of fat loss. It is not diet or medical advice.

Where the carbon goes next

Plants take carbon dioxide from the air through small pores called stomata and use it, with water and sunlight, to build energy-storing carbohydrates. 12 So some of the carbon that once sat in your fat might one day be part of a tree.