Where does fat go?

Course outline

Every lesson as plain text, if you'd rather read than click.

1. 1.Think of a campfireBABY MODE

   You lose 10 kg of fat. Where do you think most of it goes? Most of it, by weight, leaves in your breath. A campfire shows why.

   Drop a big log on a campfire and by morning most of it is gone. Only a little ash is left: about 1% of the wood's dry weight. The rest went up into the air. Burning wood makes mainly carbon dioxide and water, and they drift away as gases. The fire kept you warm, but the wood didn't turn into heat. Its atoms were rearranged, and the new molecules left.

   Now swap the log for you. Fat is your body's stored fuel. To use it, your body needs oxygen, which you breathe in. The products are the same as the fire's: carbon dioxide and water. The water leaves as urine, sweat, breath and tears. The carbon dioxide rides your blood to your lungs, and you breathe it out.

   Set the fat lost to 20 kg. How much of it do you breathe out?

2. 2.Why fat can't become energyBIG BABY MODE

   Meerman and Brown asked doctors, dietitians and personal trainers where lost fat goes. The most common answer: it turns into energy or heat. Others said it leaves in poop or turns into muscle. The authors suspect the "energy in, energy out" mantra is behind the energy answer.

   Using fat really does release energy. So why can't the fat itself become that energy? Because it would break the law of conservation of mass. In a chemical reaction no atoms are destroyed; they swap partners. Physics allows a tiny mass loss with any energy release (E = mc²), but for 10 kg of fat it works out to about 4 micrograms. Everything else has to leave as matter.

   The muscle answer fails on one atom. Muscle is made of proteins, proteins are made of amino acids, and every amino acid has an amino group with nitrogen. The average fat molecule, C55H104O6, has no nitrogen. The poop answer is only partly right: some of the water made from fat can leave that way, but the fat itself doesn't.

3. 3.Follow one fat moleculeBIG BABY MODE

   Fat sits in fat cells as triglycerides: a glycerol backbone holding three fatty acid chains. An average one has 55 carbons, 104 hydrogens and 6 oxygens. When your body needs fuel, nerve and hormone signals switch on three enzymes (ATGL, HSL, MGL) that cut the chains off one by one. That's lipolysis. The freed fatty acids ride in your blood, on a protein called albumin, to tissues around the body.

   Inside a cell, a fatty acid enters a mitochondrion. Beta-oxidation chops it into two-carbon pieces. Those feed the citric acid cycle, and every turn releases two carbon dioxide molecules: that's the fat's carbon leaving. The hydrogens' electrons pass down a chain to oxygen, which forms water. The carbon dioxide travels in your blood, mostly as bicarbonate, back to your lungs.

   Follow 10 kg of fat

   1. In: 10 kg of fat plus 29 kg of oxygen you breathe in = 39 kg.
   2. Out: 28 kg of carbon dioxide plus 11 kg of water = 39 kg. It balances.
   3. Track only the fat's own atoms: all its carbon goes into carbon dioxide, all its hydrogen into water, and its 6 oxygens split 4 to 2.
   4. Result: 84% of the fat's mass leaves as carbon dioxide (8.4 kg) and 16% as water (1.6 kg).

   So where does the 29 kg of oxygen you breathed in end up? In both products. Part of it makes up the rest of the 28 kg of carbon dioxide, beyond the fat's own atoms, and part of it ends up in the water.

4. 4.Breathing, exercise and fat cellsBIG BABY MODE

   If fat leaves through your lungs, would breathing faster burn more of it? No. Think of your kitchen trash: cooking fills the bin, and taking the bag out more often doesn't cook anything. Your cells make carbon dioxide as they use fuel. Breathing only carries it out.

   Turn breathing up to 30 breaths a minute and watch the carbon. Then add one hour of jogging.

   Where the trash picture breaks: an empty bin is harmless, but your body notices when your blood runs low on carbon dioxide. Fast, deep breathing, called hyperventilation, can leave you lightheaded, dizzy, numb or tingly.

   A resting 70 kg person breathes about 12 times a minute, and each breath carries out 8.9 mg of carbon. Over a quiet day that's about 203 g of carbon, close to the 210 g in half a kilo of sugar. Swapping one hour of rest for a jog adds about 39 g, roughly 20% more. But a single 100 g muffin covers about a fifth of a day's energy needs.

   Fat cells don't vanish when you lose weight. Adults keep a roughly constant number; the cells shrink, and about 10% are renewed each year. You can't pick which ones shrink, either: in one trial, six weeks of ab exercises alone didn't reduce belly fat, and a 2022 review of 13 studies found no spot reduction.