Where Does Fat Go?
How Fat Leaves the Body
Did you ever wonder when you lose fat, where it goes? I hadn't given it much thought until a client asked me. I didn't know, so I asked some of my colleagues what they thought.
About half the people I asked assumed it was converted into energy and then lost as heat. When you exercise, your body uses it, and that's why you get warm as you move. It turns out that's wrong.
When you workout, about 20 percent of the energy your muscles provide is used to move or contract the muscle. Eighty percent of the energy is converted to heat. So that heat you feel is muscle working, not fat burning.
Decades ago, people used to think fat turned into muscle when you exercise. But fat and muscle are two entirely different things. You can't turn fat into muscle any more than you can turn a cat into a dog. The fat cells can shrink through exercise, and muscle can grow, but one doesn't turn into the other.
Some people thought it was expelled when we went to the bathroom through urine or feces. But tests that analyzed our "output" failed to account for more than a small percentage of the fat.
Numerous other theories have tried to explain where it went or how we disposed of it, but none of them were clinically proven. And when scientists did explore those options, they turned out to be wrong. Most fat seemed to be vanishing into thin air.
That was when TV science personality Ruben Meerman and researcher Andrew Brown from the University of New South Wales decided to team up and answer the question. They devised a method to trace a single atom of fat that someone lost.
The study involved a lot of math and measurements to track everything. Here's a sample of their "novel calculation."
(661 Da (C55)+64 Da (O4))/(861 Da (C55H104O6))×100=84%
Fun right? But what does that mean? Well, it turns out, our LUNGS expel the majority of the weight we lose when we simply BREATHE OUT. You read that right. Over 80 percent of the fat molecules we burn off, leave when we exhale.
We lose the remaining 20 percent of fat molecules as water through urine, sweat, and tears. When someone says sweat is fat crying, they're at least a little right.
For those of you interested in the math, here you go. For every 10 kg of fat you burn, you have to inhale 29 kg of oxygen to metabolize (break down) the fat. The total is 39 kg. Breaking down that fat produces 28 kg of carbon dioxide and 11 kg of water. That also totals 39 kg, and since both sides of the equation match, it perfectly accounts for the lost fat.
The idea that we breathe out the majority of our fat kind of makes sense. When you workout harder, you tend to breathe more. So maybe we can lose even more fat by increasing how fast we inhale and exhale.
Unfortunately, it doesn't work that way. When we exercise, our body needs more oxygen. Since blood gets oxygen from the lungs, we have to breathe more. That draws oxygen into our lungs and increases the supply. We're not panting because our body is pushing out more fat; we're panting because we need more oxygen.
It can be dangerous if you deliberately try and increase your breathing rate more than you need. You can hyperventilate, get dizzy and pass out.
You burn fat from a workout because your body is using some of that fat for energy. Oxygen breaks up the fat molecules and releases their energy into water and carbon dioxide. Over time, we breathe most of it out. The more we break down, the more our body can dispose of through breathing.
This study answered a couple of questions. Not only how our body gets rid of fat, but why things like laxatives don't help you lose weight. Increasing how much we poop won't do any good if most fat is exhaled.
As a final note, during COVID, many people are working out in masks. Don't worry; those masks won't stop your body from getting rid of fat. Even if you're wearing an N95 mask, the holes are more than 1,900 times larger than needed for carbon dioxide molecules to pass through.
When somebody loses weight, where does the fat go?
Ruben Meerman, researcher, Andrew J Brown, professor
BMJ, Published 16 December 2014 - Cite this as: BMJ 2014;349:g7257
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