Where Does Fat Actually Go?

When people work to lose weight, the focus is often on losing fat. But what really happens to that fat? Where does it go once it’s gone? The extra mass came into the body from the environment via food, so how does it get back out into the environment when you lose it?

Surprisingly, most people—including many health professionals—don’t fully understand the answer. While it’s common to think that fat simply “burns off,” the actual process is far more interesting. Fat doesn’t just disappear; it’s broken down and leaves the body in ways you might not expect.

A study recently came out that put a delightful new spin on the work of moving fat from within the body to the world outside. It was published in the British Medical Journal and is happily not behind a subscription wall so you can read it here. If you have the same passion for stoichiometry as we do, dive in. If not, we'll break it down in layman's terms for you.

The study started with a simple question. When someone loses biomass in the form of body fat, where exactly does that mass go? We know it's not just disappearing, the law of conservation of mass is called a law for a reason. So it must be leaving the body somehow.

The researchers asked medical doctors, nutritionists, and trainers, and almost all of them made the same mistake. The majority of them thought that the mass was converted to energy. On its face, this seems logical. When you're running a caloric deficit during your training diet, the body needs to find the energy to maintain itself. So it takes the triglyceride fat packets, breaks them apart with a catabolic reaction, and uses that energy to power metabolism. So the mass must become energy... right?

The problem is this one pesky equation that keeps popping up:

E = mc²

Energy and matter are deeply connected. The mass of an object times the squared speed of light will give you the energetic potential of that object (in joules). These numbers are always pretty shocking when you run them through the equation.

For example, let's say that Albert loses 5kg (11lb) on his training program. If that 5kg were converted directly into energy it would unleash the equivalent of 100 megatons of TNT. For perspective, that's twice the energy of the biggest nuclear bomb ever tested (Russia's Tsar Bomba) and 5000 times the energy of the bombs dropped on Hiroshima and Nagasaki.

So Albert's fat mass couldn't have been transformed to energy. If that's how things worked, every time someone went on a health kick they'd end up levelling their neighborhood.

If it’s not being converted directly into energy, where is the mass going? The researchers carefully charted the course of the atoms of an average triglyceride packet and found a surprising result. Are you ready for this?

Of all the atoms in the 5kg of fat that Albert loses, 4.2kg of it will be exhaled from the lungs in the form of CO2. In other words, you exhale 84% of lost fat through your lungs! That's where all that biomass is going. It's being broken down into carbon dioxide and dispersed into the air around you with each out-breath.

And how about that other .8kg? It turns out it’s packed up into H20 and excreted from your body through urine, feces, sweat, or other bodily fluids. And that's it! Fat turns mostly into exhaled carbon dioxide and a little water. As the research paper summed up, "The lungs are the primary excretory organ for weight loss." Isn't that cool!?

It doesn't take long for people to look at these results and think... "Hmm… If I breathe a lot more will I lose some extra fat?"

Sadly, no. The fact that the fat packets are being used at all is the result of a body doing exercise and not having enough calories in the system to meet energy needs. You can't cheat your way around that reality. This study won't spare you from the sacrifice and exertion of training, all it is shows exactly how all that biomass is getting from inside your body to the outside.

Thinking of the body at this molecular level seems cold and scientific, but it’s actually quite beautiful. We're made of simple matter and powered by the energy of our little star, and our bodies follow the same exact rules as the rest of the universe. We aren't special, our bodies aren't mysterious black boxes that we have to be superstitious about, they're bound to the same limits of all matter. And with our intellect we can understand these limits and work with them to feel happy and healthy in the days we have. All too soon not just our fat, but our skin, organs, and eventually bones will break down into their constituent atoms and be scattered across the planet. And the planet will also be broken up one day and diffused across galactic space.

This is an awe-inspiring dance of time, space, matter and energy, and we have the privilege to be a part of it. Wouldn’t it be great to do that dance without a lot of excess biomass dragging you down? Let's mobilize that fat with a clean diet and daily exercise, and end the day with a sigh of satisfaction; a sigh that carries with it the molecules that once were sticking to our bodies!

Bonus Critical Thinking Challenge

Question: If you're putting out all this extra CO2 from breaking down fat, are you contributing to global warming when you slim down?

Answer: No! Let's say you gained fat from eating sweet potatoes. Plants live on CO2. The potatoes took atmospheric carbon and incorporated it into their mass to grow. You then ate the potatoes and sequestered that carbon into your own body in the form of fat. Finally you exercised, broke up that fat, and exhaled most of it back into the atmosphere.

This is a closed carbon cycle, with no extra CO2 being added to the system, hence no global warming. Global warming is caused by accessing vast amounts of CO2 that used to be locked away in the earth's crust and shifting it to the atmosphere, not from organic plants and animals cycling it through their bodies as energy and waste.

If you wanted to split hairs, you could argue that because the sweet potatoes were tilled by a tractor and came to you on a truck they aren't really carbon neutral, but that's not the potatoes' fault. That extra carbon came from oil outside of the closed system!