Shazam! Where'd all that Dang Fat Go?

Very few people know what happens to body fat when you diet.

You probably know someone who’s an annoying smarty-pants when it comes to nutrition. Maybe it’s your hemp-wearing, matcha-pushing sister, or some know-it-all buttface on Instagram, of which there are untold thousands.

More than likely it’s some master-of-the-universe trainer at your gym who pontificates to his clients annoyingly long and loud about Paleo diets, the merits of whole-wheat toast and avocado, or anything else that crosses his intermittingly calorie-restricted brain.

Well, here’s how you can probably take them down a peg or two or ten. Just ask them this simple question:

“What actually happens to body fat when we lose it?”

Chances are, one of three things will happen: 1, they’ll say something about it being “burned up,” that it’s used to make energy -- which is not only wrong but breaks at least one law of physics, 2, they’ll tug nervously on the collar of their polo shirt and mumble something about being late for the chiropodist, or less likely, 3, they’ll ape the late Jackie Gleason, aka Ralph Kramden, and squeak out “Homina homina,” which was something Ralph said when he was caught in an embarrassing situation.

It’s amazing that the real answer is known to so few. After all, those of us in the health business – and in truth, the majority of the fat-assed country -- spend a really big chunk of time trying to figure out how to defeat fat. If Sun Tzu was a nutritionist, he’d be thrashing around in his tomb or crypt because modern nutritionists not only don’t know their enemy, they’ve completely mischaracterized it.

Okay, What’s the Real Answer?

The short answer is that you literally breathe most of it out through your lungs. The rest leaves your body via urine, sweat, and any and all other liquid bodily secretions.

Here’s exactly how it happens, step by step:

1. All fats in the body exist as triglycerides and much of it is stored as droplets of oil in the fat cells that exist throughout the body. They exist as a fuel supply. If you cut calories through diet or exercise, an enzyme called hormone-sensitive lipase is activated by hormonal messengers and breaks down triglycerides into glycerol and fatty acids.

2. These breakdown components are then released into the bloodstream where they’re absorbed by first and foremost the liver, followed by the muscles.

3. Once absorbed by the liver or muscles, the triglyceride components are further broken apart and rearranged, resulting in large amounts of acetyl-CoA.

4. The acetyl-CoA combines with oxaloacetate within the cells’ mitochondria to form citric acid.

5. This initiates the citric acid cycle, aka the Krebs cycle, which generates carbon dioxide, water, heat, and ATP.

6. The carbon dioxide then leaves the body through the lungs as you exhale, while the water exits through mostly urine and sweat. The heat that was released through the citric acid cycle maintains your body temperature, and the ATP generated powers every movement you make, whether conscious (walking, lifting a weight, chewing gum, etc.) or subconscious (the beating of your heart and breathing, among other things).

Fine, But now in English, Okay?

In less technical terms, any energy (food) you consume is stored in fat cells as triglycerides. When you expend more energy than you take in, the triglycerides are dumped into your body so they can be used as fuel.

These fatty acids get further broken down into yet smaller chemical components, and this breakdown results in energy, water, and a lot of carbon dioxide.

The British Medical Journal described the process in more concrete terms, writing that a human needs to inhale about 3 pounds of oxygen to burn 1 pound of fat. The oxygen initiates the aforementioned metabolic processes, resulting in just about 18.5 pounds of carbon dioxide and about 1.5 pounds of water.

(Oh, by the way, before you start copying your Gen Z kid who huffs mothball fumes out of a paper bag to get high, you can’t simply snort cubic meters of air in a manic attempt to lose a couple of pounds of fat before your high school reunion. No, this breathing of oxygen must be accompanied by physical exertion to trigger the metabolic process of burning fat.)

The carbon dioxide is then breathed out and the water exits the body through urine, sweat, feces, saliva, or any other bodily fluid you can think of.

Will This Info Make Dieting Easier?

Knowing the exact process through which fat loss occurs won’t do a lick in helping you lose additional body fat. However, it might raise you a tick or two on the evolutionary scale of lifters, coaches, nutritionists, and even doctors who don’t have the faintest clue about how fat loss works.

And it might give you some satisfaction that every breath you take during a hard workout, every rivulet of sweat that flows down your scalp and back, equates to a little bit of pesky fat being evicted from your body.

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Reference: Meerman R et al. When somebody loses weight, where does the fat go? BMJ. 2014 Dec 16;349:g7257. PubMed.