When You Lose Weight, Where Does It Actually Go?

Most health professionals lack a clear understanding of how body fat is lost, often subscribing to misconceptions like fat converting to energy or muscle.

The truth is, fat is actually broken down into carbon dioxide and water, with the majority of the lost fat being exhaled as carbon dioxide. This insight is crucial for understanding the real mechanics behind weight loss and dispelling common myths.

Unveiling Weight Loss Misconceptions

The world is obsessed with weight loss and fad diets, yet few people actually understand where fat goes when we lose weight.

Even among 150 doctors, dietitians, and personal trainers we surveyed, this knowledge gap was surprisingly common. The most widespread myth was that fat is converted into energy. However, this idea contradicts the law of conservation of matter, which all chemical reactions obey.

Some respondents believed fat transforms into muscle, which is impossible, while others assumed it exits through the digestive system. In reality, only three people in our survey got the correct answer. That means 98% of these health professionals couldn't accurately explain how weight loss works.

So if fat isn't turned into energy, muscle, or waste, where does it actually go?

The Surprising Science of Fat Loss

The correct answer is that fat is converted to carbon dioxide and water. You exhale the carbon dioxide and the water mixes into your circulation until it's lost as urine or sweat.

If you lose 10 pounds of fat, precisely 8.4lb comes out through your lungs, and the remaining 1.6lb turns into water. In other words, nearly all the weight we lose is exhaled.

This surprises just about everyone, but actually, almost everything we eat comes back out via the lungs. Every carbohydrate you digest and nearly all the fats are converted to carbon dioxide and water. The same goes for alcohol.

Protein shares the same fate, except for the small part that turns into urea and other solids, which you excrete as urine.

The only thing in food that makes it to your colon undigested and intact is dietary fiber (think corn). Everything else you swallow is absorbed into your bloodstream and organs and, after that, it's not going anywhere until you've vaporized it.

Understanding Energy and Weight Dynamics

We all learn that "energy in equals energy out" in high school. But energy is a notoriously confusing concept, even among health professionals and scientists who study obesity.

The reason we gain or lose weight is much less mysterious if we keep track of all the kilograms, too, not just those enigmatic kilojoules or calories.

According to the latest government figures, Australians consume 3.5kg of food and beverages every day. Of that, 415 grams is solid macronutrients, 23 grams is fiber and the remaining 3kg is water.

What's not reported is that we inhale more than 600 grams worth of oxygen, too, and this figure is equally important for your waistline.

The 415 grams of carbohydrates, fats, protein, and alcohol most Australians eat every day will produce exactly 740 grams of carbon dioxide plus 280 grams of water (about one cup) and about 35 grams of urea and other solids excreted as urine.

An average 75kg person's resting metabolic rate (the rate at which the body uses energy when the person isn't moving) produces about 590 grams of carbon dioxide per day. No pill or potion you can buy will increase that figure, despite the bold claims you might have heard.

The good news is that you exhale 200 grams of carbon dioxide while you're fast asleep every night, so you've already breathed out a quarter of your daily target before you even step out of bed.

Active Solutions to Weight Loss

So if fat turns into carbon dioxide, could simply breathing more make you lose weight? Unfortunately not. Huffing and puffing more than you need to is called hyperventilation and will only make you dizzy, or possibly faint. The only way you can consciously increase the amount of carbon dioxide your body is producing is by moving your muscles.

But here's some more good news. Simply standing up and getting dressed more than doubles your metabolic rate. In other words, if you simply tried on all your outfits for 24 hours, you'd exhale more than 1,200 grams of carbon dioxide.

More realistically, going for a walk triples your metabolic rate, and so will cooking, vacuuming, and sweeping.

Metabolizing 100 grams of fat consumes 290 grams of oxygen and produces 280 grams of carbon dioxide plus 110 grams of water. The food you eat can't change these figures.

Therefore, to lose 100 grams of fat, you have to exhale 280 grams of carbon dioxide on top of what you'll produce by vaporizing all your food, no matter what it is.

Any diet that supplies less "fuel" than you burn will do the trick, but with so many misconceptions about how weight loss works, few of us know why.

Written by:

• Ruben Meerman, Assistant scientist, UNSW Sydney
• Andrew Brown, Professor and Head, School of Biotechnology and Biomolecular Sciences, UNSW Sydney

Adapted from an article originally published in The Conversation.