Where Fat Goes When We Lose Weight?

The world is obsessed with fad diets and weight loss, yet few of us know how a kilogram of fat actually vanishes off the scales.

The Mists

Even the 150 doctors, dietitians and personal trainers we surveyed shared this surprising gap in their health literacy. The most common misconception by far was that fat is converted to energy. The problem with this theory is that it violates the law of conservation of matter, which all chemical reactions obey.

Some respondents thought fat turns into muscle, which is impossible, and others assumed it escapes via the colon. Only three of our respondents gave the right answer, which means 98% of the health professionals in our survey could not explain how weight loss works.

So if not energy, muscles or the loo, where does fat go?

The Truth 

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 (4.54kg) of fat, precisely 8.4 pounds (3.81kg) comes out through your lungs and the remaining 1.6 pounds (0.73kg) turns into water. In other words, nearly all the weight we lose is exhaled.

In summary, 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 fibre (e.g. corn). Everything else you swallow is absorbed into your bloodstream and organs and, after that, it's not going anywhere until you've vaporised it.

Tracking Energy vs Tracking Kilograms

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, Americans consume 3.55kg (125 ounces) of food and beverages every day. Of that, 0.43kg (about 15 ounces) is solid macronutrients, 0.017kg (0.6 ounces) is fibre and the remaining 3.11kg (110 ounces) is water.

What's not reported is that we inhale more than 0.66kg (23 ounces) worth of oxygen, too, and this figure is equally important for your waistline.

If you put 3.55 kg (125 ounces) of food and water into your body, plus 0.66kg of oxygen (23 ounces), then 4.2 kg (148 ounces) of stuff needs to come back out, or you'll gain weight. If you're hoping to shed some weight, more than 4.1kg will have to go.

3.55kg food & water + 0.66kg oxygen input => 4.2 kg output to maintain or loss weight

Resting Metabolic Rate vs Carbon Dioxide & Water 

The 0.43kg (15.2 ounces) of carbohydrates, fats, protein and alcohol most Americans eat every day will produce exactly 0.77kg (27.1 ounces) of carbon dioxide plus 0.29kg (10.2 ounces) of water (about one cup) and about 0.031kg (1.1 ounces) of urea and other solids excreted as urine.

0.43kg Carbo, Fats, Protein & Alcohol input => 0.77kg Carbon Dioxide + 0.29kg Water + 0.031kg Urine output

An average 75kg (165 pounds) person's resting metabolic rate, the rate at which the body uses energy when the person isn't moving, produces about 0.59kg (21 ounces) of carbon dioxide per day. No pill or potion you can buy will increase that figure, despite the bold claims you might have heard.

Sleeping

The good news is that you exhale 0.2kg (7 ounces) 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.

0.2kg Carbon Dioxide output

Eat fewer kilograms, exhale more kilograms

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 24 hours, you'd exhale more than 1.2kg (42 ounces) of carbon dioxide.

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

Metabolising 100 ounces (2.84kg) of fat consumes 290 ounces (8.22kg) of oxygen and produces 280 ounces (7.94kg) of carbon dioxide plus 110 ounces (3.12kg) of water. The food you eat can't change these figures.

2.84kg Fat + 8.22kg Oxygen input => 7.94kg Carbon Dioxide + 3.12kg Water output

Another way of putting it.

When somebody loses 10kg of fat (triglyceride), 8.4kg is exhaled as Carbon Dioxide (CO2). The remainder of the 28kg total of Carbon Dioxide (CO2) produced is contributed by inhaled oxygen. Lungs are therefore the primary excretory organ for weight loss.

(This calculation ignores fat that may be excreted as ketone bodies under particular (patho)physiological conditions or minor amounts of lean body mass, the nitrogen in which may be excreted as urea)

Any diet that supplies less "fuel" than you burn will do the trick!