The enlightening facts about fat metabolism 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 kg of fat, precisely 8.4 kg comes out through your lungs and the remaining 1.6 kg 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 fibre (think 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. Kilograms in versus kilograms out 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 3.5 kg of food and beverages every day. Of that, 415 grams is solid macronutrients, 23 grams is fibre and the remaining 3 kg is water.

Loading Replay Replay video Play video Play video 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. If you put 3.5 kg of food and water into your body, plus 600 grams of oxygen, then 4.1 kg of stuff needs to come back out, or you’ll gain weight. If you’re hoping to shed some weight, more than 4.1 kg will have to go. So how do you make this happen? 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 75 kg 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. Eat less, exhale more 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.