WHEN you lose weight, where does the fat go?

The question had never occurred to me until March 19, 2013, when I stepped on the scales and discovered I’d lost 6.5kg.

I had been following the age-old advice to eat less and move more as part of my New Year’s resolution to stop smoking and lose some weight.

The quitting part was going well, but my weight was on a notoriously slippery slope. Men gain an average of 2.8kg after they give up cigarettes, and for women the figure is 3.8kg.

When you start losing weight, everybody wants to know exactly what you’ve been eating, or not eating, in case there’s some magic ingredient or dietary devil they haven’t heard of yet. There never is. My only secret was to simply count every kilojoule.

So the first three months of my weight-loss “journey” were nothing new and really quite boring.

What was not boring for me was discovering that 6.5kg had somehow managed to escape my body without me even noticing. That’s a significant amount of your self to “misplace”, and it didn’t take long for me to start wondering how this had happened.

I obviously knew what I had done to lose the weight, but where did it go, and how did it get there?

One question led to another and I was hooked. What is fat made of, precisely? What does it turn into? And why did I not know the answer to any of these questions already? We’re in the middle of an obesity epidemic and weight loss is possibly the most talked about topic on the planet.

Shouldn’t this be common knowledge?

We often talk about “burning” kilojoules or calories, so most people think the kilograms we lose are all converted to energy, or heat, but I have a physics degree, so that idea never crossed my mind. Fat is made of atoms, so here’s the first and most important weight-loss fact of all: Your body cannot turn your atoms into energy.

You would need an equal number of antimatter atoms to convert fat atoms to energy, which sounds like a nerdy physics joke, but antimatter really does exist and when it comes into contact with ordinary matter, the two annihilate each other in a flash of light.

Your local hospital’s PET (positron emission tomography) scanner looks for those little flashes to detect metastasised cancers, but this is not how people lose weight. Positrons are anti-electrons. There is no such thing as ‘anti-fat’.

I knew next to nothing about fat metabolism in early 2013, and if I had lost those 6.5kg a few months earlier I might have assumed I’d flushed them down the toilet, and left it at that.

But by a stroke of luck, the “fat is flushed down the toilet” hypothesis was off the table too, because I had just started researching a children’s book about gross bodily functions, like farts, poo, snot and wee.

I had never given the precise composition of the substance that comes out of human bums much thought before, so I was genuinely surprised to discover that three-quarters of every bowel movement is just water, and that the only substance we eat that makes it out the other end in significant quantities is the indigestible fibre in fruits, vegetables, legumes and grains.

The rest of the solids are made up of the friendly bacteria that eat those fibres, plus a tiny bit of “ash”, a little bile and, if you’re a carnivore, perhaps a few tough bits of a particularly chewy steak or gristly sausage.

There was simply no way, then, that those 6.5kg of fat ended up in the sewers of Sydney’s Eastern Suburbs. So where on Earth had they gone, and how did they get there? I was mystified.

After a Wikipedia-fuelled crash course in the hieroglyphics of biochemistry I eventually managed to follow all the convoluted pathways, and the answer turned out to be the most enlightening and motivating weight-loss fact I have ever heard.

Fat turns into carbon dioxide and water, and nothing else. You can summarise the entire process like this: Fat + oxygen = carbon dioxide + water.

Two of the most familiar and ubiquitous chemical substances on Earth also happen to be the ultimate destination for all our unwanted flab. Who knew?

For me, learning this one simple fact changed everything. Suddenly, my body made sense. I had vaporised my fat and exhaled the atoms, and it had been happening right under my nose the whole time! So here is the second major weight-loss revelation I discovered: The air you exhale is heavier than the air you inhaled.

The extra mass in expired air consists of carbon atoms wedged between the two oxygen atoms to make carbon dioxide. Water vapour, which occasionally condenses into a fog of tiny droplets right in front of your eyes on chilly mornings, adds a little mass to exhaled breath too, but that’s easily replenished. It’s those carbon atoms you want to get rid of. Exhaling more carbon than you swallow is the real secret to weight loss.

Weight loss makes complete sense when you think about it in terms of atoms but, like most people, I simply never had. We are carbon-based life forms.

What we fail to appreciate is that approximately 9mgs of that carbon whooshes out of our carbon-based lungs every time we exhale.

Those atoms are chemically welded to oxygen inside our cells and, while 9mgs might not sound like very much, we do this 17 times per minute and all that breathing adds up to 25,000 breaths per day. Therefore, we all pump about 220 grams of carbon out of our lungs every 24 hours. The more you play with these numbers, the better it gets.

On average, your body loses about 1.5kg of carbon per week. In a whole year, a 70kg adult exhales about 80kg of carbon atoms – that’s 10kg more than their body weight on any given day!

The only reason these amazing results don’t show up on the scales, of course, is because the same 70kg person will swallow 80kg of carbon atoms per year too. Otherwise they’d wither away in a matter of months.

Because it’s so easy to forget about the atoms we exhale, some people mistakenly believe that the only way a person can lose weight is through vigorous exercise, completely ignoring the

fact that we continue breathing even when we are sitting still or taking a nap.

So here’s another rock-solid weight-loss fact to take to bed tonight: You exhale 20kg of carbon in your sleep every year.

Lose weight without exercising? Yep. In your dreams!

This is an edited extract from Big Fat Myths by Ruben Meerman, published by Ebury Australia on 19 September 2016, RRP $34.99, available to order here.