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What's the story with fat?

Where does fat go when you put it on, and where does it go when you're getting rid of it? Dr Karl has some unexpected answers to these questions.

When someone is tempted by a delicious slice of cake, you might have heard them paraphrase the words uttered by the waifish supermodel Kate Moss: "Nothing tastes as good as skinny feels."

So what's the story with fat? Where does it go when you put it on, and where does it go when you're getting rid of it? The answers are quite unexpected.

We need fat. In the body, fats are usually stored inside fat cells as a molecule called a triglyceride. The triglyceride molecule looks like the letter 'E'. The vertical stroke, or backbone of the letter E is a glyceride molecule and the three horizontal strokes of the letter E are three separate molecules called fatty acids.

Now, there's a lot of variation in these three fatty acids. They can be all the same, they can be all different, or anything in between. The average triglyceride molecule has about 160 atoms — 55 carbon atoms, 104 hydrogen atoms, but only six oxygen atoms.

We used to think that until you got to about 20 years of age, you never made any more fat cells. But that turned out to be wrong.

One study deliberately overfed some normal, healthy, overweight men and women. In their upper body they laid down 1.9 kilograms of fat, and, as expected, the fat cells in this area just swelled up and got bigger.

Surprisingly, this did not happen in the lower body where they added 1.6 kilograms of fat. No! Down there, below the belt, the fat cells stayed the same size, but they grew — an extra 2.6 billion of them.

Now that was news, but not as big as the surprising results when two researchers asked where fat goes when you lose it.

The researchers were Ruben Meerman from the Australian Broadcasting Corporation and Professor Andrew Brown from the University of New South Wales.

First, they surveyed 50 doctors, 50 dietitians and 50 personal trainers with the question: "When someone loses weight where does it go?" About 60 per cent gave the answer that the fat got converted to heat or energy — which was wrong.

The only way atoms can turn into heat or energy is via nuclear reactions. The human body doesn't do nuclear reactions, it does only chemical reactions.

Only a few per cent — and they were the dietitians — got the correct answer. The triglyceride molecules get broken down into molecules of carbon dioxide and water.

You might remember that the average triglyceride molecule has 55 carbon molecules and 104 hydrogen atoms but only a very small number of oxygen atoms — just six of them. And you might also recall the carbon dioxide — CO2 — has two atoms of oxygen, while water — H2O — has one atom of oxygen.

So, if you're going to break down triglyceride molecules into carbon dioxide and water molecules, you're going to have to add lots and lots of oxygen.

Suppose you want to lose 10 kilograms of fat. That means you're going to have to add 29 kilograms of oxygen — that's a lot of breathing and huffing and puffing. And in return, you're going to get rid of some 28 kilograms of carbon dioxide and 11 kilograms of water.

And how are you going to get rid of this 39 kilograms of carbon dioxide and water? Overwhelmingly, by breathing it out. You might lose a tiny bit of this amount in sweat or urine, but the vast majority of those atoms that made up the triglyceride molecules inside your fat cells come out of your mouth as carbon dioxide and water.

This means that your major excretory organ is your mouth. And consider this, each breath removes 33 milligrams of carbon dioxide which carries 8.9 milligrams of carbon.

Now, it is true that you are what you eat. But can you unlock the carbon in your fat cells and lose weight by breathing more? Kind of.

The most effective way to breathe more often is to exercise. So eat less and move more.

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