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What Happens To Fat When You Lose Weight?

Hint: take a deep breath.

Whether it's because you've been eating more healthily, exercising more or controlling your portions, seeing fat loss on the scales (or fitting into those old jeans) is a rewarding feeling.

But have you ever wondered: where does fat go when you lose weight? The answer is quite incredible.

First we need to take a look at what fat gain is in the first place -- other than following a diet of pizza, doughnuts and chocolate -- in a physiological sense.

"If you're looking specifically at fat gain, over muscle gain, fat gain will occur when more kilojoules are consumed than the body is able to use over a period of time, whether it's a small quantity or a big quantity," Chloe McLeod, accredited practising dietitian and sports dietitian, told HuffPost Australia.

A good example is when we go away on holiday (and are a bit more lax with our diet than usual) and come back having gained a couple of kilograms.

"That has occurred because, when looking at daily energy requirements, more food is being consumed than the body actually needs," McLeod said.


"The excess carbohydrate and protein are converted into triglycerides, which is a fat found usually in the blood, and the triglycerides are made up of carbon, hydrogen and oxygen -- like most compounds in our bodies.

"These triglycerides are then stored in our fat cells as little drops, and so when you're trying to lose weight your body is trying to get rid of those."

So, when we lose weight, where does the fat actually go?

"It's so interesting. There was a study which came out recently in the British Medical Journal about this. Essentially, the fat you lose is breathed out as carbon dioxide," McLeod said.

Indeed, the researchers found that "the lungs are the primary excretory organ for weight loss".

Researcher Ruben Meerman and professor Andrew J Brown explained:

Most people believed that fat is converted to energy or heat, which violates the law of conservation of mass. We suspect this misconception is caused by the "energy in/energy out" mantra and the focus on energy production in university biochemistry courses. Other misconceptions were that the metabolites of fat are excreted in the faeces or converted to muscle.

The lungs are the primary way our body expels fat loss.
Getty Images/iStockphoto
The lungs are the primary way our body expels fat loss.

If you're wondering what this process 'looks' like, let's go back to year 10 chemistry for one minute.

"As an example, when you're trying to lose 10 kilos of human fat, this means you're going to inhale around 29 kilograms of oxygen and then that will produce 28 kilograms of carbon dioxide and 11 kilos of water," McLeod explained.

"It's about breaking down those compounds and then they are exhaled. So when you're breathing out, it's the carbon dioxide and the moisture on your breath."

While breathing is the main way weight loss is excreted from the body, small amounts of the water which forms from this process can also be released via urine, faeces, sweat, tears or "other bodily fluids".

"Some does come out in your sweat, but it's a very, very small portion. Sweat isn't 'fat crying'," McLeod said.

"Some of it will be excreted out in your faeces, but only a small amount. Most of it is coming out with your breath."


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