Where does fat go when you lose weight? Mostly into the air you breathe out

Why do most people get this question wrong?

Ruben Meerman and Andrew Brown, of the University of New South Wales, asked family doctors, dietitians and personal trainers a simple question: when somebody loses weight, where does it go? Most answered that fat is “converted to energy or heat.”

That answer is wrong, the authors explain: matter does not vanish, it changes form (the law of conservation of mass). Others believed fat is passed in stool or turned into muscle. The authors list those answers as misconceptions too.

What did the two researchers calculate?

Your body stores fat in specialized cells called adipocytes, in the form of triglycerides: molecules made of carbon, hydrogen and oxygen. In chemical terms, losing weight while keeping your muscle means using up those triglycerides.

When your body draws on its reserves, it breaks triglycerides apart (a step called lipolysis), then “burns” them with the oxygen you breathe in. The overall reaction is well known: a triglyceride plus oxygen gives CO2, water and energy.

What the article added was to trace every atom on its way out. Carbon atoms leave in CO2, hydrogen atoms in water. For the oxygen contained in fat, the authors relied on an experiment from 1949: it is shared between CO2 and water.

So where does the fat actually go?

The result: 84% of a triglyceride's mass comes out as CO2, and 16% as water. If you lose 10 kg of fat (about 22 pounds), 8.4 kg is exhaled. To do that, you also need to breathe in 29 kg of oxygen, which is why the reaction produces 28 kg of CO2 and 11 kg of water in total.

The authors conclude that the lungs are the main organ for getting rid of fat. The water can leave in urine, sweat, breath, tears or stool.

Can you lose fat by breathing faster?

No. The CO2 you breathe out only comes from your fat if your body has drawn on it first. As the authors put it, losing weight requires “unlocking” the carbon stored in fat cells, which brings back the familiar advice to eat less and move more.

They give a sense of scale. A 70 kg person breathes out about 0.74 kg of CO2 over a day of sleep, rest and light activity, which removes 203 g of carbon from the body. Swapping one hour of rest for a jog removes an extra 39 g of carbon, about 20% more. But a single 100 g muffin is roughly 20% of a day's energy needs. Exercise is easily undone by a small amount of extra food.

Do fat cells go away?

This point does not come from The BMJ but from a study published in Nature in 2008 (Spalding and colleagues). It showed that the number of fat cells stays constant in adulthood, in lean and obese people alike, even after marked weight loss. About 10% of fat cells are renewed each year.

In other words, when you lose weight your fat cells empty out, a bit like a balloon deflating, but they stay in place and can fill up again. And because fat and muscle are two different tissues, one does not turn into the other.

What are the limits of this calculation?

This is a chemistry calculation, published in The BMJ's Christmas issue and peer reviewed, not a trial in patients. It uses an “average” triglyceride and sets aside two cases: fat excreted as ketone bodies in certain conditions, and the small amount of lean mass lost at the same time.

Above all, it explains where fat leaves the body, not how to lose weight. It says nothing about the diet or activity that suits you: that is a question for your doctor.