Where Does Fat Go When You Lose It?

Ask people where fat goes when it is lost, and you will hear many answers: it is burned off as heat, it turns into muscle, it is sweated out, it leaves in the toilet. Two scientists asked the same question and worked out the answer with chemistry. The surprise is that most of it leaves through your lungs. This is general education, not medical advice.

Matter does not vanish

In 2014, physicist Ruben Meerman and biochemist Andrew Brown published a short paper in the BMJ. They started with a basic rule: matter cannot disappear. If a body loses weight, the atoms in the fat have to go somewhere. They then followed the atoms of a typical fat molecule (a triglyceride) through the chemical reactions the body uses when it burns fat for energy.

The numbers

When fat is burned, the products are carbon dioxide and water. The authors worked through the numbers for a person who loses 10 kilograms of fat:

• Oxygen in: about 29 kilograms of oxygen are needed to break down the fat, which the body gets by breathing over time.
• Carbon dioxide out: about 28 kilograms of carbon dioxide are produced and breathed out.
• Water out: about 11 kilograms of water are produced, which leaves through urine, sweat, tears and other fluids.

Put another way, about 84 percent of the fat’s atoms leave as carbon dioxide through the lungs, and about 16 percent leave as water. The oxygen taken in explains why the input and output figures add up (10 plus 29 is 39; 28 plus 11 is 39).

The common myths, answered

• “It turns into muscle.” No. Fat and muscle are different tissues, and one does not convert into the other.
• “It is sweated or passed in stool.” Only a small share leaves as water. Fat is not mainly eliminated that way.
• “It is burned as heat.” Heat and energy are released, but the matter itself has to leave in a physical form. Energy and mass are two different things to account for.

What this does not mean

• Breathing harder does not burn fat. The lungs are the exit, not the engine. Fat is only broken down when the body needs more energy than it is getting from food, as described in our guide to the thermic effect of food and the calories you burn without trying.
• It does not make weight loss easy. Understanding where the atoms go does not change how many calories it takes to lose them.
• The numbers are a model. They describe a typical triglyceride and idealized chemistry; real bodies vary a little.

Why it is worth knowing

It is a small, satisfying fact: when you lose fat, you are quite literally breathing it out. It also reinforces a practical point that runs through the evidence: weight changes are about energy in and out over time, and plateaus and fluctuations in the short term are often water, not fat. See why weight-loss plateaus happen.

Sources

• Meerman R, Brown AJ. When somebody loses weight, where does the fat go? BMJ, 2014;349:g7257.