When a Person Loses Weight, Where Does the Fat go?

Most new year's resolutions involve some type of weight loss activity.  But have you ever wondered where the fat goes as you lose it?  A recent study from Australia, published in the British Medical Journal, has found out exactly what it is that the body does when it loses weight.  Take a deep breath... most of the excess fat is converted into CO2 and exhaled!

Fat is your body's way of storing energy. When you eat a lot, your body can't possibly use all that energy at once, so it stores some of it in the form of fat. Fats are made up of fatty acids, which are carbon chains, and they can be saturated (no carbon-carbon double-bonds, meaning the carbons are saturated with hydrogens) or unsaturated (with one or more carbon-carbon double-bonds). Quick biochemistry lesson here: since saturated fatty acids have straight tails, they can pack in really tightly together, which is why butter is a solid at room temperature. Unsaturated fatty acids have one or more kinks in the tails caused by the double-bonds, and so these types of fats don't pack together, making oils liquid at room temperature.

Anyway, according to this study, as you lose weight, your body breaks down the stored fats, which can be described by this equation:

The two researchers, Ruben Meerman and Andrew Brown, thought that this equation was good at telling us what happens to fat as we lose it, but didn't really tell us the proportion of fat that was lost as CO2. They decided to look at the pathway of each individual atom out of the body, and found that the lungs are the major excretory pathway during weight loss. In fact, 84% of fat that is lost is exhaled, and the remaining 16% is excreted as water (in urine or sweat).

That's really interesting, because Meerman polled 150 doctors, dieticians, and personal trainers, and more than half of them thought that fat was converted into energy as it was broken down. But Meerman is a physicist, and he knew that this was a violation of the Law of Conservation of Mass.

The researchers found that, in order to lose 10 kg of fat (22 lbs), a person needs to inhale 29 kg (64 lbs) of oxygen and burn 94 000 calories.