When you lose weight, where does it go? Turns out, most of it is exhaled. In a new study, scientists explain the fate of fat in a human body and through precise calculations, debunk some common misconceptions. Fat doesn't simply "turn into" energy or heat, and it doesn't break into smaller parts and get excreted, the researchers say.
In reality, the body stores the excess protein or carbs in a person's diet in form of fat, specifically, as triglyceride molecules, which consist of just three kinds of atoms: carbon, hydrogen and oxygen. For people to lose weight, their triglycerides must break up into building blocks, which happens in a process known as oxidation.
When a triglyceride is burned up, the process consumes many molecules of oxygen while producing carbon dioxide and water as waste products.
The researchers showed that during weight loss, 84 percent of the fat that is lost turns into carbon dioxide and leaves the body through the lungs, whereas the remaining 16 percent becomes water.
These results show that the lungs are the primary excretory organ for weight loss. The water formed may be excreted in the urine, faeces, sweat, breath, tears or other bodily fluids and is readily replenished.
The calculations also show the frightening power of, for example, a small muffin over an hour of exercise: At rest, a person who weighs 154 pounds (70 kg) exhales just 8.9 mg of carbon with each breath. Even after an entire day, if this person only sits, sleeps, and does light activities, he or she exhales about 200 grams of carbon, the researchers calculated.
A 100 g muffin can cover 20 percent of what was lost.
On the other hand, replacing one hour of rest with exercise such as jogging, removes an additional 40 g of carbon from the body, the researchers said.
Even if one traces the fates of all the atoms in the body, the secret to weight loss remains the same: In order to lose weight, one needs to either eat less carbon or exercise more to remove carbon from the body.