When You Lose Weight, Where Does the Fat Go?

Maybe you have started eating healthy or have been exercising more or are controlling your portions! Seeing your weight go down on the scale is a rewarding feeling.

But what intrigues us here is where does the fat go when you lose weight. The answer is unbelievable. First, we must understand what fat gain is- we need to know it beyond having a diet comprising of dough nuts, pizza and chocolates, in a physiological sense. When we talk about fat gain over gain in muscle, we must understand that it occurs when we consume more kilo joules than what we are capable of using over a period of time. A great example is when we go out on a holiday and we become laxer with our diet than normal and what do we see? We come back with more kilograms. This happens because we are consuming more food than what the body actually requires.

The excess protein and carbs get converted into triglycerides which is a fat present in blood. Triglycerides are made of hydrogen, carbon and oxygen much like the many compounds in the body. The triglycerides get stored in the fat cells as tiny drops so when we are trying to lose weight, the body wants to get rid of these.

So, upon losing weight, where does the fat go? ^[1] It is really interesting. A study regarding this was issued in “British Medical Journal”. The fat that you lose is breathed out as CO₂. The researchers said that lungs are the primary excretory organ for weight loss. If you are losing 10 kg of fat, exactly 8.4 kg comes out from the lungs and 1.6 kg gets converted into water. So almost all the weight we lose is exhaled.

Fat gets stored as triglycerides. When it is required for energy, enzymes in blood break it down into glycerol and fatty acid chains ^[2]. Cells absorb fatty acids, and these are then broken down into smaller molecules and are fed to the mitochondria. Finally, the waste products of this process are carbon dioxide which we breathe out and water which we excrete. So, when you get rid of the fat you are basically converting it into panting and puffing.

It is believed by most people that fat gets converted to energy or heat, but this violates the law of conservation of mass. Some of the misconceptions are the metabolites of fat get excreted in feces or gets converted to muscle.

Now, if you are trying to know what the process looks like, we may have to take you back to year 10 chemistry. What we are essentially trying to say is that if you want to lose 10kg of fat, you will inhale 29 kg of oxygen and produce 11 kg of water and 28kg of CO2. It is about breaking those compounds before they are exhaled. CO2 and moisture are there on your breath when you breathe out.

So, breathing is the primary way weight loss happens in the body, small quantities of water formed in the process can be excreted through feces, urine, tears, sweats or other body fluids. A very small portion comes out in the sweat. A small amount gets excreted in feces. Majorly it is coming out of the breath you release.

It is surprising but nearly all that we eat comes back out from the lungs. All carbs and fat get converted to water and CO₂. Same happens with alcohol. Likewise, for protein except for some part that gets converted to urea and solids that get excreted as urine. The only part of food which goes to the colon intact is dietary fiber like corn. All else is absorbed in the blood and organ and then it does not go anywhere until it gets vaporized.