Biochemistry answers to weight loss questions: Where does the weight go?

Aaron_K123 wrote: »

I see this question on the thread a lot:

"Okay I get that if I eat less calories than I use I will lose weight, but where does that weight actually go?"

The assumption seems to be that you either poop it out somehow or that you literally convert mass into energy like you were some sort of nuclear reactor. Neither one of these is correct. The answer comes from biochemistry and I thought I'd try to give a layman explanation of what is going on when you eat food and do work. By explaining that it becomes evident where the weight goes.

First lets address the idea that you lose weight by essentially pooping it out. When you eat food enters your mouth and goes through your gastrointestinal tract. That is basically a tube from your mouth to your *kitten* that, if you think about it, is actually "outside" of your body. The parts you can digest are broken down into smaller soluble pieces and transferred through your intestinal wall to your blood. What remains behind just passes through you and as such was never really inside you in the first place. Nothing substantial crosses that lining in the opposite direction so what you poop out is just what you couldn't digest mixed with the bacteria that are in your gut. The weight you lose from pooping is just whatever you ate that you couldn't digest mixed with bacteria that is in your gut, it is not weight that was ever a part of you. Therefore you do not lose weight from your body through excrement.

The next idea is that you convert fat into energy and fat has mass and energy doesn't so you are literally just destroying mass inside of you. The more fat you convert the more weight you lose as a result. That doesn't happen. We do breakdown fats and other biomolecules to get energy but the mass is conserved, you are just breaking it down into parts that weigh the same amount as the whole. The only thing that converts mass to energy is a nuclear reaction and our bodies aren't nuclear reactors. Our body temperature is about 37 degrees celsius, not the 100 million degrees nuclear fusion would require. Therefore you don't lose weight from conversion of mass to energy.

So how do we lose weight? Well to understand that we have to look at how we do convert the food we eat into usable energy. The food we eat has one thing in common, the part that gives us sustenance in the form of calories are molecules comprised of carbon (C), hydrogen (H) and oxygen (O). Fats, or hydrocarbons, are made of just C and H. Protein is made from units called amino acids that are composed of carbon, hydrogen, oxygen and nitrogen. Carbohydrates are carbon, C, combined with water, H20, to form a carbo-hydrate CxH2x0x. Glucose for example is C6H12O6 which is essentially 6 carbons and 6 waters. Molecules that are based around carbon as these are are referred to as organic molecules.

When you eat pasta your GI tract breaks down the starches into their constitutive soluble parts which are glucose molecules. Those molecules then transfer across your intestinal lining to your blood raising your blood sugar which triggers insulin which triggers your cells to uptake glucose. Glucose is then broken down by a long series of enzymatic reactions which are oxidative, they use oxygen to break bonds in the molecule. When a chemical bond is broken some energy is released and these enzymes capture that release of energy by forming of a new bond in a "currency" molecule converting adenosine diphosphate into adenosine triphosphate (ATP). ATP is used almost like a battery for universal energy storage. Other processes in our body can be powered by these batteries which allows the production of ATP to essentially power our bodies by breaking the bond in ATP to form ADP which can then be converted back to ATP by the breakdown of bonds in food molecules.

Most of that energy capture and coupling happens through the electron transport chain (ETC) which is too complex to cover in a thread and would confuse/bore the hell out of people as I spent pages and pages trying to explain. I guess the analogy for the ETC would be picture a society where energy from lots of different specialized sources is used to transport water uphill and when that water flows back down through a channel it flows through a water-wheel that turns a turbine that produces energy that is put into rechargable batteries that are usable by everyone. In this analogy the water is protons and the energy input is food while the rechargable batteries are ATP molecules.

For those who are curious here is the ETC "water wheel" ATP synthase in action. https://www.youtube.com/watch?v=b_cp8MsnZFA

So what does that end up looking like in total? What happens to that glucose molecule?

Well glucose, C6H12O6 is broken apart by oxygen, O2, to smaller water and carbon dioxide molecules with the released energy captured in ATP molecules with some waste energy released as heat.

C6H12O6 + 6 O2 + 32 ADP >>>> 6 C02 + 6 H20 + 32 ATP + heat

So when we say that we "burn" calories it is just a phrase but it is a surprisingly accurate one. Burning something is also an oxidation reaction, which is why fire requires oxygen. If you literally burn glucose by setting it on fire then the reaction that is occuring is:

C6H12O6 + 6 O2 >>>> 6 C02 + 6 H20 + heat.

The only difference between this and what happens in your body is that in your body the reaction is carried out by enzymes that capture most of that energy in the form of bonds in ATP while in a fire that energy is entirely converted into heat.

So lets track the mass here. ADP >>> ATP is cyclical, the ADP and ATP stay in your body its just a way of storing energy so that isn't it. Water is kept in balance in your body, sure you excrete water in the form of urine, sweat, tears, etc but you also need to drink water to keep hydrated and keep your water levels the same so that isn't it. You breath in oxygen to carry out these oxidative reactions and you breath out carbon dioxide as a waste product. That isn't balanced, carbon dioxide is essentially oxygen plus carbon. So if you breath in 6 oxygen molecules and breath out 6 carbon dioxide molecules you have essentially lost 6 carbons from your body.

Now if all you do is burn the glucose you eat for energy then that carbon comes from the glucose you ate so you would not lose any weight (the C you breath out is balanced by the C you just ate). However if you take in less calories than you need to power your body you will break down glycogen (stored glucose) and/or fats (stored hydrocarbons) within your body by oxidizing them from breathed in oxygen and then breath out carbon dioxide. As a result, you lose weight. If, however, you eat more calorically than your body requires for energy your body will store the excess in the form of either glycogen (for glucose) or triglycerides (for fats) and since those molecules have weight you will gain weight. Although I used glucose as the example here the sample principle would hold true of fats as well. Proteins also contain nitrogen and waste nitrogen is excreted in the form of uric acid in our urine.

So what is the answer? When you lose weight the weight you are losing is being breathed out in the form of carbon in carbon dioxide. The amount of carbon dioxide you breath out is connected to how metabolically active you are which in turn is connected to how physically active you are being. If you go for a run you will breakdown molecules to create energy which will require more oxygen and expel more carbon dioxide which is why you have to breath harder. If, instead, you just sit on your couch and breath hard you are taking more breaths but you aren't actually producing more carbon dioxide all you are doing is expelling less carbon dioxide per exhale...that is just hyperventilating and won't do anything other than making you a bit light headed.