Pissing it all away

What is the actual chemical pathway of weight-loss? I tried to think this through logically and managed to get about half the story. Talking with my doctor revealed the second half.

To explain my thoughts

1. I was fairly sure that the actual weight of the food contributed very little to my long term weight. For the most part, food simply traveled through the system and any useful energy or chemicals were pulled out by various enzymes and bacteria. Basically the weight of input food was fairly close to the weight out output feces including the various enzymes excreted to digest the food. The only inputs that mattered would be those things my body actually absorbed. Fats, sugars, proteins, and minerals would be taken in while fibers and the like would simply keep on going.

2. I also figured other excretions such as sweat would be offset by input water.

3. The only input that was easily show to be less than the output was breathing. The body disproportionately absorbs oxygen compared to what it exhales, the converse being true of CO2. So I would take in O2 and exhale CO2. Losing a carbon molecule many, many times over. Silly me though, I didn't seem to make the connection that this was of course part of cellular respiration, which has water as a product also. Combined, these two products would make up most of the weight-loss caused by proper diet and exercise.

Though this reminds me of a revelation which shook up my thoughts on the body when it came. The idea that the digestive system really didn't absorb your food and the excrete the waste back into the system for elimination. Of course, my thoughts on digestion were hardly systematic before and many things fell into place after such an understanding. The moment of "Eureka!" really sticks with me though. This is about the closest I get to a religious experience, the moment of clarity when you realize a fact about the world and suddenly so many separate facts which seem to contradict each other spin around and fit together. This is why I say science is more than simply practical knowledge. Science has enriched my life even if I never find a useful thing to do with any particular piece of knowledge.