Engineer's approximation of the human body
Ok, so now we need to know what is represented by each stream. First, the easy one: Mass in. As far as I know, the only significant source of mass entering the body is food/drink from eating. Inhaling air is another source, but considering that inhaling is (nearly) always followed by exhaling approximately the same amount of mass, I'll ignore it for now.
Now let's look at the other side of the equation: Mass out. Here are all of the ways that I can think of that we lose mass from our bodies:
Urinating, defecating, flatulence, sweating, vomiting, bleeding, loss of dead skin, hair loss/cuts, nail loss/cuts, crying, spitting, boogers/snot coming out of nose, sexual fluids
That, as far as I can tell, is it, unless you count losing limbs/teeth, which, quite frankly, I think is cheating if your goal is to lose weight. Of that list, I would say that the only significant sources are urinating, defecating, and sweating. I doubt hair/dead skin loss is significant, but I could be wrong about that, and if you're losing a significant amount of weight with any of the other methods, I suggest you consult a doctor about that.
Ok, so now that we know this, a question immediately comes to my mind: Do people poop and/or pee more when they are losing weight than when not? Or is it all lost by sweating more/eating less?
To answer this, I did some light research into the other side of weight loss: energy. In other words, what happens to food/drink after we consume it? Well, obviously some part of what we eat is used to make new cells, but that would happen whether we were losing weight or not. So the key here is how the energy from that food is used.
We're all aware that food has a certain number of calories, or energy, that is associated with it. When we eat it, our body breaks down the food to release that energy to use it for whatever it is that the body needs to do. Any energy that is not used is stored as fat, and that's generally accepted as the method in which we gain weight. So to lose weight, we need to use the energy stored in those fat cells (in addition to the food we're continuing to eat). So for the purposes of our discussion, we need to know what the byproducts are of breaking down fat. And it turns out that the byproducts are two simple things: water and carbon dioxide.
The excess water from breaking down the fat can be lost in two ways, as I see it: sweating and urinating. The carbon dioxide, I assume, is all breathed out. Pooping is seemingly unaffected.
The second part came as somewhat of a revelation to me: WE LOSE WEIGHT WITH EVERY BREATH WE TAKE. Carbon dioxide is heavier than oxygen, and breathing is exchanging those two gases. That said, I did a little more research and some quick calculations and found that, assuming normal breathing the entire time, it takes about 3.3 days to lose 1 pound by breathing alone. Which really isn't bad, considering.
So in conclusion, my grade-school level analysis suggests that the majority of weight loss is from sweating and urinating, with some loss from breathing more due to exercise. The best bet to lose weight is to eat less food (which means less mass in), and expend more energy, which will cause you to sweat more and breath more and possibly pee more (which means more mass out). In other words, diet and exercise.
What a waste of time.
