Relative size of Components of net weight gain and loss

(Weight/mass used interchangeably; assume gravity is constant)

We obviously gain weight by eating, but we also have to gain a little mass when our lungs absorb oxygen. Similarly, we lose some mass by excretion and exhaling, as well as by perspiration and simple disconnection (dandruff, amputation). Is there anything known about the relative size of these effects as they pertain to whether one gains or loses weight? The best I can find is that there are some media reports on how a study found that 80% of fat is lost through breathing out. Well, of course, because generally carbon leaves your body via breathing, while the hydrogen stays around in some other form, or excreted as part of water or whatever.

There are a number of things I’m interested in here.

What is the (average daily I guess?) net weight gain or loss from inhaling oxygen and breathing out carbon dioxide? I’m assuming it’s a loss as we merely use oxygen in “burning” the carbon sources, and breathe out as many carbon dioxide molecules as oxygen molecules we breathe in (as each has 2 atoms of oxygen), and the former clearly have more mass.

What is the *total *(avg dly) net weight change from inhaling and exhaling, including things other than oxygen in and carbon dioxide out? I believe that the most important thing here is the net exhalation of water, which should occur at any time the humidity is below 100%.

How much of your net weight change is from dead skin falling off, assuming you don’t have ridiculous dandruff?

What is the percent net change in mass between items ingested and excreted? How much of the mass are we actually taking from food and later exhaling or perspiring?

How much water is created as a product of cellular respiration as a percentage of the water that we ingest in any form?

I assume that the body regulates water such that we tend to have exactly as much as we need, and that the proportion of our body that is water is relatively constant, but is there is tendency to have that proportion go up or down slightly as we add on fat? What about muscle?

I’m sure there are other interesting things related to this too that I haven’t been able to formulate, so any numerical information related to this topic would be appreciated.

Wow, that’s a lot of questions! I hope you don’t mind my numbering of them…
Now, it’s been quite a while since I took biochemistry 101, but I think I can answer some of them…

(1) ISTR that, on average, you burn about the equivalent of 200 g of glucose (C6H12O6) per day. This will turn into CO2 and H2O (excreted) and lots of heat/energy. So, assuming no calorie intake, you would lose about 200 g /day.

(2) Assuming you don’t inhale really bad stuff, like asbestos or polychlorinated carbons, the net gain/loss should equal to zero. Your water content is, for physiological reasons, pretty much constant, and should by no means be tried to mess with.

(3) No clue; make a guess!

(4) Zero; assuming you don’t lose/gain weight.

(5) Give me some time, and I’ll make out the equation…

(6) I believe there is; but I’m not an expert in this field…

On point (5) we can assume the equation C6H12O6 + 6O2 -> 6 CO2 + 6H2O.

(Actually, the empirical formula for your daily need is expressed by “222”: 200 g of carbohydrates; 20 g of proteins and 2 g of fat. But we can simplify a lot: let’s make it all carbohydrates - it doesn’t make that big a change!)

So, from that equation we get that you use up the same amount of oxygen from the air as you get from the food. So the amounts of water produced by “food alone” and by “combustion of food” are roughly equal.

Please state your definition of “need”. This adds up to 900 kilocalories, which is probably enough to keep you from dying, but most people’s ambitions for their daily activities go beyond that.

This Youtube video has the equation for burning body fat, which I think is more relevant than glucose, as we have limited glucose stores but almost infinite body fat.

Obviously glucose is exhaled. Water, the other product from burning carbohydrates and fats, can leave the body by exhaling, sweating or urinating. As with most things in life, the ratios depend.

As for excreting the solid stuff, that never got absorbed into your body anyway, so I wouldn’t count it, just like the nitrogen you inhale and then exhale the next breath.

The bottom line: when you gain weight, it’s through your stomach. When you lose weight, it’s pretty much entirely through exhaling. So if you want to lose weight, do stuff that makes you breathe faster.

Well, it’s from a textbook in biochemistry, and the figure is probably on the low side. Assume you do nothing but lying in your bed and rolling your thumbs.

Oops, my mistake. You actually get twice the amount of oxygen from the air. It’s O2! So, double the weight in water.