(09-05-2016, 11:56 PM)chill Wrote: I did mean to say that applying it to weight loss is highly misleading. It does not apply in the simplistic view that eating less and exercising more will result in weight loss.
First of all, I think you have the wrong law. The second law of thermodynamics is that entropy always increases in a closed system. Which has nothing to do with weight loss or gain.
The First law is probably what you were thinking about, namely that mass/energy is neither created nor destroyed in a closed system (is in other words conserved). Originally this was applied only to energy, but Einstein corrected it by pointing out that mass and energy are really the same thing in different forms.
Secondly I like to think of things more concretely. If you hold a pound of weight in your hand and step on a scale you are thereby one pound heavier. If you eat a pound of food, or drink a pound of liquid you are also immediately a pound heavier. Of course the body also excretes mass in the forms of feces, urine, sweat, radiated energy, and exhaled air. The radiated energy, as implied by Einstein's e=mc^2 is very low in mass and can be ignored for all practical purposes.
If over time the body excretes more mass than it takes in you lose weight, period. If you eat more than your body excretes then you gain weight, period. Your body takes in mass in the form of molecules and atoms and assembles them into other molecules that allow you to live. These molecules eventually break down and are excreted. Some are not digested and leave quickly, some hang around for longer but eventually they are all excreted and unless you replenish them with other molecules and atoms you will starve.
So we may do two things to lose weight. The first is to take in less mass than is already being excreted. The second is to excrete the mass you take in faster.
BUT, the two interact because the body responds to challenges by adapting. So if you take in less mass it is likely become more efficient in storing it in the form of fat, and so excretion slows so that the mass you do take in hangs around longer. It is thus a dynamic system, not a linear one.
None of this means that the "laws of thermodynamics" don't apply.