Dieting results in long term changes to hormones and muscle fibers.

[eridani]

230. The human body is constrained by physics. This does not mean that physics explains

--what is going on in the human body. It does not.

The human body is not a simple single system where the first law of thermodynamics can be observed in a Physics 101 manner:

calories burned - calories consumed = z
x - y = z

where z is some final state:
-z (weight loss)
+z (weight gain)
0 (homeostatis)

If you pass Physics 101, and move on to biophysics, biochemistry, and molecular physics you might begin to understand that when you add multiple recursive factors (as in human bodies) to an equation, as in:

(P * (W(P * W(x-y))) = z
where P is the endocrine system
where W is some other influencing system in the body, calculating z is now much more complicated. x-y=z is still the truth. However, the ultimate values (in Kcals.) of x and y are dependent upon the influence of P and W, which may vary because of varying influences upon them. There is no "First Law of Human Endocrine Systems."

It is true that the First Law of Thermodynamics must apply to the human body. And, given the simplistic approach which most people seem to take, they are confused about what 'consumed' and 'burn off' mean.

Mitochondrial ATP uncoupling for heat generation has been indentified as one of the major energy
utilization systems of the body and could account for 20-50lb/year of weight gain some or people. And there are active control systems which reduce the amount of energy used involuntarily for many of the body's autonomic functions. There are also significant energy excretion systems which are active in most bodies. a rough estimate is therefore--

C - N - S - I - H - E - V = 0

C = calories eaten
N = non-absorbed calories excreted in bowels
S = calories stored
I = calories calories used involuntarily (muscle maintenance, involuntary motion)
H = calories used for heat generation
V = calories used voluntarily (exercise, for example)
E = calories excreted in urine (Examples: fat converted to glucose in the liver and excreted in the urine, incompletely burned triglycerides which are excreted in the urine, and albumin excreted in the urine)

It should be noted that there is 'manual control' only on C and V. People who think of human metabolism as a bank account are willfully ignorant that these other variables adjust automatically within an active control system. All adjust when some of them change. When C and V are changed 'manually', there may be permanent alteration to the control system (as in long-term dieting).

The amount of energy stored is not 'whatever is left over'. The body actively stores or mobilizes energy from its energy store. If there is a resulting energy deficit, it tries to increase C, causes a reduction in I, H, and E, and even actively prevents V. If there is an energy surplus, it tries to decrease C, increases I and H, encourages V, and, as a last resort, increases E.

The control system for these actions is decentralized. So,it is possible for the energy store to believe that it needs to increase S, while simultaneously, the liver believes that it is necessary to increase E. This leaves I, H, and V at an extreme disadvantage.

If the individual is not lethargic and ravenous, then the control system is not unbalanced, but has a different equilibrium than the average. One may wish that the equilibrium were different, but the system is not amenable to manual control (especially by manually varying C), but there are strict limits to an individual's ability to change it.

Decreasing C (dieting) has been shown to cause a long-term decrease in H and a long term increase in S, and to prevent I from increasing when V is increased. Millions of dieters have experienced this. If you aren't capable of understanding it, you need to stay out of discussions that require it.