Is it a person's "fault" if they're fat? What does that question really mean?

[eridani]

35. You may or may not lose weight if you eat less and exercise more

It is true that the First Law of Thermodynamics must apply to the human body. And, given the simplistic approach which most people 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 20-50 lbs/year of weight gain for people whose basal temperature is less than normal.
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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 also. According to the laws of thermodynamics it must be the case that --

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 (mitochondrial ATP uncoupling)
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 "free will" 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 any one of them changes. 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 energy deficit resulting from eating less and/or exercising more, 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 a fat individual is not lethargic and ravenous, then the control system is not unbalanced, but has a different equilibrium than average. One may wish that the equilibrium were different, but the system is not amenable to manual control (especially by manually varying C), and 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. Obesity researchers have verified this. And your typical snotty fat-basher will forever refuse to understand this.