1) Human bodies are subject to the laws of mass and energy and the interconversion of the two, i.e. mass cannot be created without energy, and energy cannot be created without mass. E=mc2
2) The conversion of food to energy or to body fat is carried out by the body's biochemical processes. Like all chemical systems, there are varying degrees of efficiency. Some people's bodies process food (i.e. extract energy from food) more efficiently than others. If persons A & B each eat an identical bowl of ice cream, their actual net calorie gain may differ depending on their individual digestive efficiencies. This variance can account for SOME of the anecdotes that "Joe Blow can eat a ton of ice cream and never get fat"
4) However, in the absence of food all humans will starve and display an emaciated appearance. Look at pictures of concentration camp survivors or POW's from the Japanese camps in WW2. Going into the camps, you had a wide variety of peole, of all sizes, girths and metabolic efficiencies. Coming out, they were all emaciated. This proves that EVERYONE will lose weight in the face of a caloric deficiency.
5) Caloric deficiency (and the INEVITABLE weight loss that occurs, see #4 above) can be induced by:
a) restricting caloric intake (eating fewer calories)
b)increasing caloric output via exercise
c) blocking caloric processing by the body (drugs like Xenical block fat absorption in the intestines)
If you accept the above tenets (and you must, because they are all based on the laws of thermodynamics), you can arrive at the basic conclusion that body weight is a function of calorie intake times digestive efficiency, minus calorie consumption.
With that said, if you restrict calorie consumption and increase energy output, to the point were output exceeds input, you WILL lose weight.
How much weight you lose will depend on the magnitude of the deficit and will probably be a non-linear curve, i.e. you will lose weight at a different rate along the curve. Because of the variable of digestive efficiency for each person, your calorie input/output levels may need to be tweaked to lose weight, but you WILL lose it. You may have to eat less or exercise more to achieve the deficit.
The moral of the story is that no one is immune from the laws of thermodynamics. Some people will have to work harder to lose weight, but no one can say "I can't lose weight because of my metabolism/genetics." If that were true, you'd see pictures of fat people coming off the Bataan death march or coming out of Auschwitz. Be sure and post those pics if you have them.
Error in your death-camp comparison. Many people died long before reaching the emaciated look you so cherish. They are NOT in the liberation pictures.
That is true. Ancel Keyes did studies fifty years ago, right after World War II on starvation. The US gov't paid him and a team of researchers because they wanted to figure out how to deal with all those starving in Europe at the end of WW II.
Basically what Keyes found out is this. A starving person will eat like crazy when they finally have access to food. They will eat to the point where they regain their previous weight - and some people will end up slightly heavier than they were before the starvation.
Keyes himself hated overweight people - but his science has been proven time and again. Basically, a fat person who has been starved is not a normal, genetically thin person. He is a fat person who is in the process of starvation.
Living a life of permanent starvation is not an option for most people. Thus for most people, diets (which are in essence controlled starvation) do not work over the long run. It's been shown time and again that after five years, over 95% of dieters have regained the weight they lost - and many have regained more than what they've lost.