Despite what the diet book cartel wants people to believe, energy production and usage by the human body is not all that mysterious.
Delta in mass = inputs to the system - outputs from the system - energy expended by the system. If you desire negative deltas, decrease inputs, increase outputs, or increase energy expended. (Options #1 and #3 are probably better for most people, owing to the inability to control #2 in a safely scalable manner.)
The human body does not get a free pass from physics just because its details are complex.
Don't eat so much.
Problem is people won't believe you and it is unmarketable. So, companies go out and produce candy bars, market them as diet food by slapping a label on them that says, "eat this and not much other stuff and you'll lose weight."
Cracks me up.
Have you read: http://www.sethroberts.net/about/whatmakesfoodfattening.pdf
What about satiety of the food you eat? Eating fewer calories is easy if it's filling, less so if it isn't.
So, by analogy (and since we're talking about simple thermodynamics), eating more calories should make your metabolism run faster. More fuel, more fire, higher rate of fuel consumption. This is definitely true for those people who can eat lots of food and never get fat. Ectomorphs.
So, why doesn't this rule apply to everyone? Why do some people store what they eat as fat and others just burn it all off regardless of how little they exercise. Why aren't we all ectomophs? Genetics may come into play, sure, but all humans have the same genome.
The difference must be related to differences in our bodies. Which means differences in hormonal reflexes, differences in responses to certain types of food, etc.
It is about calories, but it's not _just_ about calories. It's about our bodies, too, and how we've conditioned them to process different kinds of foods.
Highly recommend "Good Calories, Bad Calories" by Gary Taubes.
And is this old metabolism debate still going on? Last time I checked most overweight people actually had higher metabolic rates.