Your body is what is referred to as an open system. “Open” meaning energy (calories are a measure of energy) can enter the system, and be used by the system (it takes energy for your body and your brain to function, and do things like think, walk, lift weights, take dumps, or to simply pump oxygen-rich blood around your body so you don’t die).
When a person is burning the equivalent amount of energy they are consuming in the form of calories over a period of time, they are at their basal metabolic rate. If they consume a higher number of calories, they are at a caloric surplus. If they are below the basal rate, they are at a caloric deficit.
Your body, through the wonders of evolution, has developed an ingenious system of storing surplus energy as body fat. Our prehistoric ancestors* had to adapt to survive periods of famine, and the human body evolved to consume excess body fat in times where the body was at a caloric deficit.
This is why, when operating at a caloric deficit, your body does not simply shut down and die. You are still getting the energy you need, you are simply making up the difference** by burning your stored body fat as energy. Operating at a caloric deficit of 3,500 calories over any given time will burn one pound of body fat as energy.
It is physically and biologically impossible to store body fat while at a caloric deficit.
*Modern Western man lives in a world of extreme comfort and abundance compared to our ancient ancestors. We are able to eat whatever, whenever we please, and our species has not had nearly enough time (What, a few hundred, maybe a thousand years? Probably far less if your ancestors weren’t aristocrats in the old world) to adapt to this change. Who knows. Never in the history of planet earth has a species seen such a rapid and dramatic shift in its food abundance, source, and chemical makeup.***
**When the body starts to run out of fat to burn in a caloric deficit (below about 10% body fat), shit starts to hit the fan.
I think it’s a matter of personal opinion, but most men (myself included) would be ecstatic to be maintaining a body fat percentage somewhere between about 10% and 15% (my last scan using one of those hand-held body fat calculators was 26% and I would guess at my lowest, I was around 21%). Again, it’s personal opinion, but going into the single digits sounds like a lot of suffering to look worse and be less healthy.
As far as women are concerned, I can’t speak for them, but on this chart, 25% seems to be the goal of most.
To be quite honest (and it’s not like this is some revolutionary and controversial approach), I am FAR less concerned with my weight than I am with my body fat percentage. Saying “I want to be 175 pounds” is idiotic. I honestly do not know what my weight will be when I reach, say, 12% body fat (and there’s a lot to consider, such as the fact that I’m likely going to lose some muscle mass as well, but will do my best to counteract this by making sure that my calories, while still in a deficit, will primarily come from protein sources).
***In case you’ve been living under a rock, the whole Paleo thing has been picking up a ton of steam over the past five years. From what I gather, the whole thing was adopted by Crossfitters (not that there’s anything wrong with Crossfit) and is now hot shit among 50-year old housewives (they’re the ones buying those magazine/recipe book things by the checkout at the grocery store, right?). All in all, it’s a solid idea and the general idea of what I’ve been eating (aside from the brown rice and the Quest bars, depending on who you ask).