Intake, burning, and excretion determine, in large part, how you look, how you feel, and how many years you'll live. They do this because, through simple arithmetic, they control the contents of the rubber bag. Living, as we do, inside the bag, it's worth understanding how we're affected by these processes, then using that understanding to gain control of them.
Assume you eat just enough every day to meet the needs of your body. What goes in is broken down into the molecules to power your body and the result precisely equals what you burn. The residue, what comes out, is discarded to make room for the next day's food.
This is the condition of stable weight. The entire purpose of this book is to allow you to attain this state. Regrettably, many of us have spent most of our lives oscillating between the following two situations.