If you'd like to read the most fascinating book on the science of metabolism, please pick up a copy of Why We Get Fat, by Gary Taubes. It has been both exciting and frustrating for me to read. Exciting, because finally, I'm learning there is viable scientific study to back up the claim that calories-in-calories-out is a myth. Yes, you read that right. It is a myth. As in, fiction. It doesn't work, it isn't true, it's false. Frustrating, because for my years as a heavy person (who was a child of heavy parents), there was no one who could teach me why I was fattening, and what I could do about it, other than starve myself.
There are many who will say (in fact, who do say, with religious vehemence) that Taubes' assertions are wrong, without bothering to learn the science. They will hold on to the calories-in-calories-out nonsense in the face of countless studies and evidence to the contrary, because they simply want to believe it. It seems right to them. It is usually (maybe always) the naturally lean people who claim that "America needs to lose weight." Nothing gets my dander up more. Thank you, person-whose-body-is-predisposed-to-burn-fat-molecules. You have never experienced the frustration of obesity, and yet you think you know the solution. You have never attempted to "eat less and move more" in attempt to rid yourself of excess fat, only to find that it doesn't work. Thank you for pointing out that fat people are fat. What are you, some kind of bully?
I can assure you, when you're heavy, you already know it better than anyone else. You're the one who sees your naked fat in the mirror each day before you hop in the shower. You're the one who struggles into clothes that either don't fit or make you feel frumpy. You look in the mirror and loathe who you've become. Your self-control over what you eat and how much you exercise seem to make no difference, especially as you see naturally lean people eat whatever they want and move as little as they like, and they still stay lean -- while at the same time, they extol their "virtues" to you and condemn your "sloth" and "gluttony". You feel lost and alone and shamed because you can't get lean. You don't need anyone to point out to you that you're fat. You already know, and the confusion and guilt are nearly crippling.
Enter the science. To put it in its simplest terms, insulin is responsible for how our bodies will use and store fuel. When your insulin levels are high, especially if they remain elevated, it sends the message to the rest of your cells to store fat molecules (this is fascinatingly and thoroughly explained in Taubes' book). Then, when (if) the insulin levels again decrease, your cells will release the fat and burn it for fuel. What you eat has a direct effect on how much insulin you make, and for how long. But even more enlightening, your genes and hormones (what you get from your parents, and this part is outside your control) determine how sensitive you are to that insulin. Calories-in-calories-out is a deadly "solution" to the obesity problem, because it doesn't address the real factors at work that determine fat usage or storage. The hormones and enzymes will essentially protect the fat in the body, and work to keep it, at the expense of the muscles and organs (this is why obese people tend to die of heart failure). The only way to halt this fatal problem is to learn how to effectively work with the hormones and enzymes in your body to send a better message to the cells.
Fill your brain with knowledge. And before you accuse a heavy person of not trying hard enough to lose weight, stop. Just stop. If you've never struggled with obesity, there's no way you can begin to understand how hard they're already trying. They're failing because they don't have the correct solution. And no amount of bullying from lean people is going to help them.