One: What is metabolism?

It refers to the amount of energy (calories) your body burns up in a day. Sixty to 70 percent of your daily caloric expenditure goes into your resting metabolic rate (RMR), the energy required to keep your body functioning. People who eat like a horse and never gain weight—said to have fast metabolisms—are born with high RMRs, meaning it takes a lot of food for their particular bodies just to stay alive.