When we shed pounds, we trigger biological mechanisms that make it hard to keep the weight off. Understanding these systems might help you be kinder to your body.Here's what to remember:- Metabolism slows when you lose weight.- If you choose to try to lose weight, make changes that you can live with for the long haul.- Hormones in your brain conspire to make you hungrier when you lose weight.- To lose weight, what you eat is more important than how much you exercise.- But exercise seems to play a big role in maintaining a lower weight.