The way I explain it to fats is like driving a car on level ground, where speed is equivalent to body mass. Sure, there may be tons of variation in things that slow you down. Friction and air resistance are like metabolism and guhnetix, brakes are like exercise. You control your brakes, you can't control the air or friction. But, you see, your speed isn't determined by those things. It's determined, 100%, by how much throttle you use. And that is determined, 100%, by the driver; like food, you control what fuel enters the tank. No one ever got off of a speeding ticket by saying their brakes failed, because the lack of brakes does not cause positive acceleration. Similarly (remember, we're on flat ground), if you let off the gas, I guarantee you, you will shed speed.