I'm lucky I have a fast metabolism... my whole family does... everyone's got a lot of nervous energy so we burn it off.
I have an incredibly high metabolism.
Neurons are living cells with a metabolism. And they need glucose in order to function. Glucose is the fuel of the brain, just like gasoline is the fuel of your car.
I have a pretty fast metabolism, so I don't really have to worry about getting pudgy. What I try to do when I'm working out is taking in a lot of heavy protein - clean chicken, clean meats.
There is a moment when you get older when your metabolism slows down and you don't feel like working out any more, so you don't want to keep yourself fit any more, but that's your decision. Why should you be judged for it?