ok, so what exactly is this "metabolism"?
my understanding of how most people define it:
One's efficiency to convert food into energy for use.
so poor efficiency=fast metabolism because people broadly said to have a "fast metabolism" are thought to be able to eat a lot without doing much exercise and not getting fat. (really, really really hard to prove, but whatever, we'll let it slide)
then presumably high efficiency=slow metabolism because we all know fat people say they eat the same amount as skinny people (even though we know they don't and we know they don't evercise). Again, whatever, we'll let it slide.
Now this all seems a bit counter-intuitive. Don't athletes train for high efficiency? why yes they do. They want to be able to utilise the most energy from their food as possible.
so then, presumably an athlete who stops training will then get fatter than a non-athlete on the same diet?
why does this not make sense? Because the increase in an athletes metabolism is due to their increased ability to supply oxygen to their muscles (aerobic metabolism is much more efficient than anaerobic) - therefore, their enhanced efficiency is due to greater cardiovascular efficiency.
So then, it's good to know that running won't make you fat when you don't run. Phew
cool. Now, the other idea is that going on a caloric deficit is going to slow down your metabolism i.e. make it more efficient, therefore when you return to eating the same diet, you'll put on more weight then before.
this doesn't seem that bad an idea. It would make sense that eating less would force your metabolism to be more efficient. But how, exactly would this happen? And then, if it can change to being more efficient, can't it change back to being less efficient again? and then, how else can you lose weight? you can't.
Personally i don't think that being on a caloric deficit increases your metabolic efficiency. Perhaps it increases your absorption of nutrients from your diet. perhaps it decreases your fidgetiness. perhaps it slows your brain activity. perhaps it increases your desire for fatty and high calory foods - but then all these factors confound this idea of "metabolism" being responsible.