Supersize to Superskinny: How do Hollywood actors starve themselves for parts and then "ping" back into shape?

There's something about Jared Leto that I just can't let go of.  He was so breathtakingly beautiful in My So Called Life that he captured my attention forever.  And even though he might be a bit of an eejit now, I still remain slightly fascinated by his doings.

We've made fun of Jared Leto before ever since GQ aptly dubbed him a High Fashion Fool and the worst dressed man in the world, but there's no doubt that he's willing to bloat and/or starve that fantastic body when an acting part calls for it.  In The Dallas Buyers Club, his latest film, himself and Matthew McConaughey have lost incredible amounts of weight.

And Hollywood of course has a long tradition of actors either stuffing their faces for a part, or starving themselves right down to zero.

I've been putting on and losing THE SAME seven pounds for ten years and I can't get happy with my weight.  How the hell do they do it in Hollywood - simply go up to 300lbs and then look red carpet hot mere months later?  And how do all these celebrity moms "ping back into shape" (oh loathsome term) so quickly? Read: how much pressure is there on new mothers to instantly ping back into shape

I'm telling you - they've got something we haven't.  And although I hate to say it, I think it's called willpower.  And possibly personal trainers.  And massive egos.

Oh damn.  How do you think you'd get on with being Renee Zellwegger playing Brigid Jones?  I would put on the weight and it would NEVER come back off!

How would you do?