Hugh Jackman (Lost 20, Gained 30)
Credit: Everett

It's hard to keep track of how often Jackman gets jacked, but he's played Wolverine six times already, so there's that. For Les Miserables, though, he needed to look the part of a prisoner in pre-Revolutionary France, so he lost about 20 pounds before filming – and then gained 30 during production as his character became stronger. At one point, he admitted, he stopped drinking liquid for 26 hours to hollow out his cheeks and eyes: "I do not recommended it."

The Bottom Line:
Sure, dehydration will make you look thinner, but it's extremely dangerous.