Matt Damon (Lost 40, Gained 30, Muscled Up)
Credit: Everett

Eighteen years after the fact, most people forget that Damon once pulled a McConaughey (or is it a Leto?) when he dropped 40 pounds to give a realistic look to the drug-addicted soldier he portrayed in Courage Under Fire. He's reportedly said the weight loss "was too much" and "I was a wreck.'' Years later, he famously chunked up for The Informant! "I just basically ate everything I could see for a few months," said Damon, who gained 30 pounds. He completed the body transformation hat trick when he bared bulked-up muscles in 2013's Elysium.

The Bottom Line:
Though guys like Damon may make it seem easy to shed weight, remember that after an actor completes a film, he can spend his days focusing on returning to his normal weight. "If that's what you do and that's all you do, you see amazing results," says Kushner. For most people, though, it's a hard, slow process.