John Goodman Debuts Dramatic Weight Loss at London Premiere

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John Goodman was looking good when he hit the red carpet on Thursday night.

The 63-year-old star appeared significantly slimmer as he posed with his Trumbo co-stars -- including Helen Mirren and Bryan Cranston -- at the BFI London Film Festival.

WATCH: Liam Neeson's Shocking Weight Loss Leaves Him Nearly Unrecognizable in NYC

Goodman looked half his size in his sharp, pinstripe navy suit that he paired with a polka-dot tie.

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The acclaimed actor's weight has fluctuated over the years. In 2010, Goodman dropped 100 pounds and admitted on the Late Show With David Letterman that he once ballooned up to 400 pounds.

WATCH: Shonda Rhimes Reveals She Lost 98 Pounds This Year

"I'd get off of Roseanne every spring. I'd lose 60 pounds every spring," he said at the time. "Then I got too fond of the barleycorn. I'd gain it back and then some, every year. It's going to be an ongoing process for the rest of my life."

Matthew McConaughey's also had a major body transformation for his upcoming role in Gold. The Doctors' Dr. Travis Stork gave ET the rundown on the potential risks that come with celebrity yo-yo diets.

WATCH: Matthew McConaughey Is Bald, Pudgy and Nearly Unrecognizable on 'Gold' Set

"In general with that rapid weight loss, you can get malnutrition -- certainly if you're not getting enough protein,"  Dr. Stork said. "[It can] even put you at risk for gallstones, which certainly anyone who's had them knows that's no fun."

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