Americans know John Goodman best as Dan Conner, the lovable husband and father from Roseanne. Throughout Goodman's long career, which stretches back more than 30 years, his big frame and even bigger personality have helped make him famous.

But in 2013 the star decided it was time for a change. Over the years he had tried fad diets and had some success, but never for very long. At his heaviest in 2010, the actor says he was nearly 400 pounds.

With the help of Serena Williams' trainer Mackie Shilstone, the actor has managed to pull off a dramatic transformation, looking like half the man he used to be.

But there's a painful reason for the shocking change.

While there's no telling how much he lost exactly (Goodman doesn't use a scale) his trainer says it's over 100 pounds at least.

Goodman has struggled with drug and alcohol addiction in the past, and says he found changing his diet to be just as hard as giving up drugs.

"I was just eating all the time. I was just eating alcoholically," he told ABC News.

"In the old days, I would take three months out, lose 60 or 70 pounds, and then reward myself with a 6-pack of bud or whatever and just go back to my old habits."

After struggling to lose weight for decades, something changed in Goodman's life a few years ago that has made him stick to a healthy lifestyle ever since.

Click the next page to find out why Goodman lost so much weight!

It turns out the only thing that could motivate Goodman to change his life for the better was his own sense of disappointment.

"I just got tired, sick and tired of looking at myself," he said, "you're shaving in the mirror and you don't want to look at yourself. It gets dangerous."

Since then he's started exercising 6 days a week, making sure he gets 10,000 steps a day, and sticking to a Mediterranean diet of veggies and healthy protein.

Goodman also says that he's been limiting his sugar intake.

The star jokes that his life hasn't been totally perfect since losing the weight - he had to replace his whole wardrobe, after all - but he's feeling much better about himself.

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