The Oscar-winning star - whose weight has fluctuated dramatically during his movie career - has revealed he no longer wants to torture his body by adding and losing substantial amounts of weight for film roles.
He told CBS: "I keep saying I'm done with it. I really think I'm done with it, yeah!"
Christian, 45, stars alongside Matt Damon in his newest movie, 'Ford v Ferrari', which tells the story of a racing team seeking to win the 1966 24 Hours of Le Mans race in France.
And Matt relished the experience of working with the Hollywood actor, hailing his co-star's "monk-like discipline".
He shared: "I had a great time watching him. He's got [an] incredible monk-like discipline, like, when he went from Dick Cheney to this guy. So, he had to lose 70 pounds."
Christian previously piled on the pounds to play former US Vice President Dick Cheney in 'Vice'.
And he subsequently admitted to hating the experience of adding the extra weight, saying he needed to "start being a little bit aware of my mortality".
Christian - who also lost a dramatic amount of weight for movies like 'The Fighter' - explained: "It's miserable. It's horrible. If you're losing weight, there's nothing to it except for going to bed hungry and feeling miserable ... [Gaining weight] is the complete opposite.
"You're absolutely just losing all discipline and gorging like crazy. I need to stop it, it's not healthy for your body. I'm in my mid-40s now, it's going to start catching up with me if I don't start being a little bit aware of my mortality."