"It was fun for about an hour," the actor said of his diet for the film Stu.

Mark Wahlberg went all in on his transformation for his upcoming movie Stu.

Appearing virtually on NBC's The Tonight Show Starring Jimmy Fallon on Thursday, Wahlberg talked in-depth about his 20-pound weight gain for the true story of the boxer-turned-priest. When Fallon showed a side-by-side photograph showcasing Wahlberg's three-week transformation — which the actor had previously shared on Instagram a few months ago — he followed up by asking how it was possible.

"It's the mustache," Wahlberg joked, before admitting the truth. "Unfortunately, I had to consume, for two weeks, 7,000 calories, and then for another two weeks, 11,000 calories," he said. "It was fun for about an hour. And then it's such a hard, physical thing to do. Losing weight, you just kind of tough it out, you just don't eat, and exercise. And this, even when you're full, you'd wake up after a meal. I was eating every three hours."

Mark Wahlberg
Mark Whalberg on Fallon
| Credit: NBC

When Wahlberg visited Jimmy Kimmel Live back in April at the start of filming, he opened up about his regimen for gaining enough weight for the role.

"I'm challenging myself to put on 30 pounds in the next six weeks," he told Kimmel. "I actually have someone who's consulting, they want me to do it as healthy as possible."

But during his recent visit to Fallon, the actor — who just turned 50 — admitted that gaining and losing weight wasn't as easy now that he was older.

So what kept Wahlberg motivated? The drive for making (and producing) a film he was passionate about.

"Once the metabolism starts to slow down, it gets really difficult. I was trying to get this movie made for six years, we only had 30 days to shoot it, and so I wanted to really make it happen," he told Fallon. "And I was also crazy to have financed a lot of it myself. So I knew I was on a ticking clock on my own dime, and you get things done pretty quickly when that's the case."

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