Mark Wahlberg was initially excited about dropping his unbelievably strict regimen to pack on 20 pounds in 3 weeks for his upcoming film Stu, but he quickly changed his tune. He explained to Jimmy Fallon last night that he quickly found out it was not going to be a walk in the park.

"Unfortunately, I had to consume, for two weeks, 7,000 calories, and then for another two weeks, 11,000 calories," the star shared. "And it was fun for about an hour. 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, I would wake up after a meal and have another meal. I was eating every three hours. It was not fun."

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When Fallon asked the 50 year old if it's harder now at his age to drop and gain weight for his films, Mark Wahlberg answered, "Yeah, absolutely. 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. 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."

He posted his transformation process and result on Instagram.

The body transformation inspired tons of responses, including one from the star's wife, Rhea. "And it looks just as hot in person baby," she commented. Mario Lopez posted, "Yo. You wear mad chubby well homie!" Meanwhile, Baywatch alum Gena Lee Nolin wrote, "Goes to show us all - we are what we eat! The effort in moving and eating healthy!"

Earlier this year he couldn't have been happier about the fact telling Jimmy Kimmel on Jimmy Kimmel Live, "After we do the boxing scenes, I get to put on as much weight as possible over the course of the film, so I'm challenging myself to put on 30 pounds in the next six weeks," he revealed. "They want me to do it as healthy as possible. I'm like, 'Dude, I've been on such a regimen for so long, I just want to eat everything in sight.' I want to go to bakeries, I want to go to Denny's, I want to get pancakes. I want to get everything that I could possibly get my hands on."

When Jimmy Kimmel asked what Mark planned for his first meal of the gluttonous journey, the actor said it would include "maybe a 20-piece chicken nugget, hot wings from Kentucky Fried Chicken with a six-pack of beer. Let's start with a bang." He's now back on the road hitting the gym hard to regain his previous shape.

The film tells the story of Stuart Long, who got his start in boxing and moved to Los Angeles to start a career in acting when boxing didn't pan out. After a near-fatal motorcycle accident left him in the hospital, Long turned to religion and eventually joined Oregon's Mount Angel Seminary. The release date has yet to be announced.