American actor and director Rob McElhenney has unveiled a seriously jaw-dropping body transformation — and teasingly insisted that his journey from beer gut to six pack was "not that hard".

The Always Sunny In Philadelphia star, 41, posted a startling before-and-after photo on Instagram over the weekend, showing the results of a strict diet and exercise regimen.

The new look was required for his role as Mac in the long-running comedy series — and in an accompanying caption, McElenney explained exactly how he did it.

"Look, it's not that hard," he wrote.

Advertisement

"All you need to do is lift weights six days a week, stop drinking alcohol, don't eat anything after 7pm, don't eat any carbs or sugar at all, in fact just don't eat anything you like, get the personal trainer from Magic Mike, sleep nine hours a night, run three miles a day, and have a studio pay for the whole thing over a six to seven month span. I don't know why everyone's not doing this. It's a super realistic lifestyle and an appropriate body image to compare oneself to."

You have to admire his honesty.

It's not the first time McElhenney has drastically altered his body — the "before" shot he posted came from a period when he gained more than 25kgs in the pursuit of a gag on Always Sunny about his character letting himself go. Similarly, his new action hero bod will be used in the new 13th season of the show for comedic effect.

"It feels like the last eight years, every time a man takes his shirt off in a scene, he tends to be, like, insanely ripped, like a Greek God," McElhenney explained in a recent interview.

"Unless you're Chris Hemsworth and you're playing Thor or Superman, in that case it makes sense. I don't understand why guys that are playing teachers and engineers and, like, food delivery guys, all of a sudden they take their shirt off they're just f***ing ripped.

"I thought, man, like, there's something really funny about playing with the vanity of that... and also, I just thought it would be funny if an actor put that much time, effort, and energy into looking a certain way.

"And, then, in the first episode (of season 13), I take my shirt off. None of the characters are impressed. They tell me to put it back on, and I literally never take it off again. To me it's funny that somebody would put in six months' worth of hardcore working out and diet and then only get, like, 30 seconds of glory out of it.