Boston, Massachusetts - Mark Wahlberg seems to have gone from muscle man to beer belly in a matter of only about three weeks. Now, he has taken to Instagram to show off his 20 pounds of flabby gains.

Mark Wahlberg posted two before-and-after style photos on Instagram this week, both featuring his bare-chest, and both boasting a key difference.

In the first, he's every part a 1970s boxer, his teeth bared and his handlebar mustache fluttering in the breeze. The most noticeable part of the pic, though, are his impressively flexed muscles.

The second picture, though, look very different. Taken just three weeks later, it showed the 49-year-old looking considerably... softer and flabbier.

In the post's caption, the actor thanked Lawrence Duran for his cooking skills. Wahlberg apparently owes his rapid weight gain to his private chef. It turns out the changes are very much intentional, too.

Just as he would tone up for an action flick, Wahlberg has deliberately packed on the pounds for his feature in a new film called Stu. In the movie he will play a boxer who later becomes a priest and dies of a rare disease.

Wahlberg told Jimmy Kimmel in April that he has just six weeks to put on more than a few pounds. He's up a full 20 according to his post, but he has to add that much again – and quickly.

Wahlberg is familiar with such blatant weight changes for his roles. He gained more than 45 pounds of muscle mass for his 2013 role in Pain & Gain.

Exactly one year later, he had to shed another 60 pounds to portray scrawny literature professor Jim Bennett in The Gambler.

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In order to pull off this dramatic feat, he turned to a liquid diet for six weeks. Wahlberg has access to professional dietitians, though, so can undergo these insane body transformations safely.