What a difference a month makes!

The Jersey Shore's Ronnie Ortiz-Magro posted a "before" and "after" shot of his body on Instagram earlier this week, admitting that he had "really let himself go".

But it wasn't laziness or too much pizza that led the reality star to lose the six-pack he is famous for – he fell ill last year and piled on the pounds when he was unable to exercise.

Now, just four weeks after vowing to get back in shape, the 28-year-old showed off his washboard stomach on the social media site.

ronnie and sammi

"I really let myself go after I got sick last year,' he posted. "It's been a long road. I am nowhere I want to be shape wise, but I feel better than I ever did. #DietingIsKEY #CardioSavedMyGoodYearTire #TransformationThursday #letTheAbuseBegin #WowYouGotFatAsFk #AllThatDrinkingAtTheShore #DoIneedToLookLikeICarryLugguageForALiving."

Ronnie received hospital treatment for kidney stones in April 2013, tweeting at the time, "Wouldn't wish this feeling on anyone! Kidneys. FML."

He credits his buff bod to "weight, cardio and abs 5-6 times a week," and stressing, "Dieting is such a big part! If it tastes good don't eat it lol."

ronnie hot body

The former reality star rose to fame with the rest of his housemates on MTV's Jersey Shore, where fans were glued to his tumultuous, on-again-off-again relationship with Sammi "Sweetheart" Giancola.

The couple have found more success since the camera's stopped rolling and now live together, with Sammi saying earlier this year, "Our relationship is stronger. We know everything, literally, about each other. There’s no surprises anymore."