DISGRACED gridiron great O.J. Simpson is so embarrassed about his bulging belly and flapping chin skin that he’s embarked on an extreme diet.

The 65-year-old convict, who’s serving time in a Nevada prison for armed robbery and kidnapping, made international headlines after a Las Vegas court appearance in May – for being FAT!

As The ENQUIRER reported, it was the fallen star’s first public outing in more than four years – and onlookers were stunned by how much weight he’d piled on. O.J. bragged about how women still love him even though he’s fat, but obviously, he isn’t thrilled at his tremendous weight gain.

NOW, an insider at the Lovelock Correctional Center says O.J. is working out like a madman and starving himself in an effort to get back in shape.

“When O.J. walked into that courtroom, there was an audible gasp from the gallery,” an eyewitness said. “The officers had him in shackles, but they needed an extra large chain to fit around his waist.”

When O.J. saw the mocking headlines the following day – with some calling him “Fat Albert” – he vowed to “show the world” he could get back into fighting shape.

“The next day O.J. tried to run on the track, but he had to stop and walk,” said the insider. “He’s just too fat. So now he puts on his headphones and walks around that track for a couple hours a day.”

O.J., whose weight reportedly topped out at around 300 pounds, also does pushups and crunches in his cell.

“His diet consists of more fruits and vegetables now,” noted the insider. “Before, when he was packing on the pounds, he was eating a lot of starch and foods high in carbohydrates.”

Just a month into it, O.J.’s regimen seems to be working.

Said the insider: “He’s dropped a few pounds and is already feeling better.”