Rob Kardashian is on a mission to lose weight, after putting on approximately 45 pounds.

He gained the weight after his relationship with singer Rita Ora went south.

Back in December, Kardashian posted on his Twitter account: “She cheated on me with nearly 20 dudes while we were together, I wonder how many she will sleep with now that we apart [sic]? But I mean 20?!!!”

Ora never responded to the allegation but told The Sun tabloid earlier this month, “You should have to sit an exam before you go on Twitter. To see if you’re mentally stable.

"I never actually thought it was a relationship, in all honesty. I never mentally defined it as boyfriend/girlfriend. When I split up with him I said, ‘It’s because I’m never there, I don’t know how to do it.’ That’s all I said … then the rest happened,” she continued.

At his birthday party Friday in Las Vegas, Kardashian told Us Weekly that he is slowly losing the weight. “I lost five to seven pounds last week. I’m trying to lose 40. It’s a little journey but it’s good.”

Kardashian has been working out with rapper The Game, and has frequently been photographed by the paparazzi on hikes.

“We’ve started running and doing our own little workout, it’s good” he said. “I’ll exercise with my own trainer at 8 in the morning before all that.”

“I’m like 240lbs right now, probably 235lbs.”

Look at Rob Kardashian's weight transformation over the past few years:

Rob Kardashian
Rob Kardashian before weight gain Tumblr
Rob Kardashian
Rob Kardashian and sister Kourtney Tumblr
Rob Kardashian
Rob Kardashian Tumblr
Rob Kardashian at his birthday bash March 15 Tumblr