Rob Kardashian Jokes About Weight Gain -- What's He Blame for Packing on the Pounds?

The reality TV star took to Instagram on Thursday to share a flashback photo from 2011, showing him celebrating sister Khloe Kardashian's birthday at Sugar Factory in Las Vegas.

"Started right here with these milkshakes hahaha#throwbackthursday," he captioned the shot, which shows him sucking on a straw with three milkshake glasses in front of him.

The joke comes after he and his family have spoken out about his new fitness regimen.

The reality TV star has kept a very low profile since his dramatic weight gain in 2013, when he admitted to putting on over 40 pounds after his split from Rita Ora.

Just last month, Kim opened up about her brother to Rolling Stone magazine, saying "It's not that mysterious, what's happening with Rob. He has gained weight. He feels uncomfortable being on the show, and that's OK. Do I think he smokes weed, drinks beer, hangs out and plays video games with his friends all day long? Yes."

Since then though, he's been on a mission to drop the lbs.

Earlier this week, Kim tweeted about working out with her bro:

She posted a similar message earlier this month as well:

Rob himself took to his social media pages after that August 14 tweet, responding to Kim via his Instagram page by saying "GOOD work gurrrrrrllllll‼️"

He also posted a photo while exercising on Monday -- and while he didn't show us his progress, he did reveal another family member who's all about her fitness.

"Spotted these fools on the morning workout," he captioned a photo of Kylie Jenner and Tyga.

In an interview with Complex magazine, Khloe Kardashian also gave us an update on her brother, who currently lives with her.

"I love having him with me. I have a big ass house and I wanted it so he could feel like, comfortable. He's going through a hard time now and it's behind gates," she said. "With this crazy lifestyle you have to think of places where you can still have your freedom."