Rob Kardashian jokingly blamed milkshakes for his weight gain Thursday on Instagram. The 28-year-old reality star shared a throwback photo of himself slurping one of the sweet drinks at the Sugar Factory in Las Vegas in 2011. Kardashian has since gained 100 pounds, as revealed by mom Kris Jenner on "Keeping Up with the Kardashians" in April. "Started right here with these milkshakes hahaha #throwbackthursday," the star captioned the picture. Kardashian revealed he is back on an exercise regime this week after running into half-sister Kylie Jenner and her beau Tyga during his workout. "For the first time in a long time Rob is committed to changing his diet habits," a source told E! News. "He's been working with a chef and doing really well. His regimen involves more than just cardio and weights." Kardashian is reportedly seeing personal trainer Gunnar Peterson. The reality star has shunned the spotlight in recent years, and rarely shares updates on social media. Sister Khloe Kardashian admitted her brother is "not at his happiest place" in March, and confirmed in July he is still living with her. Kardashian came to fame on E! series "Keeping Up with the Kardashians," which will return from midseason break in the fall. The reality star recently went on a hike with sister Kim Kardashian, who won big in a settlement over a leaked video of Kanye West's proposal this week.