He has been open about his struggles with his weight – openly talking about it during series nine of Keeping Up With The Kardashians.
In a recent episode he confessed: "I've been the most unhappiest person for about a year and a half and it's very, very easy to fall off the wagon. I need to literally dedicate my entire life to my fitness."
The catalyst for his problems is said to be his split with Rita Ora in 2012, after which he gained 60lbs.
Rita told Elle magazine of Calvin: "He got something out of me that I never thought I had. Yes, like falling in love. I just didn’t think I had it. I’d never experienced it before. And I was just like in the wilderness, thinking, 'Will it ever happen?'."
Rob is said to have also found the breakdown of sister Khloé's marriage with his close friend Lamar Odom difficult to come to terms with as well as his mother's separation from Bruce Jenner – the man who has been his stepdad for 22 years.