Simon Cowell arrives at the "America's Got Talent"Season 14 Kickoff at the Pasadena City Auditorium in Pasadena, Calif. Picture: AP

Media mogul Simon Cowell has denied having a "gastric band fitted" after losing 9kg, and he revealed the real secret behind his weight loss.

The "America' Got Talent" boss shocked viewers as he showed off his slimmer frame earlier this month, and now he has opened up about the real secret behind his weight loss.

He told Best Magazine: "People have said: 'Oh, he's had a gastric band fitted' - but I haven't. If I had, I would admit to it. This diet is all about what you put into your body."

Instead, the 59-year-old star listened to expert advice from a doctor in Los Angeles who sought to change Simon's "appalling" diet.

He explained: "I met this doctor in LA and he told me my diet was appalling and I had to cut out fatty foods and certain drinks.

"I could see the difference. Cutting out sugar made a massive difference. The first few weeks were difficult, but then I actually stopped craving sugar.

"I had also found a brilliant gluten-free beer, which helps."

The talent show judge admitted the most difficult part of his lifestyle changes come from watching his five-year-old son Eric snacking on tasty treats he's decided to cut out.

He said: "The hardest part is when I see Eric eat pizza in front of me. That's really tough."

Simon had previously ditched junk food and adopted a vegan diet following a health scare almost two years ago when he fell down the stairs at home in the middle of the night.

He said at the time: "Within 24 hours I changed my diet and I've not looked back since. You feel better, you look better.

"I cut out a lot of the stuff I shouldn't have been eating and that was primarily meat, dairy, wheat, sugar -- those were the four main things."