The Good Morning Britain host, 54, has said he doesn't believe the music mogul, 60, managed his transformation by simply switching to a vegan diet.
Speaking on the programme earlier today (13.10.19), Piers said: "I couldn't believe Simon was [at the ITV Palooza] actually. He is wasting away, Cowell.
"I cant believe he is actually on a vegan diet, I don't think he is. Something is going on. Something is going on."
In a chat with Best magazine, he said: "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."
Speaking further, Simon said a doctor in LA told him his diet was "appalling" and advised that he cut out certain fatty foods and drinks.
After turning his diet around and cutting out sugar, Simon said he "could see the difference", although the first few weeks without it "were difficult".
Simon previously revealed that his good pal and ex-girlfriend Sinitta was the one who originally encouraged him to lose weight.
He is wasting away, Cowell.
He told the Daily Star: "I've never felt better. It was Sinitta who pointed at my stomach last year in Malibu and I went, 'I get it'."
Sinnita has also spoke about the moment she confronted Simon about his weight, although she insisted it was nowhere near as "brutal" as Simon made it sound.
She explained: "He makes it sound much more brutal than it was. We had just finished filming X Factor last year."
Sinitta reportedly told Simon he's "too handsome" and "too fabulous" to let himself go.
The star explained how she had her arms around Simon as they were hugging, and she "just kind of grabbed him a little bit around the love handles and the waist, and his little cheeks".
She continued, in a clip aired on ITV daytime show Lorraine: "I just said, 'You know, I just want you to be healthy and I want you to look good and feel good. This is not you and you need to address this'."
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