Simon Cowell debuted a 20-pound weight loss at his summer party in London, after giving up everything that’s fun to eat.

The trim music boss told Page Six he’d been prompted into seeing a doctor after a longtime pal, singer Sinitta, pointed out his tummy last summer.

Speaking at his Syco Entertainment summer bash at the Victoria and Albert Museum, Cowell related: “Sinitta didn’t need to say anything more. I went to see this guy who’s very well-known. And he said to me, after doing some bloodwork, ‘You can’t have red meat, dairy, sugar, bread or gluten.’”

Cowell was worried what the doctor would say about his drinking — and Kool cigarette habit — but he revealed: “Actually, he was way more worried about what I was eating!”

Cowell, who will turn 60 this year, joked in a speech to guests that he was actually 10 years younger, insisting that he had lied about his age to get into the music biz.

“As you know, there’s a big birthday coming up this year. I looked in the mirror last week and I went, ‘I can’t live this lie anymore, because no one’s going to believe I’m 60, so . . . I am 50!’ It’s true.”