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.’”

SIMON COWELL REVEALS DRAMATIC WEIGHT LOSS

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.

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“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.”

This article originally appeared on Page Six.