The 51-year-old music mogul has reportedly vowed to cut out carbohydrates altogether and is tucking into vegetable smoothies for lunch in a bid to shed the weight he gained by over-indulging in his favourite home-cooked meals while visiting Britain.
Simon is now reportedly hoping to lose 14lb and get in great shape in time for the launch of the US version of his talent search show `The X Factor`.
A source told The Sun newspaper: "He`s been having meals Brits love such as spaghetti Bolognese and fish fingers and chips.
"Being back in Los Angeles, he said it`s much easier for him to lose it as he will swim every day and his fiancee Mezhgan Hussainy will make sure he eats healthily."
The pop svengali recently admitted to being "vain" and insisted it is natural for TV stars to be obsessed with grooming.
Simon said: "I am definitely vain. But to be honest with you, I can`t think of one person who is on TV who isn`t vain. It`s the nature of the beast.