The former Beatles star is trying to abstain from his favorite sweet treats in order to stay trim for American lover Nancy Shevell.
Vegetarian Paul, who was accompanied by Nancy to a party at his fashion designer daughter Stella's boutique, said: "I'm trying to lay off the chocolates. I've got to stay in shape for the other half."
The 66-year-old musician also admitted he worries about Stella's happiness even though she is 37 and has her own family.
He explained: "For some stupid reason I feel I have to keep checking where they are all the time to make sure everything's going OK."
The party was held to turn on Stella's annual Christmas lights display.
The festive decorations were lit up by Paul and British comic Peter Kay, who was dressed as his comedy pop star alter-ego Geraldine.
Other guests to attend included Stella's supermodel pal Kate Moss and her boyfriend Jamie Hince and 60s singer Lulu.
Paul left the bash early, telling guests at 9pm: "This is the witching hour for me these days. It's way past my bedtime."
