The 36-year-old singer - who has famously battled with his weight throughout his career - insists his new toned physique is all down to bedroom antics with fiancee Ayda Field.
When questioned about his new buff body, Robbie said: "You`re talking abut sex now. Yes we do it, we do it, we really do it!"
However, it isn`t just sex that keeps him slim - the `Bodies` singer also finds the time to run every day - and has used his celebrity-filled Soccer Aid charity event, which took place on Sunday as a target for getting fitter.
He told the Daily Star newspaper: "I`ve been running six or seven miles a day, playing a couple of 90 minute games a week, so I am knackered.
"I don`t think I`ve lost weight but I have toned up. It`s been a good process and it`s been great to have something to aim for."
Robbie isn`t the only celebrity who uses to sex to look good - `Shrek Forever After` star Cameron Diaz also credits it as keeping her youthful.
The 37-year-old beauty said: "I don`t want to look 25 again. The fountain of youth for me, let`s see. I guess it`s exercise, healthy diet, lots of water, lots of laughter, lots of sex - yes, sex, we need that as human beings. It`s healthy, it`s natural, it`s what we are here to do!"