Billy Zane's fiancée, British model Kelly Brook, has admitted the pair enjoy lots of sex to keep them fit.The beautiful brunette told Britain's Star magazine: "I hardly do anything at all to keep in shape. I rarely watch what I eat or go to the gym."Sex keeps me fit and healthy. What can be better as part of your diet?"The 'Titanic' star's girl also said she refuses to be too thin and thinks her curves are what make her sexy. She said: "I think I'm different because I'm not one of those skinny girls like Sarah Jessica Parker - they are thin, thin, thin!"Billy is certainly smitten with his fiancée. The Hollywood hunk was so distraught at being parted from the starlet while away filming, that they set up a satellite link-up to stay in constant touch.A source revealed to Britain's Mirror newspaper: "Since they met, Kelly and Billy have rarely been parted for more than a few days."He's running up huge phone bills calling her all the time and he's constantly telling anyone who'll listen how talented he thinks she is."