The hunky actor - who is famed for his muscular physique and hardman reputation - never used to be concerned about what he ate but as he`s getting older her pays more attention and tries not to go over 2,000 calories a day by eating six small meals.
The 44-year-old star said: "I never gave a f**k about a calorie. An apple? It`s good for me. I`d have five. Bananas? Eat the bunch.
"But it`s different now and writing everything down makes it impossible for you to muck up."
Jason - who is dating model Rosie Huntington-Whiteley - also revealed he is drawn to roles that showcase his tough side rather than the more delicate ones.
The `Killer Elite` actor told The Guardian newspaper: "Luckily, I get to play some good bits that make people think I`m tough. I just gravitate to those more testosterone-filled sort of parts than me playing something a bit more fairy-like."