The country singer is reportedly always left hungry because he doesn't have an appetite for his wife Nicole's super-healthy meals and often visits his favourite fast food outlets rather than face her bland food.
A source said: "Nicole cooks with no sugar, salt, butter or oil."
The 42-year-old singer - who married the actress in 2006 - is also being pushed into exercise by his wife, who previously spoke about important exercise was to her.
The Australian actress - who has a 17-month-old daughter Sunday Rose with the star - said her father taught her the benefits of jogging at an early age and it's a skill she is hoping to pass on to Keith.
She said: "It's what my dad taught us - he's a marathon runner. He's 71 and goes on 10-mile runs. He takes Sunday hiking on his back for hours."
The Oscar-winning actress insisted she doesn't count calories however, thanks to her spouse.
She said: "As Keith's a musician there are pit-stops at Wendy's. But I'm six foot tall so it's not weight that's an issue for me, but things like cholesterol."