Britney Spears is sick for ''three days'' if she eats steak.

The 36-year-old singer likes to follow a clean-eating plan when she's on tour because she has a ''sensitive stomach'' and spends multiple days in bed vomiting if she chomps down on a hefty piece of beef - but she can have hamburgers.

She told 'Entertainment Tonight': ''Sometimes I'm just like, 'I want a hamburger!' but when I'm in work mode, I want [to eat] very clean. I want clean chicken and veggies that's it. My stomach is so sensitive, if I have a steak, I will be sick for three days. The burger is fine, but I have the small burgers from McDonald's. The happy meal!''

The blonde beauty will kick off the North American and European leg of her 'Piece of Me' tour next week and, although it's important she's in good shape for her performances, Britney doesn't have time to workout when she's on the road.

She said: ''This round of shows that we are doing on tour, they are back to back, so each show is like... I don't have time apart, besides flying. My show is a workout in itself. It really keeps you strong.''

And she's really looking forward to having her sons Sean Preston, 12, and Jayden James, 11, whom she has with her ex-husband Kevin Federline, on tour with her.

She explained: ''They will come with me. They enjoy it. My kids, they have been putting new remixes into my show and stuff... there is one of the new songs on the radio, and [my son is] like, 'Mom, that is so cool!' So I'm putting all these different new songs in my show, so they are coming along.''