The 38-year-old 5ft 9in actress, who weighs just 8.5 stone, has a body a lot of people are envious of.
But it seems she doesn't have to work too hard to maintain her slimline figure as she says she struggles to put weight on.
The Green Hornet star told Scottish newspaper the Daily Record: 'When I'm filming, I have to eat a lot more.
'I'm constantly eating because I'm burning so much energy doing what we're doing.'
She added: 'Even if we're just sitting around on the set of a movie, we're talking about the next scene and burning energy.
'However, I try to have the right kinds of food which I can eat throughout the day. I have a really good metabolism but sometimes I want to put weight on and I can't. That's my burden.'
Diaz added that she loves to cook and her specialty dish is breakfast.
She said: 'I like hearty, savoury breakfasts.
'I don't make pancakes but I can do eggs. I can do a lot of eggs and they can be, as anybody knows who cooks them, the trickiest things.'