Why Angelina Jolie Is Starving Herself

Unlike some of her peers, Angelina Jolie's pin thin frame has nothing to do with vanity.

PHOTOS: Angelina's humanitarian efforts in Bosnia

The U.N. goodwill ambassador, 36, who travels the globe meeting the poor, "puts herself on fasts to make statements for the children she visits," a Jolie source tells the new issue of Us Weekly (on stands Friday).

VIDEO: Angelina Jolie meets with refugees in Malta

"She says, 'If they can't eat, I can't eat,'" the source adds. The busy mother of six "does different cleanses from around the world. It's very dramatic, but that's how she gets."

The actress -- currently promoting her directorial debut, In the Land of Blood and Honey -- has partner Brad Pitt, 47, worrying about her health, says a second insider.

PHOTOS: Angie's hottest red carpet looks ever!

"He's worried about her and has made her see a zillion doctors, but they keep telling her she's fine."

For more on Jolie's "strange diets," pick up the new issue of Us Weekly, on stands Friday!