Tests on 150 customers at her gym confirmed her belief that she could "take any woman from any genetic background and turn her into this teeny-tiny dancer type". For five years she devised individual routines that changed every 10 days for those 150 people. The result was a huge database of routines.

It wasn't long before Hollywood came calling; three years ago, she was hired to work with Paltrow on the set of Iron Man. Soon afterwards Paltrow agreed to share her with Madonna. They are the only two people with whom Anderson now works personally. When she first started training them, Anderson's method was so effective she could eat whatever she wanted and still maintain her shape. "When I first met with them [Madonna and Paltrow], I literally was dunking a double-stuffed Oreo into a can of processed icing, and I can't remember which one it was, but whichever one I was with called the other one that minute and was, like, 'This is insane. Do you know what she's doing right now? Let me tell you what she's doing - she's killing herself!"'

Now, however, 80 per cent of the time she eats no dairy, wheat or processed food and is writing a book of recipes that can "whip stubborn fat off you in the click of a finger". She insists, however, that she eats a brownie every morning, and is "buttercream-frosting-obsessed".

It's hard to believe someone so driven in their quest for perfection would risk such a thing, and there is no sign of those brownies on her body as she poses for the photo shoot. But perhaps that's because however hard she expects clients to work, you can be certain Tracy Anderson is working herself even harder.

Reproduced in Sunday Life from The Telegraph Magazine, UK .