Describing her condition, David says in Maureen Callahan's new book 'Poker Face: The Rise and Rise of Lady Gaga': "When I say she was sick, I mean physically and mentally."
Talent scout Wendy Starland added: "The pressure on her to lose weight was very high."
Also in the book, David's wife Angela claims she used to accompany Gaga on tour alongside her husband, and the 'Paparazzi' singer would cuddle up to her in bed before performances.
She said: "I would say, 'Gaga, I have a husband to go home to. I'll be in the room next door.'
"She'd call and text me, 'I miss you, Ange, can you come back?' So I would.
"We literally would do our make-up together every morning and get ready for bed together at night."
Despite six hospital stints last year, the 24-year-old star's health problems have continued into 2010, with her being forced to cancel four US shows in January suffering from an irregular heartbeat, brought on by exhaustion. She also broke down in New Zealand in March and has recently tested "borderline positive" for Lupus, a tissue disease which killed her aunt.
Gaga has previously admitted to being "insecure", but claims she hides behind her onstage persona.
She said: "When I wake up in the morning, I feel just like any other insecure 24-year-old girl. Then I say, 'B***h, you're Lady Gaga, you get up and walk the walk today."