"I'm writing a book on my journey of weight loss and weight gain," People magazine quoted the 41-year-old star as saying at the Hollywood premier of Tyler Perry's 'Why Did I Get Married?'.
"(I'm) really coming from the soul with it, and what I was going through at different times in my life and what possibly brought it about for myself," she said.
Janet said that she would reveal in her yet-untitled book what she did "nutrition-wise" to slim down "when she needed to".
She, however, cautioned that her suggestions would not have universal application.
"It's different for everyone," she said.
Last year, Janet had put on 60 lbs, which she said had a debilitating effect on her.
"I could not believe how big I was, how heavy I really was. I would see myself in the mirror, but I wouldn't look too often. I didn't want to get used to that image," she told Extra.