Diane Warren wrote "The Truth Is In There" and the former Fifth Harmony star is the first person she thought of to sing the track when Winfrey called, looking for something to soundtrack the For Every Body campaign.
"When Oprah called me and asked me for a song, I had just written 'The Truth Is In There'," Warren says. "I thought this would be the perfect song and Ally Brooke would be the perfect artist for it".
"Ally is all about positivity, as is Oprah," she adds. "We need a positive message in the world now more than ever and Ally Brooke delivers it in her own special way. I'm so excited about this...!".
The single, released on Friday, December 21, will be featured in commercials for the weightloss brand next week, begs Decemeber 24.
The news emerges days after actress Hudson signed on as the diet firm's new ambassador.
She recently revealed her post-baby work-out look as she set herself the goal of losing 25 pounds (11.3 kilograms) before she starts work on a new movie in the spring (19). The actress welcomed her daughter, Rani Rose, with boyfriend Danny Fujikawa two months ago. She also has two older sons from previous relationships.