Jennifer Hudson, before and after.

The results of Jennifer Hudson's body transformation have never been more apparent than in her latest Weight Watchers commercial, in which she sings alongside a heavier version of herself.

On the left, Hudson in 2004, at 23, on "American Idol." On the right, Hudson now, 30-years-old and 80 pounds lighter. She sings The Four Tops' "I Believe in You and Me" as Weight Watcher's slogan: Believe, Because It Works, appears behind her.

Hudson became the spokesperson for the weight loss program in 2010. Her commitment to getting fit also led her to walk away from the lead role in the Oscar-winning film "Precious." In her new book, "I Got This: How I Changed My Ways and Lost What Weighed Me Down," Hudson writes that after putting on pounds for "Dreamgirls" (which won her an Oscar) she "wanted to try a role that had nothing whatsoever to do with my weight."