Jessica Simpson Gives Up Fast Food!

Jessica Simpson attends the launch of Jessica Simpson Girls at Belk Crabtree Valley Mall on November 5, 2011 in Raleigh, North Carolina. Jamie McCarthy/

The slimdown begins!

Four weeks after giving birth to daughter Maxwell (with fiance Eric Johnson, 32), Jessica Simpson, 31, has already curbed her high-calorie habits–and not just because of that reported $4 million Weight Watchers deal.

"She's breast-feeding Max and wants her to get the best nutrition," a source tells the new issue of Us Weekly (out Friday) of the singer, whose pregnancy snacks included Pop-Tarts and Kraft mac 'n' cheese. "She has to cut down on fast food. Besides, it's a great way to jump-start the massive diet ahead."

Last November, Us Weekly broke the news that the 5-foot-3 star was in negotiations to work with Weight Watchers to get her pre-baby body back. On Wednesday, Simpson took to Twitter to confirm the deal. "So excited to be part of the Weight Watchers family," she tweeted.

Later that day, she further explained her decision to give the weight loss program a try on her Facebook page. "The cool thing about [Weight Watchers] is that it focuses on healthy habits for the long-term (and I can still indulge in my guilty pleasures every now and then, too)," Simpson wrote. "I have actually gotten a group of friends together who are going to be doing it with me."

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