Weight loss plans like Slim-Fast and Weight Watchers are competing hard against free options like calorie-counting apps on smartphones. - 

Weight Watchers is no longer a diet company. Nope, it's now a "health care company," according to the company's CEO David Kirchhoff. 

That's just one of the branding changes going on inside the roughly $40 billion American diet industry. The industry is being forced to rethink how it sells itself as it competes against new products like free calorie-counting apps on smartphones.

Unilever's nutrition supplement company Slim-Fast is turning away from the traditional diet marketing strategy and is unveiling a whole new campaign targeting sex appeal.

E.J. Schultz looked into the marketing behind the diet industry for a new article in Advertising Age and says the new provocative ads focus on the "bedroom benefits" of losing weight. 

"One ad shows a silhouette of a woman and there's a speaking cloud that comes out and she's saying 'I want to get into my new pants' and then there's a thought bubble above her head that says 'I want to get into someone else's pants' and the tag line is "Get what you really want. Slim-Fast." 

Schultz says that traditional weight loss companies need to innovate to keep customers from being lured away by free versions online.

"[Weight Watchers is] probably hoping that the people with the free apps maybe join for a day or two, or a week or month, and then just slowly forget about it," said Schultz. "I think the fact is if you're paying for something as in Weight Watchers you're a little bit more invested and you might be more likely to stick with the program if you're seeing the steady results that they promise."

Follow Sarah Gardner at @RadioGardner