There are numerous apps out there with the goal of helping the user lose weight, whether by tracking their food intake, their exercise, or their weight itself. They also take different approaches to behavior change, with rewards, gamification, or social networks. But Carrot Fit, a new iPhone app, manages to do something new: the app includes a snarky female-voiced artificial intelligence (AI) who will cheer the user on when he or she loses weight, but berate him or her for weight gains.

In most ways, the app is a straightforward weight tracking app. Users enter their weight manually and the app tracks it on a graph, and also calculates the user's BMI and allows the user to enter weight loss goals. Carrot will remind users to enter their weight, and will "pass judgment" once they do, either with silly rewards or mocking aspersions. The app also features a cartoon avatar that will change to reflect the user's weight.

"If you slough off those extra pounds, Carrot will reward you with fabulous prizes like workout tips, cat facts, and permission to watch your friend eat a bag of potato chips," the app description says. But "if you blimp up, Carrot will be upset. When Carrot is upset, she gets mean. When Carrot gets mean, there is a 74.9 percent chance that she will make you cry."

Brian Mueller, the one-man team behind Carrot Fit and its two predecessors, Carrot To Do and Carrot Alarm, has a background in screenwriting. He said the Carrot character, who he describes as "sort of a mixup of all the different psychotic AI characters out there," came after looking at some of the gamified behavior change apps already on the market.

"It just seemed like a unique perspective to take on having this character," he told MobiHealthNews, "but instead of her being this motivational peppy character that is always positive, like you see in literally every other app out there, I made her really mean and negative. Threatening to kill the user, and ridiculing them and bribing them. And surprisingly, people really responded to it."

The iOS app is the third app by Mueller to use this approach, and he says across the three apps he has more than 500,000 users. His previous two apps used the same combative personality to get users organized and get them up in the morning, respectively. The difference between the newest app and the previous two, of course, is that people tend to take their weight more seriously, and more personally, than their organizational skills or their tendency to oversleep.

"I'm definitely very conscious about that, and I definitely don't want to make people feel bad," Mueller told MobiHealthNews. "I wanted to make weight loss fun and entertaining and this is really the best that I can do in trying to make it funny. I worried when I put it out people would hate it, but so far the response has been really positive. I think people will realize just by looking at the app that you're not supposed to get a negative vibe out of it. I don't think when you see the avatar you get this feeling that she's trying to make you feel bad. I think it's supposed to be more silly and more positive."

Mueller also said he's very responsive to user feedback, and he's removed dialogue from his previous Carrot apps when users felt it "went too far."

Carrot also charges users for the privilege of being slapped around by their weight loss tracker -- it goes for $1.99 on the iOS App Store. The weight tracker is just the first module for Carrot Fit, Mueller said, which will ultimately be much more comprehensive. Future plans for the app will cast Carrot in the role of fitness coach when the app adds "Deathmarch 5K" and "The 6-Minute Workout" fitness modules. Mueller also plans to add a calorie counter and integration with some WiFi scales. He wants to add Android support as well, but there's no definite timeline.