Kimberly Calliari paid $300, lost 51 pounds, and won $1,200.

That all happened after she signed up for two challenges on HealthyWage.com, a website that lets dieters bet their own money they’ll meet a weight loss goal. HealthyWage is one of several wagering sites that have been launched in the past few years, including DietBet.com and Stickk.com. GymPact, a smartphone app, pushes people to go to the gym — or get charged for not going. All of the sites have the same premise: Get healthy or risk losing your cash.