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 that they'll meet a weight loss goal. HealthyWage is one of several wagering websites that have 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 it. All of the sites work differently, but have the same premise: get healthy or risk losing your cash.

"If I didn't have anything on the line I don't think I would have done it," says Calliari, a 29-year-old merchandise buyer for grocery stores in Milwaukee.

The services make money by taking a percentage of the total collected from those who don't meet their goals. To make sure no one fakes a weight loss, the sites ask users to photograph themselves on a scale or film a video and submit it to the site for verification.

Calliari first heard about HealthyWage after the company she worked for used the website as a way to motivate employees to lose weight. In February 2012, she started two challenges. One was to lose 10 percent of her body weight in six months. She wagered $100. She lost the weight and won $200.

The second was a year-long challenge to drop her body mass index from obese to healthy. She wagered $300. Her weight dropped from 204 pounds to 153 pounds and her body mass index, a number determined using weight and height, fell to 24 from 33.8. She'll soon receive a check for $1,000.

"I think it does motivate people," says Susan Adams, a registered dietitian and assistant professor of nutrition at La Salle University in Philadelphia. She says betting $300 might be too much, saying that many people lose the motivation even after paying cash. "How many people join gyms on Jan. 1? Go back in March and see how many are left," Adams says.

But the thought of losing such a large amount of money kept Calliari on track. "If I'm going to spend money on a purse why can't I put in $300 for my own health?" Calliari says.

If you think money will motivate you to drop the pounds, check out these four weight loss and exercise wagering programs:

DIETBET.COM

DietBet users can go on the website and join a public group, or create their own private game among family members or friends. The amount of money needed to wager is set by the person who starts the game. There are many public games that charge as little as $25. The goal is to lose 4 percent of your body weight in four weeks. Those that do, share the total amount of money the group collected. Jamie Rosen, the CEO of DietBet.com, says that about a third of the dieters who bet money actually win money. But he says about 93 percent of people who join a game lose weight, even if they don't win cash. DietBet.com takes as much as 15 percent of the total amount collected before distributing it to winners. Rosen says the average person who wagers $25 can expect to win about $50 to $75 if they shed 4 percent of their body weight.

GYMPACT

This app wants to make sure you're hitting the gym. You have to make a pact on which days of the week you plan to go to the gym. You have to wager at least $5 that will be charged if you don't make it to the gym. Every time you go to the gym, you have to check in on the GymPact app. GymPact verifies that every gym that is checked in to actually exists. The app uses your smartphone's global positioning system to make sure you are at your gym for at least 30 minutes when you check in. At the end of the week, you get cash rewards for making your goals. The reward money comes from people who didn't meet their goal. GymPact takes a 30 percent cut and then distributes it to those that met their goals. GymPact says that the average user is rewarded about 50 cents to $1 for each day they make it to the gym. The app is available for Apple Inc.'s iPhones and on smartphones that use Google Inc.'s Android operating system.

HEALTHYWAGE.COM

This website enables you to select from three different challenges. Bet $150 that you'll lose 10 percent of your body weight in six months, and you could win $300 if you drop the pounds. (The 10 percent challenge cost $100 with a $200 prize when Calliari started the challenge last year, but it was bumped up to $150 with a $300 prize in January after customers asked for a bigger prize, says HealthyWage's co-founder David Roddenberry.)

If you have a body mass index above 30, which is considered obese by the Centers for Disease Control and Prevention, you can join the 12-month challenge to lower your body mass index below 25, which is considered healthy. There's a free program that awards $100 to anyone who completes the BMI Challenge, or you can wager $150 to win $400, or wager $300 to possibly win $1,000.

The third challenge is called The Matchup, where users are put in a team of five. It costs $25 a month for three months, and the team that loses the highest percentage of their total weight wins $10,000. But about 150 teams are playing at a time, so the chances of winning any money are slimmer.

STICKK.COM

On this website, there's no money to win. Instead, users set an amount of money their credit or debit cards will be charged if they don't reach their goal of losing a certain amount of weight in a week. As a motivator, the money can be sent to a charity or friend who you wouldn't want to give the chance to hold your lack of commitment over you. You might even try what Stickk calls an anti-charity. You can pledge that the money will go to an organization whose mission you don't support. Stickk collects 29.5 percent of money sent to an anti-charity and 19.5 percent of money sent to a charity. Stickk also lets users use the site without wagering any money, and can set other goals besides losing weight, such as to quit smoking.