A researcher at the University of Texas has published a study in the Journal of Medical Internet Research looking at efficacy of a free online weight loss program. The study looked at 1,258 randomly selected users of SparkPeople.com and found that those who entered their weight into the site at least four times per month lost an average of 11 pounds per month more than those that didn't, and those who made at least one post on the site's forum during the time they used the site lost over three pounds more than those that didn't. (Correction: A previous version of this article misstated the weight entry as four times total, rather than per month.)

Dr. Kevin Hwang, the lead researcher on the study, approached SparkPeople for the dataset in 2009, and the company ended up providing him with a random sample of users who joined the site in 2008, with extreme outliers (like users who reported heights of more than 10 ft.) eliminated. The sample included data entered by users through 2010, and researchers completed the analysis in 2011. Hwang writes in the study that looking at SparkPeople's data has two advantages over previous work with online weight loss communities. First, as a "naturalized cohort" study, the subjects were not volunteers for a research study and were not aware they were being studied.

Second, SparkPeople is a free service, which Hwang suggests could make the results more relevant for public health than studying paid subscription services.

"The public health impact of an intervention is determined by efficacy and dissemination," he wrote. "Because this online program is free, scalable, and widely disseminated, the potential public health impact is significant."

The downside to the format is that the data is self-reported. But researchers found that participants entered weight gains as well as losses, and SparkPeople representatives maintain that there's no reason for users of the site to lie.

SparkPeople was founded by former eBay employees in 2001, originally as a paid site. But the founders decided to switch to a free, ad-supported model in 2005. In 2009, the site passed Weight Watchers to become the dieting site with the most unique hits in America, according to comScore.

"We felt our software was very strong, but we were competing with companies that spent a lot on marketing," Chief Operations Officers Dave Heilmann told MobiHealthNews. "We realized that going free would expose a lot more people to our technology."

The site combines written content like recipes and articles, tracking tools (primarily based on manual weight entry, but increasingly integrating with digital tools), and an online forum for social support. Members are encouraged to take advantage of all three services to meet their weight loss goals. The site's average user starts out about 35 pounds overweight, according to Heilmann, and the user base is about 90 percent female (as was the sample in the study).

"I'm proud of that 10 to 12 percent," said Heilmann, referring to the number of men on the site. Heilmann says he thinks the social aspect of the site appeals more to women. The company also offers a number of mobile apps, which they begin offering in 2009, including one that focuses solely on tracking. With that app, he says, it's more like 65 percent female.

In addition to ads, SparkPeople generates revenue by charging for its apps, selling recipe and diet books, and recently began offering a premium subscription to the website that provides direct, personal access to trainers.

Tools other than weight entry and forum usage did correlate with increased weight loss in the study, but not a statistically significant amount. In addition, SparkPeople offers tools now that weren't available when that data was collected between 2008 and 2010. Nonetheless, Heilmann said the results are encouraging for the company.

"It does reinforce how we've spent our time," he said. "Honestly, a big part of the study was to show people this works. People need to believe in a diet they're on to stick with it. We do want to do more potentially with corporate wellness and things like that."