Popular diet programs, such as Weight Watchers, have proven to be successful — but for how long?

A new study, published in Circulation: Cardiovascular Quality and Outcomes, found the success of these programs and their contributions to the multi-million dollar weight loss industry don't exactly translate to how effective they are. Specifically, there's little known on the impact they have on reducing risk for heart disease. "With such a small number of trials looking at each diet and their somewhat conflicting results, there is only modest evidence that using these diets is beneficial in the long-term," Dr. Mark J. Eisenberg, lead study author and professor of medicine at Jewish General Hospital/McGill University in Canada, said in a press release.

So Dr. Eisenberg and his team analyzed the clinical trials that have been done on four popular diet plans that promote weight loss and cardiovascular health: Weight Watchers, Atkins, South Beach, and Zone. And the differences across each diet were pretty modest. Weight Watchers dieters, for example, lost up to 13.2 pounds after one year compared to those who lost up to 11.9 pounds maintaining usual care, or weight loss methods including low-fat diets, behavioral weight loss intervention, nutritional counseling, or self-help materials. On average, the smallest amount of weight lost on each program relative to the usual methods was a few pounds difference.

As for heart health, the study's press release explained, “in studies involving head-to-head comparisons, there were no marked differences between Atkins, Weight Watchers, and Zone diets at improving cholesterol, blood pressure, blood sugar levels, or other cardiovascular risk factors." Ultimately, science should consider long-term clinical trials in order to truly (ahem) weigh the impact of these programs on weight loss and heart health. Though it's worth saying that when it comes to promoting greater heart health, diet is only one part of the equation. The American Heart Association recommends a healthy diet, as well as regular exercise and stress management.

We can't say these results are too surprising. A couple months ago, Medical Daily reported on the findings of an Australian study that found maintaing weight loss matters more than how it's lost in the first place. Study participants were assigned to either a gradual or rapid weight loss program, and while those on the rapid weight loss program met their goal quicker, both groups averaged a 71 percent weight gain after three years.

"A broader lifestyle intervention, which also involves doctors and other health professionals, may be more effective," Eisenberg said. "This also tells doctors that popular diets on their own may not be the solution to help their patients lose weight."

Source: Eisenberg MJ, Atallah R, Filion KB, et al. Circulation: Cardiovascular Quality and Outcomes. 2014.