Dallas, Texas - Popular commercial diets can help you lose some weight in the short term, but keeping the weight off after the first year and the diet’s impact on heart health are unclear, according to a study published in Circulation: Cardiovascular Quality and Outcomes, an American Heart Association journal.

Nearly 70 percent of American adults are overweight or obese – and therefore at higher risk for health problems such as heart disease, stroke, high blood pressure and diabetes. Whether a diet will be effective is an important public health question.

“Despite their popularity and important contributions to the multi-million dollar weight loss industry, we still do not know if these diets are effective to help people lose weight and decrease their risk factors for heart disease,” said Mark J. Eisenberg, M.D., M.P.H., the study’s senior author and Professor of Medicine at Jewish General Hospital/McGill University in Montreal, Quebec, Canada. “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.”

After analyzing clinical trials on four popular diet plans — Atkins, South Beach, Weight Watchers, and Zone — that promote weight loss and improved cardiovascular health, researchers found:

• In trials comparing Weight Watchers to usual care, Weight Watchers dieters lost an average 7.7 to 13.2 pounds after one year compared to 1.8 to 11.9 pounds with usual care, however at 2 years, the weight lost was partially regained. Usual care refers to traditional methods to promote weight loss such as low-fat diets, behavioral weight loss intervention, nutritional counseling, or self-help materials.
• Results from trials on the Atkins diet were inconsistent. In the only trial comparing the South Beach diet to usual care, no difference in weight loss occurred in 12 months, however the participants in this study were both severely obese and had undergone gastric bypass surgery.
• Trials involving head-to-head comparisons between Atkins, Weight Watchers, Zone and usual care suggest that all four result in a modest weight loss at one year, as did those in the control group who received the usual care. Those on the Atkins diet lost an average 4.6 to 10.3 pounds; Weight Watchers participants lost an average 6.6 pounds; Zone dieters lost an average 3.5 to 7 pounds; and control lost about 4.85 pounds.
• 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.

The longest diet studies researchers analyzed lasted for two years, and results were only available for the Atkins or Weight Watchers diets. Those studies found dieters regained some of their weight over time.

To better understand the potential benefits from any one or all of these diets, researchers need to conduct large clinical trials directly comparing all four popular diets for long-term weight loss and changes in other heart disease risk factors, he said.

“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.”

Co-authors are Renée Atallah, M.Sc.; Kristian B. Filion, Ph.D.; Susan M. Wakil, M.D.; Jacques Genest, M.D.; Lawrence Joseph, Ph.D.; Paul Poirier, M.D., Ph.D.; Stéphane Rinfret, M.D., S.M.; and Ernesto L. Schiffrin, M.D., Ph.D. Author disclosures are on the manuscript.

The Canadian Institutes of Health Research funded the study.