Weight Watchers. Atkins. South Beach. Ornish. The Zone.

There is no doubt that all of these popular diets reduce body weight and cardiac risk.

Adhering to a diet, and not the particular type of diet, however, seems to be all that matters when it comes to keeping the weight off and lowering cardiac risk.

To assess the adherence rates and effectiveness of four popular diets (Atkins, the Zone, Weight Watchers and Ornish), Boston researchers randomized 160 participants to one of the four diets.

The participants were overweight or obese, ranged in age from 22 to 72 years, and had known high blood pressure, dyslipidemia lipid disorders and fasting high blood glucose.

The dieting participants were followed between July 2000 and January 2002. After the first two months, participants controlled their degree of adherence to the respective diets.

“Overall dietary adherence rates were low,” say the researchers, “although increased adherence was associated with greater weight loss and cardiac risk factor for each diet group.”

Participants in the Ornish group lost the most weight (7.25 pounds), followed by The Zone (7 pounds), Weight Watchers (6.6 pounds) and Atkins (4.6 pounds).

The amount of weight loss was associated with self-reported dietary adherence level but not with diet type, according to the researchers. In addition, each diet significantly reduced the participants’ LDL/ HDL cholesterol ratio by about 10.

—Journal of the American Medical Association, January 5, 2005

Food editor note: The Weight Watcher and Ornish diets promote exercise as part of their “program.” In addition, the Ornish program has a relaxation/imagery/meditation component, which is an integral part of participants’ success.

Diets Followed in the Study

Atkins Diet: Carbs are initially restricted to 20 grams per day and then gradually increased to 50 grams per day.

Weight Watchers: Portion size is limited and calorie load is restricted to between 1,200 and 1,600 calories per day.

The Zone: The goal is a carbohydrate-fat-protein ratio of 40:30:30.

Ornish Diet: A vegetarian diet allowing only 10 percent of calories from fat.