Clinical Question: Which of four popular diets (Atkins, Zone, Weight Watchers, and Ornish) is the most effective for weight loss and reduction of cardiac risk factors?

Setting: Outpatient (specialty)

Study Design: Randomized controlled trial (single-blinded)

Allocation: Concealed

Synopsis: Few studies have addressed the health effects of popular diets and even fewer studies have directly compared different diets. In this trial, investigators enrolled 160 overweight or obese adults (mean body mass index = 35; range = 27 to 42) who were 22 to 72 years of age with known hypertension, dyslipidemia, or fasting hyperglycemia. Participants were randomized to one of the following diet groups: Atkins (carbohydrate restriction), Zone (macronutrient balance), Weight Watchers (calorie restriction), or Ornish (fat restriction). Persons assessing outcomes were blinded to treatment group assignment.

The study attrition rate as a result of patient dropouts was high: the number of participants who did not complete the study at months 2, 6, and 12 were 34 (21 percent), 61 (38 percent), and 67 (42 percent), respectively. The most common reason cited by patients for withdrawing was that the assigned diet was too hard to follow or was not resulting in adequate weight loss. Although the results were not statistically significant (P = .08), more patients discontinued the Atkins (48 percent) and Ornish diets (50 percent) than the less extreme Zone (35 percent) and Weight Watchers (35 percent) diets.

Using intention-to-treat analysis, all four programs resulted in similar weight loss results at one year with no statistically significant difference among the groups. In each of the groups, approximately 25 percent of participants sustained a weight loss of more than 5 percent of initial body weight, while 10 percent of participants sustained a weight loss of more than 10 percent of initial body weight at one year. Improvement in cardiac risk factors was directly proportional to the amount of weight loss and was similar among the diet groups. Self-reported dietary adherence correlated directly with the amount of weight loss and reduction in cardiac risk factors. The study was large enough to have an 80 percent chance of detecting a 3 percent difference in weight change among the diets.

Bottom Line: All four popular diets (i.e., Atkins, Zone, Weight Watchers, Ornish) are equally effective for helping adults lose weight and reduce cardiac risk factors. Because success in this study directly correlated with adherence to the diet, it makes sense to help patients choose the diet that is easiest for them to follow and not preferentially encourage one diet over any other. (Level of Evidence: 1b–)