Every few years there is a new “star” among the array of popular weight-loss diets, with the most recent attention falling on the Atkins diet and its low-carb cousin, the South Beach diet. People are always searching for the perfect diet, the one that will lead to the most weight loss. But as anyone who has ever tried to lose weight knows, it’s not easy to do. It is much easier to blame one’s persistent pounds on the failed diet and move onto the next one. There is a reason why more than 1,000 diet books compete for space on the shelves.

Nutrition experts have long been critical of these popular, or so called “fad” diets: few are backed by scientific evidence and many are extreme deviations from what is considered balanced eating. But with the current obesity crisis, health professionals have become interested in the potential for these popular weight loss diets to help their patients lose weight.

A study in the January 5, 2005 issue of the Journal of the American Medical Association set out to determine the effectiveness and sustainability of four popular weight loss diets: the Atkins, Ornish, Weight Watchers, and Zone. The results showed that after one year, all four diets led to modest reductions in body weight and improvements in several cardiovascular risk factors. Adherence rates, however, were low, with 42% of participants dropping out by the time the study ended.

About the Study

This study included 160 participants between the ages of 22 and 72. To take part in the study participants had to be overweight or obese with a body mass index (BMI, a measure of weight in relation to height) between 27 and 42. (Healthy BMI is greater than 19 and less than 25.) Participants also had to have at least one risk factor for cardiovascular disease (e.g. total cholesterol of at least 200 mg/dL [5.2 mmol/L], fasting blood sugar of at least 110 mg/dL [6.1 mmol/L], systolic blood pressure of at least 145 mm Hg).

The researchers randomly assigned participants to one of four diets: Atkins, Ornish, Weight Watchers, or Zone (Table 1). During the first two months, a dietitian and physician on the research team conducted classes for the participants to provide diet-specific counseling, give positive reinforcement, and address barriers to adherence.

Table 1. Diet Specifics


Diet Type


AtkinsCarbohydrate restrictionLess than 20 grams (g) of carbohydrate daily, with a gradual increase toward 50 g
ZoneMacronutrient balance40% of calories from carbohydrates, 30% from fat, and 30% from protein
Weight WatchersCalorie restriction; portion controlKeep total daily “points” (each point is roughly 50 calories) in a range determined by current weight, for most this was between 24 and 30 points per day
OrnishFat restrictionA vegetarian diet containing only 10% of calories from fat

To measure diet adherence, the researchers analyzed three-day food records, which the participants were asked to keep at baseline, one, two, six, and 12 months. The researchers also telephoned participants monthly and asked them to rate their dietary adherence level on a scale of one to 10, with 10 being a perfect score and one being their baseline diet.

The researchers assessed outcome measures, including body weight, waist size, and other cardiovascular risk factors, at baseline, two, six, and 12 months. Participants were asked about exercise level and medication use at these times.

The Findings

At the end of one year, all four diets resulted in modest, but statistically significant weight loss (Table 2). Because 65 (42%) of the participants dropped out before the end of the study, the researchers used their baseline measurements to calculate the final 12-month outcomes. When these participants were excluded from the final outcome calculation, the average weight losses were slightly greater. All diets also led to modest, but statistically significant, improvements in several cardiovascular risk factors.

Table 2. Group outcomes at 12 months



Weight Watchers


Average weight loss (pounds)





Average calorie reduction





Participants completing 12 months

21 (53%)

26 (65%)

26 (65%)

20 (50%)

Interestingly, there was no significant difference between the four diets; all led to similar degrees of weight loss and cardiovascular risk reduction. The drop out rates suggest, however, that participants found the two less extreme diets (Zone and Weight Watchers) easier to stick to.

How Does This Affect You?

This study showed that following any one of the four diets tested can lead to weight loss and a reduction in cardiovascular risk—the catch was staying on the diets long enough to reap the benefits.

The results of this study suggest that it is not which diet is best in general, but which diet is best for you . If you find that any one of these popular diets fits well with your temperament and lifestyle, you will likely succeed—as long as you stick with it. The researchers would probably have seen higher adherence rates if the participants had been allowed to choose which diet they wanted to follow. A bread lover, for instance, would be setting him- or herself up for failure by going on the Atkins diet.

If you are interested in trying one of the popular weight loss diets to lose weight, you might benefit by talking it over with a nutritionist or dietitian beforehand. He or she can advise you on whether this diet is a good match for you, and suggest modifications to make it more healthful and easier to follow.

Also, before you set out to lose weight, it’s a good idea to enlist the support of others. Research has shown that people who receive regular support—whether through group counseling sessions, individual consultations, or online networks—are more likely to lose the weight and keep it off.

In the end, the measure of a successful diet is not about how much weight you can lose or whether you can adhere to it for a year; it’s about establishing lifelong behaviors around eating and exercising that lead you to achieve or maintain a healthy weight.