Many of the most widely known diet plans do indeed take off a few pounds, but only for true devotees, according to new research released Tuesday.

A study in the Journal of the American Medical Association found that four popular diets - Atkins, Weight Watchers, The Zone and Ornish - helped people who stuck with them lose up to seven pounds in one year. However, nearly half of the dieters dropped out of the study before the year was up, most often because they felt the diets were too hard to follow or weren’t working.

“We found that a variety of popular diets can reduce weight and several cardiac risk factors under realistic clinical conditions, but only for the minority of individuals who can sustain a high dietary adherence level,” the study authors write.

To investigate how well the most popular diets work, Dr. Michael L. Dansinger of the Tufts-New England Medical Center in Boston and his colleagues asked 160 overweight and obese adults to try one of the four plans for one year. All participants had at least one obesity-related health risk, such as High cholesterol or high blood pressure.

People following the Atkins “low-carb” diet reduced daily intake of carbohydrates to 20 grams, gradually increasing to 50 grams, and could eat large amounts of protein and fat. Zone dieters, in contrast, aimed for a 40-30-30 ratio of carbohydrates, fat and protein, respectively, in terms of percentage of calories.

Followers of the Weight Watchers plan limited “points” every day, with each portion of food representing a certain number of points (roughly 50 calories each). The fourth plan, known as the Ornish diet, is a vegetarian program in which 10 percent of calories come from fat.

Researchers also encouraged participants to exercise at least 60 minutes every week and to keep a regular food diary during the study.

Overall, only 58 percent of the dieters kept up with their food regimen for one year.

Among those who abandoned their diets, a slightly higher percentage gave up on the Atkins or Ornish plans, which the researchers characterized as being “the more extreme diets” than quit the more moderate Zone and Weight Watchers programs.

Over the course of the year, people who stuck with their diets “cheated” more and more as time went on, regardless of which diet they followed.

“No single diet produced satisfactory adherence rates, and the progressively decreasing mean adherence scores were practically identical among the four diets,” Dansinger and his team note.

Still, the dieters lost between 4.6 and 7.3 pounds during the year.

The better people stuck to the plans, the more weight they lost - indeed, 10 percent of participants lost at least 10 percent of their initial weight.

All of the diets appeared to help lower LDL - so-called “bad” cholesterol - and raise HDL, the “good” cholesterol.

In an accompanying editorial, Dr. Robert H. Eckel of the University of Colorado at Denver writes that more dietary research is clearly needed. In the meantime, he recommended that people who want to lose weight and truly keep it off should try the “Low Fad” approach - “modest, but persistent caloric restriction,” and exercise.

SOURCE: Journal of the American Medical Association, January 5, 2005.