The first head-to-head trial of four popular diet plans — Atkins, Dean Ornish, Weight Watchers and the Zone — has found that people who stick with any of them for a year lose about 5 percent of their body weight, far fewer pounds than most dieters hope for, and at least a third drop out of all of them before the year is up.

“This is the first real assessment of popular diet books to see what results they will produce,” said Thomas Wadden, director of the Weight and Eating Disorders Program at the University of Pennsylvania School of Medicine. “The results are modest. The study shows that no single approach has a monopoly on weight loss.”

But the study also found that even modest weight loss pays off: All the diets cut the risk of heart disease by 7 to 15 percent. “It shows yet again that modest weight loss can improve risk factors,” said Kelly Brownell, director of the Yale Center for Eating and Weight Disorders.

Researchers said the results, announced at a news conference at the American Heart Assn.’s annual meeting Sunday in Orlando, might help settle the growing debate about the best way to shed unwanted pounds for good.

The study

Led by Tufts University researchers and funded by the Tufts-New England Medical Center with federal support, the study randomly assigned 160 overweight and obese men and women to follow one of the four diets for a year.

Participants ranged in age from 22 to 72. All had tried to lose weight before, and all had at least one major risk factor for heart disease — high blood pressure, elevated blood cholesterol, abnormal blood sugar level or full-blown diabetes. Their average body mass index was 35, equal to about 216 pounds for someone 5 feet 6 inches tall.

The Atkins, Ornish and Zone groups each received a book describing their program. Since no book describing the Weight Watchers program exists, this group received a cookbook published by Weight Watchers International. All groups received instruction in their programs and four counseling sessions during the first two months. They were assessed for their ability to follow the regimens and then left to their own devices for the next 10 months.

Atkins, Ornish dropouts

About half of those in both the Atkins group (very low carbohydrate, high fat) and the Ornish group (very low-fat, high-carbohydrate vegetarian) dropped out before the study was completed. About a third dropped out of the Weight Watchers group (low fat, moderate calorie, similar to the diet advocated by the U.S. Dietary Guidelines) and the Zone (which relies on a system to track how much food raises blood sugar levels).

All four groups showed about 3 percent weight loss overall over the year — equivalent to a 200-pound person trimming about 6 pounds. But among those who stuck with their diets for the entire year, results were slightly better, ranging from a 4 percent loss for Atkins to 6 percent for the Ornish group. Both the Weight Watchers and the Zone dieters lost about 5 percent.

Modest loss, big benefits

Although this modest weight loss may not be what dieters are looking for, it brought significant health benefits. Risk of heart disease dropped 7 percent in the Ornish group, 11 percent in the Zone group, 12 percent on Atkins and 15 percent for Weight Watchers.

“A lot of people have been concerned that the Atkins diet will cause cholesterol levels to go through the roof,” said Michael Dansinger, director of obesity research at the Tufts atherosclerosis research lab and the lead author of the study. “But we found that total cholesterol dropped by about 3 percent, and low-density lipoprotein went down about 8 percent.”