NEW YORK, May 8 (Reuters Life!) - Eating a low-calorie soup or salad at the start of a meal to curb hunger could be the key to dieters' success, according to a U.S. rating of eight popular diet plans.

Consumer Reports magazine, published by a non-profit consumer research group, rated eight diet plans that have been studied in clinical trials, with results after three to six months and after 12 months and at least 40 subjects per diet.

Top of the list came the Volumetrics diet created by U.S. nutritionist Barbara Rolls. It encourages dieters to fill up on low-calorie foods like fruits and vegetables and consume a low fat soup or salad before a meal to take the edge off their hunger.

"Other diets, while not as explicit about employing this promising strategy, recommend ways to reduce calories while consuming larger volumes of food to stay satiated," said a statement from Consumer Reports.

The magazine also gave high marks to Weight Watchers, which uses weekly meetings and weigh-ins for motivation and to support diet and exercise changes. This diet scored average on weight loss but first in long-term adherence.

The Jenny Craig diet, which enlists dieters to sign up for individual counseling and meal plans at company outlets, by phone, or online, ranked highly.

But a study of client histories revealed high dropout rates despite minimal food preparation. But people who stuck with the plan lost considerable weight.

Slim-Fast closely followed Weight Watchers and Jenny Craig in the ratings list. Slim-Fast is a brand line of controlled calorie shakes and bars, widely available in drugstores and supermarkets.

Consumer Reports said clinical studies showed above-average long-term weight loss but a high long-term dropout rate.

Consumer Reports listed top dieting strategies based on research and statistics gleaned from the National Weight Control Registry, which enrolls people who have lost 30 pounds and kept the weight off for at least a year. They were:

1. Eat a substantial morning meal

2. Crank up the activity

3. Fill up on "low density" or low calorie foods

4. Step regularly on the scale

5. Bore yourself thin as the more monotonous the diet, the less one will eat