Very low-calorie liquid diets do not appear to change metabolism in a way that causes the dieter to regain the weight later, according to a study published recently in the Journal of the American Medical Association.

The finding is based on a study of 18 obese women, half of whom consumed only Optifast, a 420 calorie per day liquid diet manufactured by Sandoz Nutrition Co. The other half of the subjects were placed on a more traditional weight-loss regimen that was limited to 1,200 calories per day.

"End-of-treatment changes in the resting metabolic rate . . . did not differ significantly from baseline in either condition," concluded a team of researchers led by Thomas A. Wadden of the University of Pennsylvania School of Medicine.

Some previous studies have found that severe caloric restriction depresses the metabolic rate, "which is likely to contribute to the regaining of weight so frequently observed after treatment." Other studies have found no such effect, they said.

The women in the Pennsylvania study who were given Optifast lost more weight than those on the less severe diet and their metabolic rate dropped more, the researchers said. After the 48-week study ended, the rates were not significantly different.

The variable that does appear to matter is exercise. Researchers said that exercise in combination with diet seems to result in more favorable results than exercise alone. All of the women involved in the experiment increased their level of physical activity, primarily by walking.