A group of Portland researchers hopes to judge the long-term effect by putting 240 area residents on a diet for 21/2 years.
The study should show which plan works better, even if the difference between the groups' average weight loss is as little as 2 pounds, said Njeri Karanja, the scientist leading the effort.
"Basically, we just wanted to follow what the population was doing" by studying low-carb diets, said Karanja, a senior investigator at Portland's Kaiser Permanente Center for Health Research.
"A lot of patients were coming to their doctors and saying they were following this diet, and the doctors didn't know what to tell them."
Short-term studies have eased some early concerns that low-carb diets would be useless, or would quickly cause kidney, liver or heart problems.
In fact, low-carb diets have beaten other plans for short-term weight loss in several studies.
However, what long-term data exist suggest that low-carb diets are hard to stick to and may lead to less long-term weight loss than traditional low-fat diets.
That plan, developed in large part by Karanja and Kaiser colleagues, was included as an example of healthy eating in the federal Dietary Guidelines for Americans released last week.
Volunteers will be asked to follow their diet's principles, but they will get to make their own choices about buying and eating food.
Health workers will check patients throughout the study, looking both at weight loss and at measures of health, such as cholesterol levels.
She hopes that half the volunteers enrolled will be men, and that 48 volunteers, or 20 percent, will be members of ethnic or racial minorities.
Researchers at Kaiser and Oregon Health & Science University are conducting the study, which is supported by a $3.3 million grant from the National Center for Complementary and Alternative Medicine.