Johnston agreed that with such small differences, the best weight-loss choice is the one you think you can stick with. "Choose the one that gives you the fewest challenges as far as adherence," he said.
But as far as researchers are concerned, Johnston said, "what we really need to understand is, how can people best maintain the initial weight loss?"
For the study, Johnston's team analyzed data from the clinical trials testing various diets -- sometimes in combination with exercise and behavioral counseling. Some studies included people who were obese but healthy; in others, people had obesity-related ills, like type 2 diabetes or heart disease.
Many of the studies focused on lower-carb diets, like Atkins, South Beach and Zone -- where people were told to get, at most, 40 percent of their calories from carbs. Low-fat diets, like Ornish and Rosemary Conley, required people to get no more than 20 percent of their calories from fat, and about 60 percent from carbs.
At six months, people in those trials lost a few pounds more than people in studies of Weight Watchers and other more moderate diets -- which capped fat intake at about 30 percent of daily calories, and carbs at 55 percent to 60 percent.
Across the studies, people generally lost a few extra pounds if the program explicitly told them to exercise, or offered behavioral counseling at least twice a month for the first three months.
According to Van Horn, the findings show there is nothing "magic" about cutting carbs or fat, or adding protein. "The laws of thermodynamics still apply," she said. "Weight loss happens when you consume [fewer calories] than you need. Increasing physical activity helps to lose that weight more steadily, but only if you do not compensate by eating more."
But while shedding pounds is healthy for obese people, it's not everything. "Our study looked only at weight loss," Johnston pointed out. "So we're not necessarily talking about what's best for your health."
Van Horn agreed. "Good health is more than weight loss or weight control," she said.
People need to eat a variety of foods -- including fruits, vegetables, fiber-rich grains, lean proteins and unsaturated fats (from sources like vegetable oil, fatty fish and nuts) -- to get the nutrients that support good health, Van Horn said.