Dr. Michael Dansinger, a nutrition expert at Tufts Medical Center, said having less freedom in choosing meals seems to help people meet their dieting goals.
The diet the researchers studied was the Medifast 5 & 1 Plan. It consists of five pre-packaged meals each day, along with one meal of vegetables and protein prepared by the dieter.
Dieters can pick from 70 different packaged foods to create five meals totaling about 1,000 calories a day.
People on the plan can also purchase different levels of support along with the meals, such as access to dieticians and recipes. Typically, the plan costs about $300 a month.
To see how effective the Medifast diet is in helping people lose weight and keep it off, the researchers asked 60 people to join the plan for free.
They compared these dieters to another 60 people who were given advice on how to meet a 1,000 calorie-per-day target, but who continued to buy and prepare their own food.
For everyone in the study, the goal for the first 26 weeks was to lose weight, and during the second half of the study the goal was to maintain weight.
All of the participants were obese, having a body mass index (BMI) - a measure of body size relative to height - between 35 and 50.
A BMI of 35 represents, for example, a person who is 5-feet 8-inches tall and 230 pounds.
By the end of the study, 15 people dropped out of the regular-food group and 10 dropped out of the Medifast group.
After six months, those who stuck with either dieting approach lost weight.
People in the Medifast group shed an average of 16.5 pounds, or 6.7 percent of their starting weight.
Dansinger said such a reduction in weight is modest, but it can have a meaningful impact on people's health.
"That's been demonstrated in numerous studies to be effective for improving blood sugar in people with diabetes or delaying the progression from pre-diabetes to type 2 diabetes and to reduce other heart disease risk factors, including blood pressure, cholesterol and inflammation," he told Reuters Health.
The regular-food group lost an average of 8.4 pounds, or 3.4 percent of their body weight.
The Medifast dieters also had greater reductions in body fat, waist circumference and cholesterol than the other group.
Of the average 16.5 pounds Medifast dieters lost, 14 were fat mass. Regular-food dieters lost an average of 8.14 pounds of fat. The study did not examine the diets' effects on bone and muscle mass.