In a head-to-head comparison with a traditional diet, people who stuck to a diet of portion-controlled packaged foods lost almost twice as much weight as those who only got advice on how to trim calories, according to a new study.
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.
"When there's less structure, then participants are making more decisions, and I think too often they're making decisions that undermine their goal to stick to a calorie target," said Dansinger, who was not part of the study.
Special: Losing Weight After Age 50 -- Six Tips to Shed Pounds
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.
Medifast users lost an average 2.24 inches around their waists and their total cholesterol dropped an average 8.4 milligrams, compared to 1.46 inches and 1.1 milligrams, respectively, in the regular diet group.
Over time, though, the dieters in both groups tended to rebound somewhat toward their original weights.
After one year in the study, the Medifast group ended up about 10 pounds, or 4.2 percent, lighter than when they started, while the other group was about four pounds, or 1.7 percent, lighter.
James Shikany, the lead author of the study, said the results show that sticking to the Medifast diet makes it harder for dieters to eat more calories than they should.
"It's more of a regimented type of diet, and some people find that's what they need in order to limit their portion intake," said Shikany, who is a professor at the University of Alabama at Birmingham Division of Preventive Medicine.
Dansinger said the weight loss results from the Medifast diet appear to fall in the middle of the pack of other weight loss interventions.
Some approaches, such as weight loss surgery or more extreme diets can have more dramatic results.
But in terms of dietary interventions or FDA-approved weight loss pills, the effect is roughly comparable, he said.
Lisa Davis, the vice president of science and clinical affairs at Medifast, said earlier studies of the diet actually found greater weight reductions.
One explanation she offered is that the current study participants' demographics, such as race, appeared to be different in Shikany's study than in earlier ones.
In particular, Shikany's study included a majority of African Americans whereas other studies had mostly caucasians, Davis said.
"I think that's worth investigating in the future," she told Reuters Health.
Medifast funded the current study, and one of the researchers has been a consultant to Medifast.
Dansinger said that based on this and other studies, he would recommend meal-replacement diets.
"In my opinion, for patients who have had repeated failures with other attempts at sustaining weight loss, the Medifast plan or other similar plans appear to be a good option," he said.
Shikany agreed, but said it's unclear whether cost could be a factor in the diet's appeal.
"If you take (cost) out of the equation, I think it would be a good option for some people, especially for people who need that extra assistance in weight loss efforts as far as determining portions," he said.
© 2014 Thomson/Reuters. All rights reserved.