Weight loss loves company, hints new research.

For every additional 10 people signed up at a clinical center for a weight loss trial, the average person loses half a percent more weight, the study found.

Dr. Arne Astrup of the University of Copenhagen in Denmark and his colleagues delved into a database of 22 weight-loss centers across five countries, gathered in a previous large clinical trial.

Each center served between 4 and 85 men and women, and all of the more than 600 total participants were instructed to follow a nutritionally balanced diet of 800 to 1,000 calories a day. (Typical recommendations for non-dieters range from 1,600 to 2,400 calories per day, depending on gender, age, and level of activity.)

The average participant started the trial weighing 242 pounds, and lost 24 pounds (10 percent of their body weight) after 2 months on the low-calorie diet. Average weight loss among the centers ranged from 6 percent to 12 percent.

Taking into account age, initial body mass index (a measure of weight in relation to height) and sex, the researchers found that 10 more participants at a center translated into a modest half a percent climb in weight loss for the average person at that center. An average 200-pound woman, for example, would have dropped one extra pound over those 2 months.

The idea to investigate such a link came to the team of Danish and Belgian researchers after they noticed the relative performance of weight loss, or bariatric, surgeons. Those who have more experience, they write in the journal Obesity, tend to provide safer and more effective treatments.

Weight loss center size may reflect quality of care in a similar way, Astrup told Reuters Health by email. He noted that investigators, study coordinators and dieticians working at weight loss centers serving large numbers of participants would tend to be more experienced than those at smaller centers.

While he notes that the study is “the first of its kind,” Pedro Teixeira of the Technical University of Lisbon, in Portugal, is concerned that 2 months is too short to determine a diet’s effectiveness.

Teixeira, who was not involved in the study, also suggests that other factors left unaccounted for could explain some of the differences seen across centers. In separate studies, he has identified a set of weight loss predictors, including self-motivation and realistic expectations.

The researchers, too, point to other possible explanations for their preliminary findings. A larger center may have a larger pool of subjects, for example, allowing them to recruit more highly motivated individuals than smaller centers.

Regardless, if an obese or overweight person seeking treatment is given the option, “larger centers with more patient flow are probably the best,” advised Astrup. “Go for the big ones.”

SOURCE: Obesity, online June 10, 2010.