This diet might also help people whose obesity makes it impossible to exercise, the researchers added.
"Obese individuals withshould be urged to lose weight," said lead researcher , head of the Musculoskeletal Statistics Unit at The Parker Institute at Copenhagen University Hospital at Frederiksberg in Denmark.
Samantha Heller, a dietitian and clinical nutrition coordinator at the Center for Cancer Care at Griffin Hospital in Derby, Conn., said that "the question this study brings up is whether the participants can maintain the weight loss they achieved on a formula-based, very low-calorie, supervised diet, in real life, with real food."
For the study, Christensen's team followed 175 obese people suffering from knee osteoarthritis. During the first eight weeks, the participants had only the formula diet, called the , which includes soups and shakes and was limited to 800 calories a day. The participants stayed on this diet for eight weeks.
Following this diet, the patients lost a lot of weight, but also increased their bone mineral density, Christensen said.
Dieticians supervised the participants, Christensen noted.
During the first eight weeks, people lost an average of 26 pounds, according to the study.
"This is achievable for all people with knee osteoarthritis, because 91 percent of all the people who started the trial completed 16 weeks of the trial, so it's feasible," he said.
The findings appear in the Dec. 21 issue of the European Journal of Clinical Nutrition.
While the study's main funding came from two Norwegian foundations, it was funded in part by the makers of the Cambridge Weight Plan. Study co-author Dr. Anthony Leeds is the medical director of the program. The company paid for the dieticians and donated their products, Christensen said.
The diets included the recommended daily intake of amino acids, fatty acids, vitamins, and minerals, the researchers said. It also increased levels of vitamin D, which is essential for bone growth. Levels of vitamin B12 were also boosted, to improve nervous-system functioning of the nervous system and blood production.
Losing weight helped more than 60 percent of the participants reduce their knee pain and improved their ability to walk, the researchers found.
Osteoarthritis results in degradation of joints causing joint pain, tenderness, stiffness and locking. According to Christensen, many weight-loss diets decrease bone mineral density, which can weaken bones, especially among people who can't exercise.
The researchers have followed these patients for a year to see if they have maintained theirand whether their has improved, Christensen said. "The results are looking good," he said.
Maintaining weight loss over time is the challenge, Heller said.
Without learning strategies for managing life's daily obstacle course of stressors, frustrations, temptations and social interactions, it is highly likely that people who lose weight on a formula diet will regain the weight they lost when they stop the program, she said.
"Losing weight is difficult at best, and for people who are overweight or obese the struggle is complex and involves environmental, physiological, psychological and health issues," Heller said.
"People who are motivated to make lifestyle changes should work with trained, accredited, health professionals such as a registered dietitian and their physician," she added.
For more on osteoarthritis, visit the U.S. National Library of Medicine.