Over the long term, there appears to be little difference between the weights of overweight people who take part in diet programs and people who do not, a study showed Friday.

In their study, University of Iowa doctors compared long-term success rates of participants in managed weight programs with the normal weight fluctuations of 332 overweight Iowans who were not in programs.Little difference was found between the two groups, the doctors concluded in their study, published in the December issue of the International Journal of Obesity. Only 12 percent of the people in weight loss programs lost as much as 20 pounds and only 17 percent of that group kept the weight off two years.

The study examined all available published data for the long-term effectiveness of various weight loss programs, including protein diets, carbohydrate diets, dieting support groups and even physician-monitored programs.

"The weight loss programs are relatively unsuccessful," said Dr. Paul Williamson, one of the study's authors. "I'm discouraged about their prospects for success."

The Iowa researchers were frustrated in their efforts to track long-term weight loss for individuals involved in popular commercial weight loss plans. Williamson said all of the weight plan providers refused to divulge any data or said they did not keep track of clients after they ended the program.

Williamson said the study's findings do not mean that weight loss programs cannot be helpful, but consumers should approach them with their eyes wide open.

"If a program motivates or helps a person exercise, then it can be helpful. The program won't lose the weight. It's just a way to help with motivation," Williamson said.

Williamsn said dieters should be committed and motivated before investing time, money and effort into a weight loss program.

"Consumers should ask will it be worth the effort in the long run, not how much will I lose in the next six to 12 weeks," he said.

While gathering data, the Iowa researchers found that dieters who did successfully maintain their weight loss for a long period of time were those who were knowledgeable about nutrition, self-motivated and who exercised regularly.