And about 55 percent of the ads included at least one representation that was very likely to be false or lacked adequate substantiation of its promises, the analysis by Federal Trade Commission staff said.
"There are no fast and easy fixes," Surgeon General Richard Carmona wrote in a preface to the study. "The public must adopt a healthy skepticism about advertising that promises miracles and scientific breakthroughs."
Carmona said companies should use real weight-loss results in their promotions and publishers and broadcasters should screen ads they run to ensure they "are based on science and not on wishful thinking."
About 61 percent of U.S. adults are overweight or obese, the report said, and more than two-thirds of all Americans are trying to lose or keep off weight. Consumers spent about $35 billion in 2000 on weight-loss products ranging from books and videos to drugs and diet shakes, the report said.
As CBS News Correspondent Sharyl Attkisson reports, Americans are falling hard for the promise of easy solutions. Weight loss magazine ads have doubled in the past decade. Americans are spending billions of dollars a year on weight loss supplements, but are getting fatter by the moment.
Timothy Muris, Federal Trade Commission Chairman says, "Whether pills, potions, lotions or devices, the only things these quick fix leave lighter are consumer wallets."
Lynn McAfee, of the Council on Size and Weight Discrimination, told CBS, "As long as people buy these products, as long as media outlets are willing to sell them space and lend them their credibility, this industry has little or no incentive to improve."
The Federal Trade Commission today issued a new warning for consumers to look out for these common red flags:
Because there are so many bad ads, the Federal Trade Commission says it can't possibly police all of them. So the government says it's up to consumers to be wary of weight loss ads full of empty promises.
Consumer testimonials and before-and-after photos were common in diet promotions but "rarely portrayed realistic weight loss," the report said.
The FTC conducted the study with the Partnership for Healthy Weight Management, a coalition that includes scientists, government agencies and weight-loss companies.
The groups evaluated 300 advertisements from broadcast and cable television, infomercials, radio, magazines, newspapers, supermarket tabloids, direct mail, commercial e-mail and Internet Web sites. The staff also compared 1992 ads from eight national magazines to 2001 ads in the same publications.
"False or misleading claims are common in weight-loss advertising, and, based on our comparison of 1992 magazine ads with magazine ads for 2001, the number of products and the amount of advertising, much of it deceptive, appears to have increased dramatically over the last decade," the report said.
Many of the ads with likely false claims appeared in mainstream magazines such as Family Circle and Cosmopolitan, the report said.
The FTC staff did not evaluate whether specific claims were substantiated, but the report said many of the promised effects clearly were unsupported by scientific evidence.
Claims that are too good to be true include assertions that a user can lose a pound a day or more for extended periods; that substantial weight loss, without surgery, can be achieved without diet or exercise; and that users can lose weight regardless of how much they eat, the report said.
Consumers "must become more knowledgeable about the importance of achieving and maintaining healthy weight, more informed about how to shop for weight-loss products and services, and more skeptical of ads promising quick fixes," the report said.
Media screening standards also would reduce the amount of blatantly deceptive advertising, the report said.
The FTC also said its efforts against deceptive marketing for weight-loss products have increased.
In April 2000, Enforma Natural Products, which advertised and sold "The Enforma System," agreed to repay $10 million to customers to settle FTC charges that they used false claims about scientific testing.
"With Enforma you can eat what you want and never, ever, ever, ever have to diet again," said the company's ad.
The company promoted its two products - "Fat Trapper" and "Exercise In A Bottle" - primarily with 30-minute infomercials featuring former baseball player Steve Garvey. The company claimed the system could block fat from being absorbed and increase the body's capacity to burn it off.
"By promoting unrealistic expectations and false hopes, they doom current weight-loss efforts to failure and make future attempts less likely to succeed," said Dr. George Blackburn, a professor of nutrition at Harvard Medical School-Beth Israel Deaconess Medical Center.
Blackburn said some weight-loss supplements lack safety warnings and can be dangerous.
Ephedra, a popular herb commonly used for weight loss and bodybuilding, has long been controversial. The FDA has reports of 100 deaths among ephedra users.
The Justice Department said last month that it is conducting a criminal investigation into whether Metabolife International, the nation's leading seller of the supplement, lied about the safety of ephedra.
While the FTC study did not criticize specific products, it provided many examples of false or exaggerated claims.
One product made from the ground-up shells of shrimps, crabs and lobsters was promoted with statements like "Have you ever seen an overweight fish? Or an oyster with a few pounds too many?"
"To lose weight and not regain it, ongoing changes in thinking, eating and exercise are essential," Blackburn said. He said that when people know more about the realities of weight-loss, "fewer will be inclined to waste their money, time and effort on dangerous fads."