For many dieters, reducing their weight is a never-ending cycle of dabbling with diets.

About one-quarter of the U.S. public is more than 20 percent overweight, according to the National Center for Health Statistics. About half of all U.S. women diet; 27 percent of U.S. men do.Perhaps more significantly, women who try to lose weight average five weight-loss efforts per year, according to a study by Marketdata Enterprises, a market research and consulting firm.

"Yo-yo" dieting - that on-again, off-again, on-again cycle - is not the way to lose weight, say experts. In fact, too much dieting may well cause weight gain, as so many dispirited dieters have discovered.

In desperation, many turn to the $33 billion-a-year weight-loss industry. This ever-growing market includes weight-loss products and services such as spas, exercise clubs, artificial sweeteners and diet drinks.

Commercial weight-loss clinics constitute about $1.8 billion of this market; non-commercial hospital-affiliated programs about $2 billion. Marketdata estimates that 13,000 weight-loss programs operate nationwide.

Increasing concern about the unregulated nature of the weight-loss industry led to congressional hearings last year chaired by Rep. Ron Wyden, D-Ore., of the House small business subcommittee on regulation, business opportunities and energy.

At a hearing, Wyden said, "For those dieters who are not morbidly obese, slow and steady is the way to go, but you wouldn't know it from the hard-sell advertising employed by many diet companies."

At the same time, more than a dozen well-known diet programs are being investigated by the Federal Trade Commission in regard to their advertising and promotional claims about weight loss.

If the claim sounds too good to be true, that's because it is, most experts say.

"A lot of ads promise lots of wonderful things. But the only way to lose 30 pounds in three weeks permanently is to cut off your leg," said Paula Nessa, a registered dietitian who works as a private consultant. "Realistically, the most you can lose a week is 2 pounds. If you have a lot to lose, it's going to take a long time."

For many health professionals, the whole concept of dieting is inappropriate.

"The phrase `going on a diet' means there's the full intention of going off it," said Sue Hanson, registered dietitian and nutrition program manager for the Park-Nicollet Medical Foundation, which promotes low-fat diets combined with moderate exercise.

Weight loss can be maintained only if a diet is permanently incorporated into one's lifestyle, experts said.

"Weight loss is not difficult. Keeping it off is," said Karen Holtmeyer, registered dietitian in private practice and past president of the Twin Cities Dietetic association. "Anyone can grit their teeth for two to three weeks (on a low-calorie diet). But if you're going to keep it off, it has to be something you can stick with for the rest of your life," added Nessa.

The rate of permanent weight loss is not encouraging. About 95 percent of those who lose weight will regain it in one to two years, according to many scholarly studies.

This figure may overstate the difficulty of maintaining weight loss, said Hanson, because the data is gathered at centers that work with repetitive, diet-resistant clientele. Still, the number reflects the difficulty dieters have in maintaining their weight loss.

Chronic dieting may be ineffective because of its effect on the body. Some research indicates the more one diets, the less one loses. That's because the human body can't tell the difference between starvation and dieting.

So what's a desperate dieter to do?

Let the dieter beware: There are no official standards for weight-loss programs, which are not licensed or regulated in any other state.

But the FTC has developed guidelines for the public to use when evaluating weight-loss options. The free brochure, "Diet Programs," is available from the FTC's Public Reference Branch, Room 130, 6th and Pennsylvania Ave., N.W., Washington, D.C. 20580. The FTC recommends that dieters ask these questions of diet programs before participating:

- What does the diet regimen include as far as calorie restrictions, special foods or vitamin supplements, counseling and exercise services?

- What health risks are associated with a diet program? Some diet programs are riskier than others, or may be better suited to certain individuals.

- What are the costs and how are they paid? Many people drop out of diet programs early but may have already paid a large fee.

- Is there professional supervision? Don't be swayed by the appearance of white smocks or the titles of "nutritionist" or "dietitian." Those phrases are not regulated and can be used by anyone. A registered dietitian, on the other hand, requires accreditation, a college degree in food science and nutrition, and continuing education.

- What type of maintenance program is offered, if any, and at what cost? Long-term success usually depends on this.