The mad need for Americans to diet has produced a booming business in exercise studios, fat clinics and self-help groups like Overeaters Anonymous.

Many of these are helpful to some people some of the time, but according to one speaker at last week's National Institutes of Health consensus development conference, they have a huge dropout rate -- an estimated 50 percent quit in the first few weeks and another chunk fails to finish.

The preoccupation with thinness has produced a subculture of purveyers of magical cures, easy answers and instant results, almost none of which do anybody any good, and some of which are dangerous.

* Over-the-counter diet pills have been shown to be useless in long-range weight programs and contain drugs and chemicals that some believe are potentially dangerous. They were cited by Dr. Jules Hirsch at his press conference presenting the NIH consensus statement last week as an example of fast-selling weight products whose effectiveness is dubious.

* Low-calorie regimens such as the Cambridge Diet and Herbalife products are sold only through sales representatives in a multi-level marketing set-up in which customers are encouraged to become salesmen themselves, as are their customers and so on, with promises not only of healthy slimness, but healthy bankbooks as well.

*All very-low-calorie programs are potentially life-threatening because in some people, some of the time, they can induce cardiac irregularities. The Food and Drug Administration has warned that "any diet that provides fewer than 800 calories a day should be considered experimental and should be undertaken only with proper medical supervision." Herbalife maintains in a suit against the FDA that its formulas provide 1,000 calories a day.

Herbalife has sued the FDA for what it claims is "a widespread, even corrupt, 'trial by publicity' campaign to misinform the public by false and defamatory statements," among other similar charges.

In a general article on the hazards of herbs and other so-called natural ingredients in health foods and diet aids, the FDA Consumer reported that some herbs can be toxic. And in a formal "Talk Paper," a common method used by the FDA to respond to queries about products, the agency said it had had a number of complaints that users of Herbalife products had nausea, diarrhea, constipation, headaches and stomach cramps, which went away when use was discontinued.

Herbalife literature predicts such symptoms in some users, claiming that it is the body "housecleaning" itself. At least one of the products in the Herbalife regimen contains aloe vera, known to have laxative effects.

* Products labeled as containing the peptide CCK (cholecystokinin) have been advertised recently for sale by mail. CCK researchers, including NIMH's Jacqueline Crawley and Dr. Steven Peikin of Jefferson Medical School in Philadelphia, warn that CCK, taken orally, is metabolized too quickly to have an appetite suppressing effect. Moreover, studies have demonstrated that a rapid tolerance to its appetite-suppressing effect is quickly built up even when injected in the abdomens of experimental animals.

The U.S. Postal Service is currently investigating the possibility of mail fraud charges involving some of these oral CCK products. Injunctions against using the mails to sell one of the products -- Anorex-CCK -- has been granted pending the outcome of Postal Service hearings currently under way.

Crawley has also written the Food and Drug Administration warning that even if CCK actually did reach the bloodstream, "there will be great risk of severe intestinal cramps, nausea and vomiting, common side effects of CCK."