First, the good news: They work. The Nutri/Systems, the Jenny Craigs, the Optifasts, the Weight Watchers and all the rest of the parade of mass-market, enrollment-style diets currently on the American scene. Sign up, stick with it and lose weight. If you're faithful, it's virtually guaranteed.

More good news: They're not considered dangerous. Stay within the doctor's guidelines and the diet's strictures and you'll likely stay healthy.

The inevitable catch: The real world. Real meals. Real food. Actual day-to-day eating without diet counselors, group support, weekly meetings, prepackaged foods or strictly limited calorie intake. Going it on your own in the world of restaurants and dinner parties and seemingly limitless food choices.

"These diets all have some really positive points about them," says Bettye Nowlin, a registered dietitian and a spokesperson for the American Dietetic Assn. "They all use behavior modification and all have an exercise component, and you need both of those to be successful. But they're not realistic in the long term. People aren't going to keep up these systems all their lives."

Still, say medical and diet professionals, the current crop of diet plans, many of which have appeared only in the past decade, are far superior to their gimmicky or ill-advised antecedents that emphasize practices such as food combining, single-food diets, fasting and other questionable weight-loss strategies. The newer diets, which shun such oddities and often recommend or require close medical supervision and sound, basic nutrition, have little relation to fad diets such as the nutrient-deficient Last Chance Diet that was briefly popular in the 1970s and was blamed for several deaths.

But the new diets are playing for high stakes--the enrollment-style diets are estimated to be a multibillion-dollar industry--and each offers a distinctive "hook" to attract the approximately 100 million overweight Americans. The maxim of the diet biz in the late 1980s remains: You gotta have a gimmick.

The enrollment-style diets fall into three basic types:

\o7 Liquid, or supplemented fast, diets. \f7 These plans typically are medically supervised, either by the dieter's personal physician or one designated by the diet facility. The regimen involves an initial period of abstaining from solid food and subsisting on the low-calorie, high-nutrient liquid supplement. A normal, balanced diet is then gradually reintroduced during a period of counseling and behavior modification. Optifast and the Cambridge Plan are typical examples.