She looked great in her black turtleneck, springing around the stage, showing off her new, lithe, sexy body. Oprah Winfrey seemed positively giddy about showing off the size 10 jeans she'd been saving for the day she would be thin enough to wear them again.

Winfrey caused a sensation last month on her syndicated talk show when she told her cheering audience that she had lost 67 pounds in just four months on a doctor-supervised program called Optifast.Since July, with only two exceptions, she had eaten nothing but five packets a day of Optifast powder mixed with water. She had seen a doctor weekly for tests, attended weight-loss lectures, and gradually worked up to running 61/2 miles a day.

This woman, who has triumphed over a miserable childhood of abuse to become one of the nation's richest, most successful entertainers, said losing this weight has been the biggest accomplishment of her life. She even wheeled 67 pounds of animal fat in front of the camera to heighten the drama.

For Sandoz Nutrition Corp. of Minneapolis, which makes Optifast, the result was a publicity goldmine. Sandoz knew the show was coming; it bought time midway through to announce a toll-free number for referrals to hospitals or clinics with Optifast programs.

Wave on wave, the calls came.

One million in all, according to telephone records. One million. The first day. Most didn't get through, but 100,000 did the first day and another 100,000 soon after.

Winfrey's example may help lots of those people shed pounds, but weight-loss researchers say the timing and the content of her message were less than ideal. One who feels this way is Dr. George L. Blackburn of New England Deaconess Hospital, who was a consultant to Winfrey on her weight loss.

Winfrey probably didn't mean to, but what she said fed two of the biggest weight-loss myths. The first is that it's losing the weight that matters. And second, that thin is good, but thinner is better.

If only Winfrey had waited a while longer before doing the show, Blackburn says, she could have championed the real fight, the real triumph: keeping off the weight you've lost.

In fact, researchers say, losing weight is fairly easy. It's keeping it off that counts. Anyone who is overweight has probably dieted away more than his whole body weight over the years and regained it. Winfrey began her show with a long list of the diets she's been on; she said that in seven years of dieting, she had gained 70 pounds.

And while Winfrey was significantly overweight, making her weight a health concern, most Americans don't need drastic diets like Optifast or similar programs.

If you're healthy and just five, 10 or 20 pounds overweight, especially if the weight is concentrated on your hips, buttocks and thighs, you probably don't have a health reason for losing weight, says Thomas A. Wadden, a University of Pennsylvania psychologist. Wadden thinks such people usually are better off accepting their bodies the way they are instead of trying to look like skinny teen-agers.

It's not that Optifast is dangerous, researchers say. For the right people, very-low-calorie diets, either milk shake-type formulas like Optifast or sparse food regimens, are considered both safe and effective when undertaken by seriously overweight people under strict medical supervision and combined with behavior therapy to teach new eating habits. Only people 30 percent or 50 pounds over their ideal weight are potential candidates for Optifast.

Liquid protein diets, which caused a scandal and dozens of deaths in the late 1970s, were far less nutritious than the current milk-protein based formulas and often were undertaken without any medical care. Doctors are adamant on this point: people, especially those with heart, liver or kidney disease, can die on severe diets without proper medical supervision.

The appeal of very low calorie diets is speed; the Optifast program lasts six months, starting with 12 weeks of drinking formula only, followed by a period of reintroduction to food. It costs about $400 a month, which may be covered by insurance.

Optifast has been around since 1976 and has plenty of competitors, too. But from the reaction to Winfrey's show, you'd think we'd never heard of a diet before. We have heard. And tried. Scarsdale. Liquid protein. High protein. High carbohydrate. Weight Watchers. Grapefruit. Beverly Hills. Rainbow. Pritikin. From sound to silly, from sober to stupid, we've tried everything to take off pounds.

What we have not accomplished, as a nation, is keeping it off. There is twice as much obesity in the United States now as in 1900, even though we consume an average of 3-10 percent fewer calories, says Wadden.

We are sedentary. We sit, drive, watch TV. Having an extension phone saves 70 miles a year, accounting for 2 pounds of body weight, Wadden says. Using a manual typewriter at work, if you could find one, instead of a computer would keep off 4 pounds. "All those energy-saving devices are conspiring to make us obese," says Wadden.

Our sedentary lifestyles, eating habits, and for some of us, probably our genes, too, lead most of us to regain the weight we've lost dieting.

In a recent study, Wadden found that dieters using a very-low-calorie diet like Optifast or a regular low-calorie diet of 1,200 calories a day lost about 30-40 pounds. After a year, they had kept off only a third of the weight they had lost. When their diets were combined with behavior therapy, they maintained about two-thirds of the weight loss.

After three years, the group without behavior therapy had kept only 5 pounds of their weight loss off; those who also had had behavior therapy had kept 15 pounds off.

Gaining, losing and regaining weight, or "yo-yo" dieting, is the subject of intense interest at the moment. Is it better to lose weight and regain it than never to have lost at all? It's not clear.

Nor is it clear what makes people obese in the first place. Johanna Dwyer, director of the Frances Stern Nutrition Center at New England Medical Center, says fat people do seem to be unusually sedentary, but they don't necessarily eat much more than thin people. Genetics almost certainly play a role. Moral stamina almost certainly doesn't. Says Dwyer: "It's not willpower. That willpower business is baloney. A lot of thin people have no willpower whatsoever."

Without a known cause for obesity, a cure is not in sight. Dwyer says the most optimistic five-year cure rates for massive obesity run about 15 percent. "It's really a difficult problem. There isn't any diet, whether it's brand name or not, that's going to work over the long term, at least that we know of now.... In the end, what is probably going to be necessary is a radical restructing of lifestyles for most people - much more physical activity and much more moderation in food intake."

So what should you do? Blackburn, who is chief of the nutrition metabolism laboratory at the Deaconess Hospital, advises severely obese patients to lose half their excess weight; his research shows they will gain most of the possible health benefit that way and may be better able to maintain the loss than if they shed all the overweight. He says it would have been better for Winfrey to have lost 35 pounds, kept it off for three or four months, then lost the rest of the weight.

If you only want to lose 10-30 pounds, Wadden says, the best way is to trim 500 calories a day from your diet, presumably from the fat and sugar you eat, so you'll lose about a pound a week. And get more exercise; walk, climb stairs, just move. If you change what you eat too drastically, you're unlikely to be able to stay on the diet.

If you're adamant about trying a quickie liquid formula diet, you'll find several brands in drugstores. Blackburn says these formulas are all right providing you're in good health (check with your doctor) and you don't use them as your only food. You need at least one regular meal a day in addition to the liquid formula unless you're under direct medical supervision, he says.

But Wadden says he wishes women weren't under such enormous pressure to get skinny. Women who have gained 10 or 15 pounds since their teens, he says, "should not be tyrannized by having to look like 15-year-olds . . ."