It's Day One of the diet that just won't die. And like people all over the country, I have embarked on an eating plan that goes against all current nutritional knowledge -- and my better judgment -- but promises astonishing weight loss. Ten to 17 pounds in a week.

Right.

Now I know this is impossible, not to mention undesirable. But the Cabbage Soup fat-burning diet is the current regimen of choice -- Cosmo featured it approvingly last month, GQ cautiously this month. So, in the name of research, of discovering whether the soup diet is a miracle or a mirage, I thought I'd give it a try.

In fact, the diet is not new. It's been around for at least 15 years -- many of the photocopies come from originals made on a typewriter -- though it often drops out of sight. But every once in a while, it surfaces with a vengeance. A one-week program built around a cabbage, onion, celery and green pepper soup, the routine has its success stories. People who swear they have never lost weight so easily. People who have managed to keep the weight off. People who go back to it every time they gain five pounds and need a quick fix.

"I'm almost evangelistic about it," says Susan Carter, an Alexandria real estate agent who two years ago lost 40 pounds on the diet in about four months. "On occasion I think I've lost 10 pounds a week."

Once I started telling people I was on the diet, I learned of others who'd experienced its wonders: a colleague's aunt in Lebanon, a hospital publicist's brother-in-law in Maine, a veteran of a spa week in Zimbabwe, a pilot who uses it to combat weight gained in fattening faraway places.

Their stories impressed me but didn't turn me into a convert. An eating plan that ignores the basic food groups can't be good for you. Nor can one that doesn't encourage portion control. And the program calls for as much of the soup as you want every day, and different accompaniments: fruit the first day; vegetables the second; fruits and vegetables the third; bananas and skim milk the fourth; chicken, fish or beef and vegetables the fifth and sixth; and brown rice and fruit juice the seventh. Sounds like one day of every quirky diet you've ever tried.

Nevertheless, figuring I can stand anything for a week, I down a large cantaloupe, a hunk of pineapple and an unsweetened baked apple and plunge in. DAY 2

It's vegetable day, and the thought of cabbage soup or leafy vegetables at breakfast doesn't do much for me. I finally opt for cooked carrots and fennel slices and realize that I am already worrying about food much too much.

The diet's devotees don't seem to have that problem. They rejoice in the knowledge the program is fast, so rigid you can't make mistakes, and the quantity of food is, for the most part, unlimited. "The thing that is saving you is that you don't ever have to be hungry," says Milly White, a former caterer who now owns a children's store in D.C. "You don't have to go around hungry."

The diet actually encourages large quantities of the allowed foods. "Eat as much {of the soup} as you want . . . This soup will not add calories . . . The more you eat, the more you lose," says the encouragement accompanying many of the versions I've seen. And on the brown rice day: "Stuff, stuff, stuff."

As the day wears on, I find I'm slightly grumpy having to organize my day around cabbage soup and vegetables. And, despite veteran dieters' testimonials to the contrary, I feel hungry. DAY 3

Fruits and vegetables are such a major improvement over just plain vegetables that my spirits have lifted. My energy level, however, has not. And an exercise class feels harder than usual -- a disturbing development that makes you wonder what the reasoning behind this diet is.

But that's hard to pin down because there's no one to ask. Nobody claims authorship.

Several versions of the diet currently circulating refer to a regimen created by Sacred Heart Hospital for overweight heart patients in Spokane. But there is no such hospital. And the well-known Sacred Heart Medical Center in Spokane adamantly disclaims it. "Absolutely 100 percent no, nothing, nada, we never did it, never did research on it, never passed it out," says Tom Sofio, a public relations writer there.

But that doesn't stop people from besieging the center with questions and requests for copies of the diet. In the past six to eight months, there's been a noticeable increase from ordinary dieters. But over time calls have come from people who should know better. From the health and wellness director of a prison in Atlanta, who thought it might help his patients, but wanted to double-check. From a representative of Arnold Schwarzenegger, when the actor was chairman of the President's Council on Physical Fitness and Sports. From doctors.

"We've had prominent doctors call us," says the center's head nutritionist, Elaine Reid, with disbelief. "I quickly say, It's not real; we don't endorse it.' But how could they possibly get through medical school and not know this is bogus?"

Reid sends out strongly worded disclaimers: "This diet plan emphasizes the consumption of fruits and vegetables primarily and excludes the consumption of meat or fish, cereal grains, and milk products on most days," it reads. "Any diet {that} includes only one or two foods and excludes other food groups is low or deficient in essential nutrients. Our experience with diets like this one is that they do not lead to permanent weight loss."

No matter: Witness the army of new recruits ready to try the diet. "I don't know why it won't die," Reid says. "I guess because it sounds like a miracle, people want to believe something so badly that they're willing to try anything." DAY 4

With its regimen of soup, skim milk and bananas, this day is the hardest so far. Breakfasting on a banana is fine, but how many more can I eat? Some versions of the diet suggest a maximum of three, others eight. Halfway through my third, I stop eating -- which naturally promotes weight loss.

"There's no magic to it," says D.C. nutritionist Katherine Tallmadge. "People on the diet are basically starving themselves and filling themselves with watery vegetables. They can't help but lose a lot of water weight. They're doing a modified fast."

If the medical center didn't concoct this torturous program -- and Reid suggests a disgruntled ex-employee may have been the culprit -- who else could have dreamed it up?

One tantalizing notion is that the soup is left over from World War II days, when cabbage and onions were combined in a concoction remembered as "skinny soup."

Reid raises another interesting possibility: Dolly Parton. Early copies of the regime referred to the Dolly Parton Diet. And Reid recalls reading an article in a women's magazine years ago that described a similar program featuring "T.J.'s Miracle Soup" (in fact, some versions that have come my way call it that). As Reid remembers it, the explanation was that T.J. was a member of Dolly Parton's band at the time she lost a great deal of weight.

Calls to Parton's publicists last week initially got denials like "impossible" and "she's been on lots of diets, but not this one." But when the question was finally put to the singer, the elusive response came back, "She has no comment."

Hmmmm. On to Day 5. DAY 5

Suddenly high-protein foods (chicken, fish or beef) are introduced, along with tomatoes, throwing my body into havoc. I don't feel hungry any more, but I don't feel good. And I wonder how a diet that swings wildly from fruit to beef to brown rice (Day 7) could actually work. By any rational nutritional analysis, the regimen makes no sense. Successful weight-loss programs generally prescribe a limited number of daily servings from each of the four food groups. This diet ricochets among them from day to day.

"A diet like this is just not something you can live with," says Tallmadge. "After basically starving yourself . . . you can't help but lose weight. But you'll lose a lot of water weight, and maybe some lean muscle mass. The true key to success is finding a way to eat that you enjoy and can live with, but that also achieves your health goals."

Reid cautions that if the diet had been designed for overweight heart patients, it could actually have been harmful to them for two reasons: The soup recipe calls for Lipton Onion Soup mix, and the high-protein days describe an enormous 20-ounce limit of beef. "The onion soup mix is almost pure salt, and if you're a heart patient eating as much as you want, you could be rushed to the hospital in congestive heart failure," she says. "And you cannot eat that kind of a fat load {in the beef}. It could kill you." DAY 6

I feel weighed down by yesterday's salmon and spinach breakfast and the steak, mushroom and tomato dinner (soup in between), but persevere in the name of research. I have a feeling this approach won't produce the results for me that others have seen -- they probably don't eat a well-balanced diet in the first place.

But the diet has worked well for many people.

James O'Shea, who recently lost 14 pounds in about two and a half weeks, is one of them. The owner of two restaurants in Litchfield, Conn. (Grappa and The West Street Grill), O'Shea is particularly happy on days five and six. That's when he bakes salmon in a little bit of water and soy sauce in a 375-degree oven for 12 minutes and scarfs it down with fresh tomatoes. Or eats steaks and vegetables (no potatoes) to his heart's content.

Bethesda private investigator Susan Giller, who lost 18 pounds by going on and off the diet three times with a few weeks in between each go-round, is another disciple. "Psychologically, it's a brilliant diet," she says. "It's different every day so you don't get bored, and you stay on it only for a week at a time, so you can put off food cravings. The other brilliant thing is that you can do it when it's convenient for you -- not when you have a lot of engagements. I think it's a great diet. I'm back to my college weight." DAY 7

It's my last day on the diet, and all I have to do is get through a day of brown rice, fruit juice, vegetables and soup. The scale reads six pounds less than when I started, but I remind myself hourly that this can't be genuine weight loss.

The experience, nonetheless, has reminded me of a few important things:

There is no diet in which you can ignore portion control -- even this one. In fact, I gained weight on the first beef, chicken and fish day because I ate more protein than I would have on an ordinary day -- even though I was well under the 20-ounce limit.

No matter what kind of life you have, some menu planning is necessary for good eating.

And maybe it wouldn't be a bad idea to adopt the once-a-week all-fruit-and-vegetable day.

As for the two big containers of cabbage soup in my freezer, I don't think I'll look at them for a long, long time. SEVEN DAYS OF THE CABBAGE SOUP DIET

This is the 7-day diet. All versions of it say to eat as much soup as you want any time you are hungry throughout the week. Add the following foods.

DAY 1: All fruits except bananas. Cantaloupe and watermelon are lower in calories than most fruit.

DAY 2: All vegetables, raw or cooked. Try to eat leafy green vegetables and stay away from dried beans, peas and corn. You may also have a large baked potato with butter on your vegetable day.

DAY 3: Fruits and vegetables. No potatoes or bananas. DAY 4: Bananas and skim milk. Eat as many as 8 bananas and 8 glasses of skim milk.

DAY 5: Beef, chicken without skin, or fish. You can eat 10 to 20 ounces of beef, chicken or fish and 6 tomatoes. Drink 8 glasses of water. Be sure to eat the soup at least once.

DAY 6: Beef, chicken without skin, or fish and vegetables. No potato. As on Day 5, drink the water, eat the soup.

DAY 7: Brown rice, vegetables and unsweetened fruit juice. THE DIET'S SOUP CABBAGE SOUP (About 6 quarts)

While the ingredients for this soup are basically the same, the quantities of each vegetable seem to change according to what worked for the cook who typed it up (some called for 6 large onions, others a bunch of scallions). Given the result, it doesn't seem to matter.

1/2 to whole head of cabbage

1 onion, or 6 onions, or 6 green onions (scallions)

16 to 28 ounces canned crushed or whole tomatoes, or fresh tomatoes

2 green bell peppers

1 stalk to 1 bunch of celery

1 to 2 packages Lipton Onion Soup mix

Black pepper to taste

Herbs to taste

6 carrots (optional)

16 ounces green beans (optional)

to 1/2 cup balsamic vinegar (optional)

Small can V-8 juice (optional)

Slice vegetables and cover with water in a large pot. Add soup mix. Boil gently for 10 minutes, cover, lower heat and simmer until vegetables are soft. Add pepper and whatever herbs you like. If you like, add fresh herbs (such as cilantro or basil) at serving times. Since the soup will last several days, when the liquid gets low, you will probably want to add more water and a beef or chicken bouillon cube or two.

Eat as much of the soup as you like as often as you want.

Per cup with the minimum additions: 34 calories, 1 gm protein, 8 gm carbohydrates, trace fat, 0 mg cholesterol, trace saturated fat, 64 mg sodium