YOU'D THINK FROM ALL the gossip items and national publicity that everyone in the country was suddenly on "The Last Chance diet." Last May, a chic Washingtonian (too chic to let her name be printed here) was sneaking up to Pennsylvania to "The Last Chance Diet" clinic and overheard Jackie Onassis call to make an appointment for herself and daughter Caroline -- a sure sign that the country's newest ploy to beat the Toledos was also the vogue way to the perfectly starved figure. In fact, it is exactly that starving. The perfection part, though, is cause for a dieting world dispute, and some consumer revenge.

In Washington, where a few months ago, "The Last Chance" people opened a swank clinic in Foxhall Square, a lot of pretty people have followed Jackie O's vanguard. Betty Lou Ourisman thinks it was "great." She lost twenty-five pounds and only gained eight back. Barbara Eagleton lost nineteen pounds in four weeks. Nancy Dutton lost twenty-three pounds in two months and Marvin Sadik, director of the National Portrait Gallery, lost forty-five pounds in two months. Rita Merthan, Rosalynn Carter's personal secretary, has taken the starvation oath and Louise Gore, at last check, was still losing, beating the scales down past thirty-five pounds. . . past forty. . .

Starving is easy enough. You just . . . close your mouth. But, no, it's not perfect.When you don't eat, eventually (sometimes right away) your good lean tissues (like muscles) break down instead of the fatty ones. "The Last Chance Diet" claims to solve that problem by prescribing a liquid protein that helps save your body and your health while you fast.

In Washington, the protein formula, first commercially developed by Dr. Robert Linn, is sold out constantly at drug stores. It's sold over the counter as a food supplement. Anyone can buy it. And, anyone can learn how to do the diet by reading Dr. Linn's The Last Chance Diet, a best-selling paperback most people think is a self-help book. Which it isn't.

The diet is most definitely not supposed to be undertaken by anyone alone, which is to say without a doctor's supervision, and a lot of people in the medical profession, notably Linn's former colleagues, question whether Linn should have written the book in the first place.

George F. Cahill Jr., M.D., a professor of medicine at Harvard University and head of Boston's Joslin Diabetes Foundation, pioneered the original protein-sparing biochemistry. Three organic compounds that are categorized as "ketone bodies" are found in the blood and urine when glucose is no longer the body's main fuel. In 1967 Dr. Cahill and his associate, Dr. Oliver Owen, determined that the brain could use these ketone bodies for energy exactly as it used glucose. Previously it had been believed that "ketosis," the production of ketone bodies, was a harmful and undesirable condition. It had also been thought that glucose was the brain's only source of energy. Ketones also act as an appetite suppressant, encouraging the faster to abstain from eating.

Dr. George Blackburn, another Harvard professor who is director of nutritional support services at New England Deaconess Hospital, turned the Cahill-Owen discovery into a weight-losing system.

When asked what he thinks of his concept growing into a fad diet with a $1.95 paperback best seller, Dr. Blackburn says, "My major concern now is all the people who are undertaking to follow this program on their own. The protein-sparing fast should only be done under medical supervision. It was ill-advised, non-productive, not constructive and not responsible of Dr. Linn to have written that book."

Linn is the first to admit that his diet is meant to be used only under medical supervision, because there are some people who should not fast under any circumstances -- among them pregnant women, people with malfunctioning livers, people with kidney failure and people with certain types of heart disease. Linn says he stresses this warning on the back cover of his book with a "Caution: the diet and the products recommended in this book should be used only under medical supervision."

But that doesn't matter, Blackburn argues. "I certainly don't think that a sub-optimal manual of a comprehensive medical program with a soft disclaimer warns the public of all the serious risks they run doing this program on their own. Why, if he warns the public so well, why did New York get that bookstore to issue refunds on Linn's book after consumers realized only on page 89, or wherever at the end, that protein-sparing wasn't a self-help diet?" (The State Attorney General's office, consumer Fraud Division, recently entered into "an administrative agreement" with Walden Books whereby consumers who wanted a full refund on Linn's book got one because Walden had not mentioned in its advertisements that The Last Chance Diet was not a self-help program.)

Dr. Cahill, while not as harsh as Blackburn in criticizing Linn, doesn't like the fact that some young women in the 110 pound range, who have tendencies towards developing anorexia nervosa (a serious aversion to eating, where the patient diets into emaciation and sometimes death), will get carried away with TLCD.

"Listen," Cahill said, "the protein-sparing fast isn't going to kill you if you go on it by yourself for say a week, but very often hidden metabolic disturbances or conditions that went undetected are aggravated by the fast -- and then, yes, you have a problem. I can understand and appreciate [Blackburn's] concern."

Affectionately called "Fat City" by staff and intimates, the Washington clinic is Dr. Linn's latest.

Since it opened in a fourteen-room suite in January, more than three hundred well-heeled Washington fatties have paid Dr. Linn and his staff anywhere from $100 to $150 for the initial visit (depending on lab work) and $60 per week for the duration of their individually designed programs. At the very least, all patients participate in Dr. Linn's program for thirty days and many patients come weekly for months.

People entering the clinic are greeted by a cluster of white-garbed minions who hover around the front desk where new clients are asked to "sign in please." Other lists have "Have you registered for our exercise class?" and "Please sign up if interested in meeting with our behavioral psychologist."

Immediately, a new patient is asked to fill out a six-page, 130-item health questionnaire: Do you have constant noises in your ears? When you catch a cold, do you always have to go to bed? Do you sometimes have severe choking sweats at night? Are you often bothered by a thumping of the heart? Have you lost more than half of your teeth? Do you usually belch a lot after eating? Have you ever had piles?

The reception room is huge, thickly-carpeted and filled with six sofas. Only three patients are at the clinic this morning. On one couch sits a young man who looks to be in his twenties, weighing a good 250 pounds. (Four out of ten patients at the clinic are men.)

He wears a bright pink polyester leisure suit and a pink and lime green nylon sport shirt that pulls at the buttonholes. He is nervous and taps his right foot incessantly while sighing low, dolorous breaths of desperation, although he has already lost twenty pounds and has been on the program just two weeks.

Another patient could be any of a hundred Washington matrons: a tall, handsome woman in her indistinguishable forties or fifties with rather "horsey" features and perfectly streaked blond hair that bespeaks professional care. She's about twenty-five pounds overweight but carries it well.

The third patient in the waiting area is a pathetic creature easily weighing 300 pounds. All her features have been distorted by rolls of fat. A nurse helps her to get off the couch.

Another white-garbed woman takes the questionnaire that has just been completed while a different nurse with a stethoscope around her neck beckons the new patient into one of the clinic's four examining rooms. Now a complete medical history and patient evaluation will be conducted before the doctor's physical.

After the physical, the patient meets with the clinic's dietician who compiles a dietary history noting, among other things, bad eating habits, the heaviest and lightest weight of the patient over the last five years and past diet success or, most likely, failure. The new patient is then given a "food intake record" which he is to complete over the next week indicating his eating patterns.

Maxene Kaizen, Linn's director of dietetics and nutrition, explains that most constant dieters suffer from appalling eating habits, general ignorance concerning nutrition basics, and eating in response to emotions rather than hunger. She goes on to say that in Washington her patients tend to be stricter with themselves during the day and then eat and drink to excess during the evening at receptions and restaurants.

Dr. Linn is a small, dark, intense man. He graduated from the Philadelphia College of Osteopathic Medicine in 1959, and for five years was a general practitioner in a small suburb outside of Philadelphia called Broomall.

At the present time there are only seven licensed osteopaths in the District including Dr. Linn, compared to 9000 doctors of medicine. Linn's District license was granted by reciprocity based on his original licensing examination in Kentucky in 1959.

Osteopaths are not allowed to practice at any hospital other than their own without prior approval, and no hospitals in the District will allow an osteopath to admit patients, which could make being treated here by an osteopath risky business. (However, Linn's Washington associate, Arthur Frank, is an M.D. and eliminates any problem for him in that regard.)

Linn says he switched from being a general practitioner to a diet specialist as a direct result of the "doctor heal thyself" syndrome.

One hot day ten years ago he says, "as I was meandering over to the eighteenth tee, my heart started tomtomming as if I'd just run a mile in under four minutes." Anxious and dizzy, he abandoned the golf course for a hospital emergency room. Although he had not had a heart attack, at thirty-two, the 235 pounds "rotundly distributed" on his five-foot-eleven frame was serious enough for him to admit to himself that he had a weight problem and had to face the reality of dieting.

"I turned to the medical texts," Linn explains, "only to find the diet literature primitive and depressing. So i began to experiment on myself. One type of dieting versus another, until finally, six months later I was down to 165.

"I know what it means to be fat," he explains with sentimentality that borders on the maudlin, "and I know how hard it is to diet and keep the weight off, or how hard it used to be. But ten years ago, George Cahill hadn't started his association with ketones and George Blackburn knew as much about protein-sparing as I did --ago."

Today, Linn says he wants to give the treatment of obesity credibility and has just organized a nonprofit, tax-free foundation called the Nutrition Research Foundation for the dissemination of information on nutrition to the general public. And he says that he's implementing an indigent program for Washington's chronic obese who can't afford Foxhall Square, and a pediatrics program. He claims that forty per cent of his practice is medical referral, all of which may be true but does not help explain away professional doubts about the protein-sparing fast and Linn's handling of "The Last Chance Diet."

Despite all the controversy, medical researchers agree on one thing: if you want to lose weight and stay slim you must eat less, and move more, in order to expend more calories than your body consumes. A pound of fat is still 3500 calories and you still have to use up 3500 calories worth of energy to make it disappear. Which brings us back to learning how to eat less over the long term.

One of Linn's most vociferous fault-finders is now Lyle Stuart, who originally published the hardcover version of his book. In January 1976, after reading an article in Vogue that mentioned the revolutionary approach to dieting at Linn's clinic in Broomall, Stuart drove "120 miles for my first appointment" because "it was my last chance." He signed Linn up as an author and wrote a glowing introduction. But now Stuart is bitter about his experience. He gained back a "considerable amount" of the weight he lost as soon as he went off the diet. He feels Linn's program "was a failure for me," because at the time the doctor wasn't concerned with behavior modification.

Now, Linn calls "the re-feeding, reentry stage of the protein-sparing fast" the most important part of his program. Because this stage is so crucial to the success of the dieter, who must re-learn how to eat solid food after weeks or months on liquids, it is where most self-helpers on this regimen run into trouble.

When you're doing this diet on your own (and I have), there is a very real fear that once you start eating again, you're going to keep on eating and eating and eating. There is a very real possibility that all the misery will have been for naught. Under Linn's supervision, however, the support system cuts down the likelihood of failure. Working with the dietitian, the patients are able to see for themselves that undesirable eating patterns must be changed. The dieter gains a new perspective: learning to control food and not use it as an emotional support.

Linn claims that over the last two years more than eighty per cent of his patients who used the protein-sparing fast and the behaviour modification retraining have kept all or almost all of their lost weight off.

Eighty per cent. The figure is amazing since in a controlled hospital experiment at Johns Hopkins in 1963 (monitored by Dr. Neil Solomon) only five per cent succeeded in holding their weight down. But Dr. Linn insists he has eighty per cent success with his patients and his diet.

Will you have success? Well, how did you do on The Egg, the Grapefruit, the Banana and Skim Milk, the Rice, the Drinking Man's, the Magic Mayo, the Dr. Stillman Quick Weight-Loss, the Dr. Stillman Quick-Inches-Off, the Dr. Atkin's Revolutionary, the Dr. Solomon Easy, the Vogue, the Redbook, the Cosmopolitan, the Ladies Home Journal, the Weight Watchers, the Ayds Plan, the Anti-Cellulite, the Calories Don't Count, the Calories Do Count, the Ice Cream, the Lecithin, B6, Apple Cider Vinegar and Kelp, the Macrobiotics, the Vegetarian, the Simeon's HCG (Human Chorionic Ganodotropin) Injections, the Diuretics, the Amphetamines, the Starvation and Deprivation, the Mini, Midi, Maxi diet! Each was the answer, the "perfect" diet until the next one came along. The the chances are you tried that one too in order to lose the weight you regained after the "perfect" diet the year before. I know I did.

As far as diets go, Linn's "Last Chance" may just be the answer. However, if upon serious reflection and critical self-analysis you discover, as I did, that patience rather than discipline is your strongest virtue, the future is ours. Doctors in Chicago have just developed a drug they claim will "make dieting obsolete." Called perfluorooctyl bromide, the drug is purportedly the miracle we have all been praying for, coating the lining of the stomach and intestines, causing food to pass out of the body rather than become deposited as fat. A thought as comforting as a hot fudge sundae with extra whipped cream.

All things come to those who wait.