In the world of fad diets, nothing lasts more than a year or two. This may account for the blank look you get when you mention the liquid protein diet to some people. After all, it has already been replaced by the Scarsdale Diet which has been replaced by the Pritikin Diet.

The public's short memory may also acount for the recent efforts of the Cambridge Diet to gain a toehold in the lucrative diet market, despite the fact that it is similar to liquid protein diet plans which are reported to have caused at least 17 deaths in 1977.

But the Consumer Protection Division of the U.S. Postal Service does have a long memory. The Post Office started mail fraud proceedings against the Cambridge Diet almost as soon as the first ad for it appeared in March. A ruling in the case is expected this week.

Since March, full-page ads have appeared in several newspapers and magazines touting this "ultimate weight loss," which caused one expert witness, Dr. Sheldon Margen, to describe the diet as the "ultimate diet because "it may be your last one."

The company has already filled 75,000 orders; the government is holding at least 50,000 additional orders, worth well over $1 million, pending outcome of the case.

The product is sold from offices in Monterey and Pebble Beach, Calif. Ten meals, enough for a little more than three days, cost $12.45; 120 meals (less than six weeks' supply) is $68.45.

Ads for the cans of powder which must be mixed with water, carry the imprimateur of a British Ph.D., Alan Howard, who is quoted in the ads as saying that the diet is "safe and superbly effective." Howard reveives royalities from the sale of the diet plan in this country. Jack and Eileen Feather, officers in the Cambridge Diet, are already the subject of a grand jury investigation for possible mail fraud violations concerning products such as Mark Eden's Bust Developer, which claims to increase bust size; Slim-Skins and Astro-Trimmer, which claims to reduce body size.

The Cambridge Diet, Howard says, was tested for eight years and according to the ad "actually reduces body fat as fast as fasting or complete starvation . . . yet so delicious and totally satisfies all the body's daily recommended needs of vitamins and minerals."

"Actual hospital tested weight losses of up to 10, 11, 13, even 15 pounds in just one week," are part of the claims.

But on June 11, at a hearing in San Francisco federal court, U.S. District Judge Robert Schnacke issued a temporary restraining order, which temporarily put the Cambridge Plan out of business.Assistant U.S. attorney Richard de Saint Phalle used what he described as "a landmark approach, a new consumer protection tool" which went much further than the usual injunction. This one not only impounded all incoming mail but halted the sale of and advertisement of the product. As De St. Phalle said, it "totally shut them down."

John F. Banker attorney for the Feathers, said the company agreed to the temporary restraining order.

In its complaint the government charged the company with defrauding the public through false advertising. When Judge Schnacke issued the temporary restraining order he said: "We aren't dealing with a poision . . . but a sustained diet with this number of calories without benefit of medical supervision is a deadly combination . . . in the long run it's going to kill somebody. The advertising in its present state is almost, per se, an invitation to disaster."

The judge told the Feathers and their attorney the advertising would have to be reformed or he would issue a preliminary injuction, the next step after the temporary restraining order.

Banker said in a telephone interview that "based on the medical advice the government got, it acted responsibly." Banker said a similar diet, called Modifast, widely sold in Europe and no one has died while on it. According to Banker, "two or three papers have been submitted [in Europe] that say this diet dosen't need medical supervision."

The marketers of the diet powder claim that their product is different from the others that caused the deaths, the most familiar of which was Prolinn, associated with the man who popularized the protein sparing fast diet, Dr. Robert Linn. The Cambridge Diet promoters say that the quality of the protein in their product is better and that the diet, has been hospital tested in England.

According to the advertisement, "The Cambridge Diet has been shown safe and enormously effective. We have had none of the harmful side effects associated with fasting or the so-called liquid protein diets -- man cannot live on protein along. The Cambridge Diet formula is scientifically based on a meatbolically optimized balance of carbohydrate and protein and essentail vitamins and minerals . . . "

But an FDA official said: "We have no reason to believe it was the quality of the protein" that caused the deaths. Taylor Quinn, associate director for compliance said, "We don't know what caused those deaths," but the agency believes it is the amount of calories. "We question anything below 800 calories," Quinn said.

The Cambridge Diet contains only 330 calories a day.

The biggest concern of the judge hearing the case was the advertising and labeling. He emphasized that "the harm lies in the advertising that says it may be used safetly without harm and without medical attention."

Banker agreed that the advertising has not made certain restrictions clear.

"If it had been clear to anyone reading it, we would not have had this big lawsuit."

In the booklet that accompanies the product, the first page contains this notice: "We advise anyone starting on any reduction plan to check with the doctor. We feel sure your own doctor, when he reviews the literature accommpanying your diet, will give you his enthusiastic go ahead and then can monitor and advise you when you have reched your ideal weight . . . "

According to one of the government attorneys on the case, the judge felt that notice was not sufficent. All it means, the attourney said, is "See your doctor, but why bother, he's going to love it!"

Dr. George Blackburn, chief of nutrition metabolism at New England Deaconess Hospital in Boston and one of the country's leading experts on obesity, says that even if people see a doctor, it won't make much difference. Blackburn feels most of them aren't qualified to superivse this drastic diet. In an affidavit Blackburn says: "Americas doctors are not trained in metabolism and nutrition involved with semistarvation and cannot and should not approve of its use." Blackburn believes there is a significent risk in a diet which severley restricts calories.

So, it appears, does Judge Schnacke. The government and the Feathers have been negotiating a settlement, a consent decree, which the judge was expected to rule on yesterday. If the settlement is accepted the Cambridge Diet will have to make substantial modifications in its advertising.

The Cambridge Diet and the government have reached an agreement on most points. They have agreed the diet plan could no longer make any claims which are not supported by reliable sceintific or reliable medical data:

That it's safe to stay on the diet for more than four consecutive weeks,

That the diet will provide all the nutrition the body needs to sustain life

That the formula is a nutritionally balanced diet

That it produces no side effects

That it fulfills 75 percent of the U.S. Recommended Daily Allowances for protein

The company will be required to put a notice in a box in future ads and in the package insert the following statement: "Consult your doctor before starting this diet."

The notice would contain a list of poeple who should use this diet under no circumstances: lactating mothers, pregnant women, growing children, elderly people and those with certain illnesses.

People who have unfilled orders pending will be given the option of reading all the new material and then reordering or getting their money back.