Somehow the image of pounds just slipping away with hardly any effort is irresistible.

It doesn't matter that most of us now know we're creatures of often irrational cultural dictates, battling with often insurmountable physiological constraints. It doesn't matter how many times we've lost before, we usually gain it back. (As about 97 percent of dieters do.)

If there's a promise of magic, we're apt to try it. And it may work. Or does it?

Now comes the Cambridge Diet, "guaranteed" to get the weight off and, its promoters promise, keep it off. Sure, you lose weight; the diet -- 330 calories a day -- is a modified fast. But because it has been marketed only since March 1980, its promise of forever-thin is as yet unsubstantiated. More important, many medical specialists believe it could be dangerous, if not used under medical supervision.

The Cambridge Diet is a powder consisting of less protein and more carbohydrate than similar concoctions, now used mainly for the grossly obese. Like its relatives, Cambridge is a well-balanced mixture of high-quality nutrients.

When mixed with liquid, it becomes a very low-calorie meal that nevertheless, its purveyors say, provides safe, adequate and balanced nutrition with about a fourth of the usual daily calories for weight-loss diets. People on the Cambridge Diet have been known to lose as much as 20 pounds a week.

Any quick weight-loss plan can have serious consequences for the dieter, and in the long run, most experts and dieters have found, such plans do not work.

"The body doesn't know it is fasting," says Aaron Altschul, who heads the Georgetown University Diet Management Clinic. "It thinks it's starving, so it mobilizes its defenses to protect its fat stores, making it harder and harder for the weight to come off." And harder and harder, he adds, for any subsequent diets to work.

Much of the medical concern about the Cambridge Diet centers on the methods by which it is marketed.

Fasting diets, well-established research shows, can wreak havoc with the dieter's physiology -- literally pull the plug on the heart -- unless the dieter is monitored meticulously. There can be imbalances in the body's delicate electro-chemical functions which, among other things, keep the heart beating. Physicians can spot -- and head off -- potential trouble through regular electrocardiograms, or in blood tests.

The Cambridge Diet is being sold to the general public -- about a million people are on it, promoters say -- through specifically designated "Cambridge counselors," whose principal qualification apparently is at least two weeks on the diet themselves. Some may be health professionals, but many are not.

FDA spokesman Emil Corwin says FDA staff nutritionists are "especially concerned" that the counselors may not have the knowledge to counsel effectively.

According to Suzanne McKean, the Cambridge account executive at Hill & Knowlton, which took on Cambridge's press relations about six weeks ago, counselors buy the 21-meal cans -- a week's supply -- wholesale from the company at about $12 a can and resell them to "patrons" for about $18.

Counselors with "leadership qualities," says McKean, "may become group counselors," and eventually "chancellors." They continue to sell the diet but also train new counselor-salesmen.

Counselors are supposed to warn people to check with their doctors before they go on the diet, McKean says, but beyond that, they serve mainly as "a support group, an extended family."

Vaughn Feather, the 33-year-old Californian who heads the Cambridge effort in this country, says criticism of the product is unfounded. He offers literature from European clinicians to support this position and says that testing is underway in this country as well.

After an agreement with the Post Office and the FDA, Cambridge literature contains warnings against use without strict medical supervision by those with hypertension, diabetes, heart or kidney problems, gout, hypoglycemia or by the elderly, growing children, pregnant women, nursing mothers, and cautions that it should not be used as the sole source of nutrition for more than a month.

Feather, however, says that tests (in England where it was developed) have shown that the diet can be used safely for much longer, that it causes the loss of fat -- rather than protein -- and that it causes no problem with the system's electrolytes, including potassium, which is the substance required for proper cardiac functioning.

"That," snaps Washington internist Dr. Arthur Frank, "is absurd."

Frank, who specializes in weight problems, uses a soy-based supplement in a modified fast, but only with constant monitoring.

"If you do statistical studies on large numbers of people," he says, "you still won't be able to predict who will be in trouble and who won't. It's impossible without careful individual metabolic studies." (Potassium needs can vary widely; too much can be as dangerous as too little.)

Another concern, says FDA spokesman Corwin, is that some people on the diet could have underlying ailments of which they are unaware.

Corwin says the agency feels "impelled to warn people about the diet," and is monitoring complaints. It has case studies of three women hospitalized within the past few months for heart irregularities and who were, each said, on the Cambridge Diet.

McKean says they had pre-existing health problems that should have kept them off the diet. Cambridge Diet attorney John Banker adds that they had not followed instructions. He notes that a warning against using the diet with other medications like diuretics is now included in the package.

In other words, the diet was misused. "But of course that is a big part of the problem," says Corwin. "Misuse was much of the problem with the liquid protein diets" that were eventually linked to some 58 deaths of mostly young women in the '70s.

Randall Lee of the American Society of Bariatric Physicians (diet doctors) says that the company apparently "feels that if people don't follow directions it's not their fault. Our feeling is that diets of that nature mandate built-in protections."

Says Frank, "Look, if a million people do anything, you know some of them will screw it up.