Pinch a rat's tail and it will overeat.

Stress a human and what do you think happens?

That's one part of the story of obesity or, rather, of being overweight -- the experts are in the process of redefining the terms. But it is only one part.

Doctors have been defining and redefining and examining and experimenting and hypothesizing about fatness for years.

They've produced dozens of theories, some accepted, some still controversial, that attribute obesity to genetics, to metabolic disorders, to brain disorders, to hormonal disorders, to thermogenesis (energy burning) disorders, to having too few brown fat cells, to having too many other fat cells, to having fat cells that are too fat.

Other theories suggest that obesity is mostly or all psychological -- you eat too much because you hated your mother or because your boss yelled at you or because you're sexually inhibited. Or that it is mostly or all attributable to bad eating habits. Or piggishness. Or probably the most popular and least rational explanation: weak will. That last includes "letting yourself go," "no self discipline" and "slatternliness."

Despite all the professional attention, obesity is becoming a bigger problem, and overweight people keep dropping in the public's estimation and, sadly, in their own.

A few years ago, for example, some 6-year-olds were shown silhouettes of obese children and described them in terms including "lazy," "dirty," "stupid," "ugly," "cheater" and "liar."

In a related study, both children and adults were shown silhouettes of a normal child, an obese child and children with handicaps such as missing hands and disfigured faces. Children and adults alike said they would be least likely to befriend the obese child. According to Dr. Albert J. Stunkard, psychiatry professor at Philadelphia's University of Pennsylvania and well-known authority on obesity, this negative reaction to the obese child was "not only uniform among blacks and whites and persons from rural and urban settings, but it is observed among the obese themselves."

The cultural signals are everywhere. Thin is in and fat is out out out. It's been that way since the end of the age of Rubens, but it seems to worsen each year.

The cultural imperatives for thinness also have been linked to an increase in eating disorders such as bulimia -- the binge-purge syndrome that seems almost epidemic these days -- and the self-starvation disorder known as anorexia. These are the products of conflicting signals like the TV ads full of fat-laden junk food and mouth-watering desserts sold by boyishly slim models. America is not only obsessed with fat, it is obsessed with food.

The only fat people on television are objects of derision. Where is television's tubby hero? Its fat heroine? Even when characters are supposed to be fat, they're not. For example, a major plot line of Judith Krantz's book "Scruples" was the gradual, unnoticed, but massive weight loss of its heroine. So who did TV get to play the part a couple of seasons ago? The bionic woman herself, Lindsay Wagner. Fat? Well, they padded her tummy a bit, and dressed her frumpy, but so much for that plot line.

Whatever began it, however, Americans are such creatures of the advertising and pop-cultural onslaught that they seem willing to do and buy almost anything that promises to deliver down-to-the-bone slimness -- to the tune of some $200 million a year. Virtually none of the diets works in the long run, some are out-and-out quackery, some are genuinely dangerous, and some are healthy but hopelessly rigid. OO nly a few years ago, most of those trying to treat weight problems believed O psychological problems were the main cause of obesity. Now most of them are leaning to the theory that, as Stunkard told a National Institues of Health consensus development conference on obesity last week, "these disturbances are more likely to be the results of obesity" both because of the negative prejudice and "through the effects of dieting."

A few have tried to swim against the cultural tide to convince, if not the public at large, at least those with the most unyielding weight problems that their self-image should not be affected by the conflicting cultural cues. Among these are Marcia Millman in her book "Such a Pretty Face," Kim Chernin in "The Obsession," Sandra Edwards in "Too Much Is Not Enough" and most recently Ellyn Satter in "Child of Mine."

Satter, especially, has something to say about the treatment accorded fat children. A long-time diet counselor, she finally concluded that the loving relationship between parent and child was more important than imposing a diet and risk delivering a message that "the child is not any good unless he is thin."

Although it is true that fat children are more likely to grow into fat adults than are thin children, this is by no means inevitable. Some evidence presented at the NIH conference suggested that birth weight has no relevance to later weight problems, and although weight at age 1 and older does, it does so for fewer than half the children. There are very few scientific studies of childhood obesity, much less on the effects of dieting, but some new animal evidence presented by M.R.C. "Marcie" Greenwood of the Vassar College biology department suggests that restricting calories in the very young may result in stunted growth but not necessarily less obesity.

Psychiatrists have noted a link between some clinical depressions and eating disorders -- either overeating or undereating -- and some researchers speculate that mood and appetite are regulated by some common mechanisms. Stunkard's work shows that people who diet often report irritability, depression, anxiety and hostility, suggesting a relationship to a weight "set point," below which a person's brain-body metabolism might conclude the body was starving and move to protect fat stores, thereby making weight loss well nigh impossible.

Any diet or diet therapy that claims you will "take it off and keep it off" is simply untrue, although dozens make this claim. Despite all the concern about controlling body weight -- culturally, clinically, medically and personally -- the statistics remain dismal: No more than 10 percent of those who lose weight in clinics where such things are measured, regardless of what technique was used, actually keep it off for longer than a year or two at best. And even fewer keep it off for a longer period. Both commercial weight programs and nonprofit clinics have large dropout rates.

Some specialists are beginning to believe that these statistics are misleadingly low, that people who fail either to lose or to keep lost weight from returning on their first try may return in another weight loss program and be among the 10 percent who succeed. If this is the case, cumulative "cure" rates may be closer to 15 percent, maybe even higher. They also believe that, as in stopping smoking, many self-dieters may be successful. It is a tricky statistical exercise, but it encourages the clinicians and should encourage the overweight who failed once to try, try again.

Nevertheless, obesity expert Theodore B. van Itallie told the NIH conference that nearly 34 million Americans are overweight and of these, 11.5 million are seriously so and at extreme medical risk.

The mission of the conference was principally to reach agreement on the link of obesity to specific health problems -- cancers of the breast and uterus in women, the prostate in men, and in both sexes colorectral cancer as well as hypertension, cardiovascular disease and, in susceptible individuals, Type-II (adult onset) diabetes. Some studies, including a new analysis of 30 years of data from the extensive heart study being conducted in Framingham, Mass., are suggesting that obesity is an "independent risk factor" -- it can lead to earlier death in men who are otherwise healthy. Although the analysis of the data for women is incomplete, there is a suggestion that it may be only slightly less so.

The consensus panel concluded that obesity was indeed a disease, "not," its chairman, Dr. Jules Hirsch, said, "a condition, like loneliness." Even five pounds above desirable weight, the panel said, could be of concern when, for example, there was a family history of hypertension or Type II diabetes.

When pushed, members of the medical community will concede that some moderately overweight people are possibly not at risk. But there has been, for the most part, no real way to determine who is and who isn't, so the only sensible approach, they believe, is to designate obesity as a general health problem. Almost all specialists agree that the morbidly obese are certainly at risk for a multitude of problems and those that do not appear to be at risk today may indeed be so tomorrow. Any obesity, the consensus panel agreed, appears to be life-shortening. SS ome new research suggests that where the excess fat is located on the body, rather S than how much of it exists, is a clue to potential health problems.

Dr. Per Bjorntorp, obesity specialist at the University of Goteborg in Sweden, has done a number of studies suggesting that people with abdominal fat are more at risk for cardiovascular disease than those whose fatty tissue is around the hips. In other words, if your waistline is the same as or larger than your hips, "the risk for cardiovascular disease seems to increase sharply," says Bjorntorp.

Which doesn't help people lose weight.

So far, the most successful programs are those that combine good nutritionally balanced diets with psychological counseling and heavy doses of exercise.

Aaron Altschul, director of one such program, the Georgetown University Diet Management Center, cites one study that set out to settle the question of whether the size or the number of fat cells were critical in weight loss programs. In that study, they took a number of patients and compared a weight loss program that included psychological counseling to a weight loss program either without therapy or with simple behavior modification. They found the weight loss program with psychotherapy was better than all other programs, and, says Altschul, "it didn't make a bit of difference whether the patients were young or old or had a large number or small number of fat cells."

The fat cells may be important to the researcher, says Altschul, "but for the practitioner or the patient they are really irrelevant."

And the older you get, the longer you've dieted, as a general rule, the heavier you will be.

In fact, research has identified automatic mechanisms that scientists believe may have helped keep mankind's prehistoric forebears alive, but which now are unneeded, unwanted and in many cases unhealthy anachronisms.

Here is how this might have worked: The caveman ate whenever food was available because it was available only intermittantly. Some of today's overweight individuals may notice that their appetites are stimulated by food ads in magazines or especially on TV. The term is "mouth-watering." The reaction is as vestigial as the appendix. Few cavemen lived much beyond their twenties or early thirties, and the older the tribe member, the less likely he or she was able to compete successfully for food. The tendency of people to gain weight with age, some believe, is another characteristic that aided survival a millenium ago but adds nothing but girth now. Dr. Reubin Andres believes that "average healthy individuals should not be concerned if they gain some weight as they move from early adult years to late middle age," and proposes factoring age into currently used height-weight charts. Animals, including man, developed "redundant systems to assure adequate feeding," believes Dr. Steven M. Paul, chief of the Clinical Neuroscience Branch of the National Institute of Mental Health. Paul and his colleagues are engaged in a series of experiments with rats that are genetically bred to either obesity or leanness. Their object is to trace the role of parts of the brain in regulating appetite and food intake.

Paul hypothesizes: "If we had only one system and it was destroyed and you didn't eat anymore, chances of survival of that species would be precarious. My own view is that the body has many ways of reducing food intake, a number of peripheral and perhaps central satiety signals, none of which could be changed in a way that would allow the organism to either starve to death or become morbidly obese . . ."

Paul's own work has produced evidence of a kind of thermostat for sugar or carbohydrate intake in the brain. Work done at the Massachusetts Institute of Technology by neuroendocrinologist Dr. Richard Wurtman and research scientist Judith Wurtman already has suggested that the brain chemical serotonin functions to regulate cravings for carbohydrates, acting as a satiety signal -- a signal that says "stop eating. You've had enough. You don't want any more now." Serotonin deficiency is also implicated in certain depressions, and the link between depression and appetite is also being studied.

Another substance that appears to have a similar function is cholecystokinin, known as CCK, which is secreted in the intestine but works in the brain. Some work by Jacqueline N. Crawley at the NIMH Clinical Neuroscience Branch has shown that rats injected in the abdomen with CCK demonstrated the same behavior as rats that had filled up on chocolate chip cookies. CCK was less effective on the genetically obese rats and, in any case, the rats built up a tolerance to it very quickly, suggesting that it would be less than useful in a dietary regimen for humans.

"Nevertheless," Crawley told a group of her colleagues recently, "it does help answer the old question of what is a gut feeling."

Dr. Steven Peikin, associate professor of medicine and pharmacology at the Jefferson Medical School in Philadelphia, has found evidence that CCK does not work in fat rats (except at extremely high levels) because the rats have too few CCK receptors. He likens the defect to that found in humans with Type II diabetes, in which plenty of insulin is produced but cannot attach to the cells that control blood sugar because they lack sufficient receptors, or specialized cell-surface molecules.

Peikin also has demonstrated that the pancreatic enzyme trypsin blocks release of CCK. By giving the fatty rats a substance that blocks the activity of trypsin, the secretion of CCK -- and its signal of satiety -- can be enhanced. So far his work is only with rats, but he expects to begin human trials within the year. At least, he believes he has found "a brand new biochemical defect responsible for overeating."

Because it appears that the body's appetite and perhaps energy regulatory centers are "all wired together," says Steven Paul, "it seems less and less likely we'll find the holy grail," a single solution or pill that will solve all obesity problems.

"On the other hand, it may be possible to find why certain obese individuals are obese with respect to one or more of these regulatory centers."

In other words, there may well be many causes of obesity -- and as many potential cures. As Peikin notes, even in the fatty rat there are unknown elements. "They eat 40 percent more than their lean litter mates, but they are 70 percent more overweight." Tail pinching is a good way to get a rat to eat, but what about the rest of it? Sources of Help

Two nonprofit comprehensive diet management programs in the Washington area:

* Georgetown University Medical Center Diet Management Program. Diet and behavior modification combined with psychotherapy and nutritional education. For weight problems including eating disorders and morbid obesity. 625-3674.

* George Washington University Weight Management Program, Department of Medicine. Behavioral, fitness and nutritional techniques. Director: Dr. Chariklia Speigel. 676-8828.