The news about getting fat and getting thin and staying that way is depressing. But not altogether hopeless.

The more scientists find out about obesity, the more it seems that to the adage "the rich get richer and the poor get poorer" one can safely add, "and the fat get fatter."Fat people, it turns out, are not only fat when they're fat, they're fat when they're thin.

And because most fat people get fat without overeating, contrary to some of the most ingrained public perceptions, including those of doctors, it is excruciatingly hard for them to get thin by undereating. This has been known about rats for years. Now it is beginning to be shown to be true in people, too. Not in everybody, perhaps, but in many.

The sad truth is that the people who complain that everything they eat turns to fat may be absolutely correct. Another sad truth is that by whatever mechanism, fat tends to protect itself, to perpetuate itself. It is one of those outmoded survival characteristics, from the days in prehistory when survival depended on reserves of energy. Our primitive brains, it seems, don't know about Sara Lee.

For a day and a half recently, some of the top researchers and clinicians in the obesity field compared notes and presented new findings at a symposium co-sponsored by the Institute of Human Nutrition at the Columbia University College of Physicians and Surgeons and Weight Watchers International Inc.

Participants in the symposium were an eclectic group of researchers and clinicians.

Not surprisingly, endocrinologists - doctors who treat hormone disorders - point to studies suggesting that the problem is basically hormonal.

Neuroscientists see it as a brain problem.

Nutritionists see it as a problem of food intake.

Psychologists and psychiatrists maintain their conviction that appropriate behavioral controls may hold the answer, that psychological and psychosocial factors contribute to the problem and may aid in its control.

New drugs are in the offing, some of which increase sympathetic nervous system activity, the fight-or-flight system that seems to be blunted in the obese; some deal with neurotransmitters - chemical messengers - in the brain or, in some cases, in the intestines, but none is near ready to use.

The one thing the specialists seem to agree on is that weight reduction is of value if someone is dangerously obese, but that weight loss merely for cosmetic reasons carries more risk than benefit.

The bottom line, as Dr. Jules Hirsch, senior physician at the Rockefeller University put it, is that "from the treatment standpoint, nothing much has changed. The data (on taking it off and keeping it off) are still just terrible." About 90 percent of women regain lost weight, as do about 85 percent of men.

Nevertheless, he notes, "with the entrance into the field not only of neurosciences but of molecular biology, biochemical genetics, tissue culture, endocrinology and metabolic studies, you're going to see another revolution of understanding." Researchers expect another wave like the one that brought in psychosocial and behavioral aspects a few years ago, which altered the time-honored but wrong idea that fat was simply caused by overeating.

The scientists also warned that yo-yo dieting - repeatedly taking off weight only to put it back on - may actually promote weight gain in the long run. This increasingly common practice of yo-yo dieting may also promote the abdominal fat deposits that carry with them the greatest health risks. "It used to be thought that if an individual lost weight and sustained it, it was all for the good," said psychologist Kelly D. Brownell of the University of Pennsylvania School of Medicine. "And if the person was unfortunate enough to regain the lost weight, well, there was always another diet."

Now, however, he said, it appears that "this may not be the case."

A collaborative research project involving scientists from Harvard, Yale, Vassar and the University of Pennsylvania, among others, is producing evidence of what Brownell calls a "diet sensitive" population, for whom each bout of weight loss and weight gain mobilizes the body's defenses against losing.

New methods of measuring resting metabolism in specially constructed respiration chambers show in some studies that persistent weight fluctuations lead to significantly lower metabolic rates - a slower burning of calories.

In laboratory rats, and in some human studies, researchers are also finding that when formerly fat patients become thin, they are actually still fat, in the sense that their proportion of fat to lean tissue is about the same as it was when they were more obviously obese. Researchers are also focusing in on the defect, probably genetic, that directs too great a proportion of nutrients to fat rather than lean tissue.

Some of the other findings and studies reported at the symposium:

- Claude Bouchard, from the Physical Activity Science Laboratory at Laval University in Quebec, reported on studies of sets of identical twins who were intentionally overfed. Both twins tended to gain the same amount of weight, and in the same places although there was a wide variation between the pairs of twins. But genetics was not the only factor at work. For example, if one twin expended more energy with exercise, the genetic effect could be overcome. According to Dr. Albert J. Stunkard of the University of Pennsylvania, "In any case, what has been inherited is clearly not a trait like eye color or hair color that is unmodifiable, but there is probably some kind of interaction with the environment."

- Stunkard and his team looked at environmental influences (in one case winning the "golden fleece" award, that dubious honor bestowed by outgoing Sen. William Proxmire, D-Wis. In that study, the Pennsylvania team looked at people who went to restaurants on smorgasbord nights, when it was all-you-can-eat-for-five-bucks. Rather to their surprise, they found that fat people did not eat more than thinner people, nor did they eat faster, according to the number of chews per swallow.

- According to studies reported by Dr. Per Bjorntorp of Sweden's University of Goteborg, abdominal fat "is a risk factor for cardiovascular disease, premature death, stroke, diabetes and possibly some forms of cancer. Moreover, some of the major problems occur in thin people who, however, have most of their fat centered in the abdominal region."

- Abdominal fat is also linked to sex hormones. Testosterone in men and progesterone in women seem to inhibit fat deposits in this area, Bjorntorp said. But there is also a clear relationship with the stress hormones, which tend to direct fat to the gut area. Smoking and alcohol use, even in a social context, can increase the stress hormones, decrease the sex hormones and lay down the fat in the "beer belly" or "middle-age spread" configuration, even in non-obese individuals. Even fat suction, a technique condemned by most of the specialists as a treatment for any sort of obesity, by its very nature leaves enough fat cells, with their enzymes and peptides, to virtually guarantee that the suctioned fat will be regenerated.

- A study of the Pima Indians in Arizona, where obesity and diabetes is rampant, so astonished researchers at the National Institutes of Health that they withheld the results from publication until they could berechecked.

According to Dr. Clifton Bogardus, a member of the NIH Arizona team, the Pima population, when metabolism was measured in a respiration chamber, demonstrated clearly familial low rates that predicted obesity.

- Exercise is a major factor in weight gain and loss, but not by itself. Laval University's Bouchard described a study in which 50 obese women volunteers exercised strenuously over a period of 14 months. Their weight loss was negligible until they were put on a diet very high in carbohydrates and virtually free of fats. "The weight loss," said Bouchard, "was spectacular."

Exercising before a meal seemed to have the greatest effect.

- And in studies of men of the same weight but who had different proportions of fat to lean tissue, Dr. F. Xavier Pi-Sunyer of Columbia University and St. Luke's-Roosevelt Hospital in New York found that the obese men had blunted responses to food, burning it more slowly than the lean men of the same weight (who were weightlifters).

The researchers noted that fat is more easily stored than burned, so whatever diets are used, they should contain minimal fat and increased carbohydrates. "Vegetarians," noted Stunkard, "are rarely obese."

-So should people diet or not? According to researcher M.R.C. Greenwood, chairman of the Vassar department of biology, "The average American women who read women's magazines are mostly not obese in terms of medical risks. Most are concerned about weight for cosmetic reasons, and I think the issue the weight cycling project raises is whether we are in fact acculturating a whole generation of women to a behavior that we now have reason to believe may have long-term health consequences.

"We don't want to tell obese people not to diet, although we need to work more on preventing relapses, but what we are saying to the non-obese female is, no, dieting is not necessarily good for you," she said.

Up to about 20 percent above ideal body weight, she noted, there is almost no health risk. There may be a social or cosmetic risk, the risk of not being able to wear a size 6 instead of a size 8. But, she said, "the intelligent way to go after it is to improve physical fitness, something nobody wants to hear.