Every doctor who casually has told a patient "you ought to lose weight," every personnel director who doesn't hire fat people "because they have no self-discipline," every thin person who has sniffed at a fatter neighbor "for letting herself go," should watch NOVA tonight at 8 on Channel 26.

Most people with real weight problems already pretty much know what "Fat Chance in a Thin World" has to say.

That's because their almost universally hopeless obsession has led them to read every book, every article, every medical journal, virtually every word ever written on the subject of weight and diet in the hopes of stumbling on the magic key.

Nobody has to tell a lifelong dieter that 95 percent of dieters regain lost weight in a year (and, indeed, another 2 or 3 percent in the next four years)--regardless of life-style changes and modified behavior.

Nor does the dieter need to be told that most people feel miserable and hostile when they are dieting--a major reason for not completing a weight-loss regimen.

But it's nice to know science is beginning to concede that fat people don't necessarily eat more than thin people. With some overweight people it is a question of how fast they burn off what they eat; with others it may be a matter of faulty brain messages. With still others it may be a "set-point," genetically coded too high; no matter how much one gains or loses, the feedback systems in the body will draw you back to the starting weight.

"Fat Chance in a Thin World" is a report on the kind of state-of-the-art research into the causes of excess weight and obesity that should be shown to every patient in every obesity clinic in America. It belies conventional wisdom, even in many so-called obesity or diet clinics, that all fat people can be treated the same. It pretty well achieves its goal of letting "facts about fat replace myths about obesity."

During World War II some conscientious objectors in Minnesota agreed to participate in a study of the effects of starvation on humans. The scientists conducting the study found metabolic changes. For example, after an initial rapid weight loss, weight stablized after a six-month period. Further, they found major psychological changes. The men became less sociable, lost interest in sex, became depressed, withdrawn, obsessed with food, obsessed with self, isolated.

In addition, dieters, speculates a psychiatrist on the program, often show the same symptoms of "semi-starvation neuroses," even though they may be consuming twice as many calories as were the subjects in the Minnesota group. Dieters also experience a metabolic stabilization, known to the practiced dieter as "a plateau," one of the body's ways to counter what it perceives as starvation, but a source of misery and frustration to someone trying to lose weight.

One of NOVA's most interesting segments concerns some new scientific research on the differences between hunger (feeling empty) and satiety (feeling satisfied but not necessarily feeling full). Some excess weight may be caused by a combination of too much of the former and not enough of the latter. Or by one or the other.

But, alas, although new research suggests that gluttony and low willpower have less to do with excess weight than biochemical or bioelectrical variations or malfunctions, there is very little to suggest that society's preoccupation with thinness is lessening. For example, in one group of several hundred 6- to 10-year-old girls, about 60 percent said they already had been on a diet to lose weight.

And finally, so far no one anywhere has come up with that elusive take-it-off-and-keep-it-off answer.