It's bathing-suit time, and wherever you turn, somebody is reminding you that you are heavier than you want to be, or ought to be, that NOW you can get back to your beautiful slim sylph-self (even if there never was one) if you'll just follow this New Revolutionary Diet Plan . . . or that one . . .

Shape Up in 10 Days, Shape Up in a Day, in an hour . . . Eat all you want! The first-chance diet. The last-chance. Scarsdale, Pritikin, Atkins. Be thin! Be beautiful! Be happy! Never be hungry!

You will find no such revolutionary diet here. No matter. How many of those never-be-fat-again promises have you fallen for? How many have worked? Do you believe you're weak-willed? Or that deep down inside you'd really rather be fat?

Something may be the matter with you, but it's probably not your moral stamina. And so long as you're healthy, exercise regularly, drink in moderation and don't smoke at all, there's probably no reason to spend your life guilt-ridden, hungry, miserable, self-hating, ill-tempered and always on a diet that never works.

What's wrong for sure, are today's cultural signals. What may be wrong--or at least different--are certain subtle biochemical workings. There is progress on both fronts.

The bottom line is this: If you have no health problems, your psychic energies may be better turned toward pursuits other than losing weight. On the other hand, we are still creatures of our culture. The psychological need to be thinner, especially for young people, may be paramount.

Weight-control is simply not a do-it-yourself proposition. Whether it is waged with support group or physician, whether the goal is to make peace with one's shape, or to fight off poundage, the battle to override your biochemical controls cannot be won without outside assistance.

The first problem about the weight problem is all those people who don't have a problem who think they do: those size-7s who would rather die (or diet) than buy a size 9.

The second problem is all those thin people who have gained five or 10, even 20 pounds and then quite easily lost it, and now think they have the answer for everybody. If they can do it, any fat slob ought to. Those people should stop reading this right now. They've got their own hang-ups about body image, and their weight relative to anybody else's. This column is for people, especially women, society has dubbed fat. Fat people, especially fat women, feel discriminated against--and they usually are.

A personnel director of a major publishing business has said flat-out he wouldn't hire a fat person "because it shows they have no self-discipline."

This is a thin culture and it is increasingly so. Cover girls in national magazines have lost significant weight over the years. Kim Chernin in her book, "The Obsession," directs attention to the early 1950s Marilyn Monroe in "Some Like It Hot." At the height of her reign as American sex goddess, she was downright pudgy by today's standards.

One therapist involved with some of the eating disorders that are an outgrowth of a preoccupation with thin and thinner suggests the preoccupation may be decreed by "a tyrannical group of homosexual dress designers whose preferences tend to boyishness." Others attribute our diminishing tastes to the liberation movement, a move to equality of the sexes in form as well as substance.

The brainwashing starts early. Judith Asner, a social worker specializing in treatment of bulimia at the Growth Center for Women in Rockville, talks about a toddler who bounced happily in front of a mirror until she noticed some padding on her thighs. "I'll just have to go on a diet," she pronounced.

A survey of 76 youngsters between the ages of 5 and 11, published in the February Journal of the American Dietetic Association, found that although none mentioned health risks, most noted "unfavorable social consequences" of being overweight. The report said that "100 percent believed that people can lose weight if they really want to."

Whatever its genesis, this cultural preoccupation with weight is as profitable as it is unhealthy. It is estimated that around 20 million Americans are "seriously dieting" at any given moment. They spend an estimated $200 million a year on diets, books, aids, pills--whatever anybody suggests may help, or has helped somebody, somewhere. And that's not counting the untold millions for programs like Weight Watchers, Diet Workshop Inc. and other commercial programs.

But Americans are not only obsessed with thinness, they're obsessed with food. Bulimic women at a recent support group session were unanimous that a combination of conflicting messages fed their twin compulsions to gorge and then to purge themselves of what they had eaten, usually by self-induced vomiting. As one women said, "It was the Sara Lee ads on one page of a magazine and the diets on the next."

Granted, some of those magazine diets can work for some people. A friend, for example, eats as she pleases for about three months; she gains about 5 pounds, sometimes 7, occasionally 10. Then she diets madly for a week or two and starts all over. It works for her. It wouldn't for many people.

But whether she knows it or not, my friend is following one of the basic tenets of a behavior-modification diet: Never eat anything you haven't planned for. (And eat anything you want so long as you have planned it in advance and factored it into your overall consumption.)

Again, that strategy can work for some people some of the time.

But behavior-modification, although it has major advantages over virtually all so-called popular, quick (read quack), all-fruit, all-meat, all-fat or all-anything diets, is still not the answer. Even behavior-mod can't keep it off most people who manage to get it off, even when eating behavior stays modified. The sad fact is that 95 percent or more of people who lose substantial poundage will gain it back within one or two years, or sooner. No matter the diet. No matter the claims. No matter what.

Although the ads continue to tempt and the diets exhort, there are signs that even the popular magazines are beginning to recognize that many of the old myths about fat people are simply not so. For example:

* Fat people lie about what they eat. Physicians no longer disbelieve a person who insists he or she gains weight on 1,200 calories a day.

* All you have to do to lose weight is eat less. Intake and output determine weight gain and loss, of course, but scientists now know that some people must cut their caloric intake to virtually starvation levels before they can lose an appreciable amount. Some nutritionists have expressed concern about the lack of certain essential elements in the diets of the dieters, especially as they age.

* Fat people have no self-discipline. Many overweight people must exert the most prodigious self-discipline to stay the same, remindful of the Red Queen in Lewis Carroll's "Through the Looking Glass": " . . . now here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"

* Being thin is healthier than being fat. Some health problems--mainly hypertension, diabetes, heart disease, arthritis--are exacerbated by overweight. Weight loss often can lower blood pressure, eliminate or lessen the need for supplemental insulin in some diabetics and perhaps decrease the risk of heart disease. But underweight people, whose Jack Sprat-ian battle is as difficult as any fat person's, are also at risk. Physicians are beginning to understand that weight may not in itself pose a serious health hazard if all other functions are normal.

* Exercise is the answer. Exercise is certainly part of an answer. Although not everyone will lose weight more easily through exercise, fitness is a healthful goal unto itself. Exercise can raise insulin levels, strengthen the heart muscle and lower blood pressure even in those people whose weight loss may be minimal.

Research into the causes of fat and thin is part of today's explosion of biochemical and neural discoveries, already affecting the medical and psychological perception of fatness, if not the cultural. And as the physiological causes of fat are understood better, some people may conquer their feelings of guilt and self-disgust and others actually may lose weight.

Much of the research tends to support what is known as the "Set-Point" theory. This holds that a combination of biochemical events, feedback systems and internal thermostats determines what an individual's weight may be. In one 6-foot-tall man it may be 175 pounds; in another, with the same bone structure, 340. No matter how the former stuffs himself, no matter how the latter starves, once they stop paying attention to what they are eating, they will revert to their "set-point" weights.

Possibly the most persuasive demonstration of the set-point was a mid-'60s study of 25 normal-weight male prisoner-volunteers in Vermont. They were overfed and underexercised for about six months until they had gained 25 percent over their starting weight. Gaining wasn't easy for most of them--they had to eat between 2,000 to (in one case) 7,000 extra calories a day.

Then, left to their own eating and exercise patterns, virtually all slid back to within a few pounds of their starting weights, in about half the time it took to fatten them up.

There are almost as many reasons for being fat--psychological as well as biochemical--as there are fat people.

Even if a "thin pill" were invented, therapists still would have plenty to do. There are enough culturally induced, behaviorally layered fat neuroses to keep them in business for generations to come.

It will still be a while before everyone can believe that, as therapist Asner likes to say, "Zaftig is not a dirty word."