DESPITE the national obsession with diets and dieting, 80 million Americans are overweight and, a recent national survey showed, the average person today is fatter than ever before. So are the coffers of a largely unregulated $10-billion industry that purports to assist people in shedding unwanted pounds via drugs, devices, salons, special foods and a never-ending stream of best-selling diet schemes.

While the desperate resort to such extremes as prolonged fasts, stapling the stomach, jaw wiring and intestinal bypass surgery, the majority of weight-conscious Americans hop from one fad diet to another searching vainly for a shortcut to slimness, and sometimes jeopardizing their health in the process.

Yet, experts report, 80 to 90 percent of dieters soon regain lost pounds and often then some, only to try still another diet in what the nutritionist Dr. Jean Mayer has dubbed ''the rhythm method of girth control.''

''If there was an effective diet,'' said Dr. George A. Bray of the University of California at Los Angeles, ''there would be no need for the continuous introduction of new diets: the 'Grapefruit Diet,' the 'Drinking Man's Diet,' the 'Air Force Diet,' the 'Mayo Diet,' the 'Quick Weight Loss Diet' and so on. It seems obvious from the number of diets that have been made available and are continuing to appear, none of them provides the answer to obesity.''

Most diet fads are a recycled version of a high-protein, lowcarbohydrate scheme first promulgated in the 1860s. Since 1950, it has resurfaced as the Calories-Don't-Count, Air Force, Mayo, Drinking Man's, Stillman's Quick-Weight-Loss, Atkins Diet Revolution and Scarsdale diets.

No matter what their name and constituents, the various fad diets share two characteristics, according to Dr. Theodore Van Itallie, director of the Obesity Research Center at St. Luke's Hospital in New York City and adviser to the Surgeon General on nutrition:

- They are nutritionally unbalanced and some may be dangerous to health. In fact, some caution that only healthy persons should follow the diet and advise against staying on it for more than a few weeks at a time.

- All, including the all-you-can-eat diets, are designed to reduce the caloric intake of the typical overweight person. Some, like the Scarsdale diet, specify the kinds and amounts of foods to eat. Others so restrict the dieter's choices that boredom and distaste automatically produce a calorie cutback.

Basically, fad diets make daily food decisions for the dieter who, upon returning to his or her old eating habits, nearly always regains the weight. Further, some of the quick weight-loss schemes are wholly illusory, producing mostly a loss of body water, not fat. In fact, on any diet, Dr. Bray said, the initial rapid loss during the first week or two is ''primarily of fluids as the body adjusts to utilizing its stored fat.''

Even more illusory are the supposed benefits of drugs and hormones purported to assist in weight reduction. Numerous studies have shown that hormones, including thyroid, HCG and growth hormone, have no effect on weight loss. Dr. Lyn Howard, director of the nutrition program at Albany Medical College, said that although appetite suppressants may help at first, ''it is our experience that weight lost with pills is always regained as soon as the pills are discontinued.''

Recent evidence suggests that the best-intentioned efforts to lose body fat and keep it off are sometimes thwarted by an energyefficient metabolic system that evolution built into the human species. This system helped the species survive a feast-or-famine existence by automatically increasing energy storage when food was plentiful and reducing energy expenditure when calories were in short supply.

Studies of people who have achieved a seemingly permanent weight loss indicate that a particular diet is only a small part of success. Of greater importance are the adoption of longterm goals, personal determination and discipline, and a restructuring of eating and exercise habits.

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