It may come as a surprise to many, but there are tens of thousands of Americans who are actually trying to gain weight. Granted, there are many millions more who have the opposite problem, and are constantly stuggling to lose unwanted pounds. But for those who picture themselves as being too thin, the struggle to change to their body image is just as desperate as it is for their overweight counterparts.

First, we should emphasize that for most thin people, weight is not a health problem, even though many of them see a doctor or nutritionist with the idea that if they could gain a few pound, they might "feel better" or have more energy. But there is no scientific basis for this belief; in fact, a number of studies have found that thin people tend to live longer and have fewer problems with degenerative diseases, such as heart disorders and diabetes, than people who are overweight.

Instead, the major motive for wanting to gain weight is a psychological one. Let's look at the typical person who thinks he's too thin. In our experience, most males who fall into this category are between 17 and 25 years old. They think their slender bodies, sticklike arms, and long, spindly legs make them look frail and young. They generally work hard at body building and often themselves to eat rich desserts and other foods they don't need or want. Still, they fail to transform themselves from "98-pound weaklings" into - their ideal - 150-pound muscle men.

The girls are usually the Twiggie type, and they aspire to the plumper Sophia Loren look. They, too, waste time, money and emotional energy on various schemes that promise added inches to their bustlines and fuller curves to their derrieres and legs. They also force themselves to eat what they consider gargantuan meals. However, when their caloric intake is actually measured, we usually find that it is considerably below the overweight person's. The fact is that many light-weights are light eaters, and many are also physically active.

In addition, there seems to be a genetic determination of thinness. In some of our own studies, we have found that thin people seem to have fewer adipose cells - the cells that accumulate and store fat - than normal and overweight individuals. People with limited fat cells or depots find it very difficult to overeat. When they have had enough, it is almost impossible for them to eat a bite more.

Most truly thin people belong to the physical type called ectomorphs - people with thin, elongated skeletons, narrow hands and feet, and long fingers and toes. True ectomorphs are usually slender all their lives. But one of our former colleagues, C.C, Seltzer of the Nutrition Department at the Harvard School of Public Health, found that many people who look like ectomorphs when they are young are, in reality, "late-blooming endomorphs." These late bloomers begin to gain weight in early middle age and are often quite overweight by the time they reach their late 40s and 50s.

Still, none of this solves the problem of a young person who desperately wants to gain 15 or 20 pounds. Our best advice to these people is to first have a thorough physical checkup to make sure that their thinness is not caused by a chronic disease such as diabetes, tuberculosis or anemia. Once their good health is confirmed, the next step should be to stay as physically fit as possible. Even a very thin person will look stronger and healthier if he or she is in the peak of physical fitness. Remember, too, that posture is important; standing tall and straight makes everybody look better.

Finally, the thin person should be careful to wear clothes that will accent his best points. A very thin man looks ridiculous in heavily padded jackets, but a well-cut blazer or sports jacket works wonders. And most of today's women's fashions are tailor-made for the tall, thin woman. Professional models, both male and female, have learned to make the most of their reedlike figures, and many of their techniques can be adopted by thin people.