The "desirable weight" charts in doctor's offices all over the country are about to be revised -- possibly pushing desirable weights up by 15 pounds -- and if we want to live longer, we all should revise our ideas about fat and health, according to Dr. Reubin Andres, a professor at John Hopkins and clinical director of the National Institute on Aging.

Popular belief says that thin people live longer and fat people will be brought down sooner by their weight. But that belief, though it is hawked by scores of diet books and even doctors, may be flat wrong. In fact, skinny people may be the ones who die sooner, according to some studies received by Andres.

Andres, whose field is obesity and aging, has reviewed more than 40 studies of weight and longevity, covering some 6 million people. The studies ranged from Helsinki policemen and Italian villagers to 750,000 Americans by the American Cancer Society and 5 million people by American insurance companies.

"The populations were extremely diverse, but what's important is that the results all point in the same direction -- the desirable weight if you want to live longer has been underestimated. The current chart on doctor's walls, and our own ideas of desirable weight fixed by a sense of esthetics, are not desirable if you want to live longer," Dr. Andres said.

Among the surprises in studies that Andres looked at were these:

Highest longevity among middle-aged workers at a Chicago utility company occurred in men who were 25 to 32 percent over their "desirable" weight.

There was minimal mortality among 70-year-old Californians who were 10 to 20 percent "overweight."

Lowest mortality among San Francisco longshoremen was found in the group that was 30 percent overweight.

In a long-term study of residents of Framingham, Mass., men in similar age groups were found with lowest mortality toward the heaviest end of the scale. Among women in the same town, longevity was greatest in a broad range of middle weights -- including some previously thought to be overweight -- and mortality highest among the very thinnest and fattest women.

Andres offers two explanations for longer life at somewhat higher weights: heavier people can better tolerate chemotherapy for cancer, and their extra weight may also help them fight diseases that leave others emaciated.

The basic chart American doctors and dieters have used for the past two decades was created by Metropolitan Life Insurance Co., based on surveys of weight and longevity by many insurance companies. But now out of a large new insurance survey is coming a new chart.

This chart is expected to add 15 pounds to desirable weights, and Andres says his data indicate that the shift should be even greater -- perhaps as much as 10 to 20 percent of body weight.

Andres is 5 feet 10 and weights 178 pounds; the "desirable" weight for him would be 154 by the old chart.

There is no single desirable weight, of course, and extremes of obesity and thinness are both dangerous to health, Andres said. "But the question is, if you want to live longer, where do you put that middle range of weight? Most of us go by esthetics because we just don't like the look of fat people. But the best esthetic weight may not be what is desirable if you want to live longer."

Another researcher has confirming evidence of the Andres survey. Dr. Ancil Keys, developer of the K-ration and now professor emeritus of physiology at the University of Minnesota, has taken statistics on more than 12,000 men in the United States, Japan, and Europe for the past two decades.

He has found that "in the absence of hypertension [high blood pressure], overweight is not a risk factor at all."

A recent article in Johns Hopkins Magazine surveying the new results in weight and health also quotes Dr. David Levitsky, associate professor of psychology and of nutrition at Cornell University.

He says that perhaps half the people who are overweight will suffer ill health effects from it, and half will not. "I'm not saying it's okay to be fat. If you are fat you have to look out for certain pathologies. . . ." But if you don't suffer from any of the obesity-exacerbated diseases, he says, then fat is not necessarily bad and losing weight probably will not gain you a longer life.