New drugs that promise to fulfill one of the sweetest dreams of 31 million overweight Americans are quietly moving toward reality, a scientist said this week.

The drugs, long regarded as impossible, make the body burn fat faster by speeding up metabolism, so that fat disappears without exercise. A whole family of the compounds, termed "beta-3 adrener-gic agonists," are being developed by several big pharmaceutical companies.No existing anti-obesity drug can speed up metabolism, the process in which the body uses food to produce energy. Energy not needed by the body for life is stored as fat.

Traditional drugs are appetite suppressants that decrease hunger. People who take them eat fewer calories, forcing their bodies to burn calories stored in fat.

Claims that a drug, food supplement or diet can kick the body's fat-burning machinery into high gear long have been regarded as evidence of quackery.

Dr. Robert L. Dow, of the central research division of Pfizer, Inc., Groton, Conn., provided a peek at early research on one experimental compound.

"This is a very active area of research, and about a half dozen pharmaceutical companies have compounds in the early stages of development," he said in an interview.

In a presentation at a national meeting of the American Chemical Society, Dow said laboratory animals given the drug in preliminary tests showed a 10-30 percent increase in metabolic rate.

"If you have a 10 percent increase in energy expenditure, you're potentially looking at about a half pound a week drop in weight," Dow said.

Tests, however, have not yet gone beyond the laboratory-animal stage. Dow could not predict how soon the first "fat burners" might be used on humans.

He described beta-3 agonists as a third fundamental approach to obesity, which public health officials describe as an increasingly common and serious problem.

An estimated 58 million American adults aged 20-74 are overweight. They have an increased risk of heart disease, cancer, diabetes and other health problems. Americans spend more than $33 billion per year on weight-loss products and services. Yet new studies indicate that excess weight affects more adults and children than ever before.

One proven weight-loss approach involves eating less. The other, exercise, hikes metabolism by burning more calories to meet muscles' demands for energy.