Go ahead, say it to her face. Nancy Marshello doesn't mind.

At 5-foot-2 and 250 pounds, Marshello is not plump or plus-size, but fat.

F-A-T.

"It's just an adjective, like short, tall, thin," shrugs Marshello, 40, a UCLA administrative assistant who lives in Hansen Hills in the northeast San Fernando Valley of Southern California.

Marshello recently reactivated the Los Angeles chapter of the National Association to Advance Fat Acceptance, a Sacramento-based group dedicated to wiping out "fatism," which it condemns as America's last, safe prejudice.

Never mind that Marshello lives in a city that celebrates boyish hips and protruding ribs as the feminine ideal. It doesn't matter that her message comes at a time when people all over the country are sipping champagne and resolving again to get thin in the new year, or that many of her ideas are heretical to the medical establishment.

She believes that fat can be fun, fit and sexy.

"The other day, a man walked into my office and said, `Mmmmmm. I like a woman with curves," said Marshello, over a lunch of Chinese chicken salad and diet soda at a Westwood restaurant.

After two decades of yo-yo dieting--which she blames for her obesity--Marshello made peace with her body.

Now her goal is to get other fat people to do the same and to get everybody else off their backs.

That means no more fat jokes, no more tittering, no more treating fat people like they're stupid, lazy and enslaved by creature comforts.

NAAFA even discourages fat people from going on diets, most of which the group denounces as costly, doomed to fail and dangerous.

"We deserve the space that our bodies take. We shouldn't apologize for it," said Elaine Murgado, who sells bathing suits for fat women through her Redlands-based Big Day at the Beach mail-order business.

Pamela Lynn of Long Beach, who unsuccessfully sued an airline several years after a ticket agent allegedly asked her to buy a second seat, runs Los Angeles-based Big Difference, which stages dances and parties for fat people.

"Why are we treated like second-class citizens? I believe beauty comes in all different shapes and sizes, not just 5-foot-2, eyes o' blue and 118 pounds," said Lynn, who weighs more than 400 pounds.

Most fat people, the group claims, eat the same amounts and types of food as thin or average-size people; it's heredity and metabolism that cause them to tip the scales at more than the average.

"Most fat people have no more choice in their size than a person does in the color of their skin," said Sally Smith, NAAFA executive director.

But why should fat people surrender to cellulite when the conventional medical wisdom says that excess fat is bad and thin is healthy? Marshello and many other NAAFA members believe that much of the fat gospel is bunk, promulgated in large part by the $33 billion diet industry.

Even many of the diseases typically linked to obesity--diabetes, hypertension, coronary artery disease and breast cancer--are unrelated to weight or the result of yo-yo dieting, according to NAAFA activist Michael Loewy, a psychologist at San Diego State University.

Loewy, who is 5-foot-11 and 310 pounds, believes that most fat people are just fine the way they are. "What needs to change is their minds, not their bodies," he said.

Even if they wanted to lose weight, it would be virtually impossible, NAAFA members say. At least 95 percent of all diets fail within three to five years, fat activists claim.

Worse yet, low-calorie diets actually cause weight gain, they insist.

That happens because the body responds to calorie deprivation by slowing down its metabolism; once the dieter resumes eating normally, the body converts extra calories to fat in anticipation of the next famine, according to NAAFA literature.

Marshello said that's exactly what happened to her. The more she dieted, the more weight she gained. Since she quit "obsessing about food" in her mid-30s, Marshello said her weight has stabilized.

NAAFA--which led the charge to stop the marketing of the weight-loss drugs Redux and fenfluramine--also emphasizes that diets injure and kill. Witness the toll of anorexia, bulimia, diet drugs and weight-loss surgery, fat activists say.

NAAFA has a long way to go to persuade the medical establishment that most of its arguments are valid.

Dr. Morton Maxwell, director of the UCLA Obesity Center, says that fat activists are simply trying to pass on the blame to Mother Nature.

"It's ridiculous," he said. "They're responsible for their own obesity."

Maxwell said that obesity is the second leading cause of preventable death in the United States, behind smoking.