San Francisco ? The only thing that could get me out of bed at 8 on a Saturday morning is the chance to work out with the best-known aerobics instructor in the Bay Area. So I find myself standing here, in a community hall, with a baker’s dozen of would-be fitniks, grateful that I mainlined a double latte in the cab.

The red-haired woman before me is wearing the predictable Lycra of her profession. She’s oozing good cheer as we all warm up to the tune of “Here Comes the Sun.”

But it’s not the uniform or the attitude that’s made Jennifer Portnick famous. It’s her shape. The 38-year-old who has done aerobics six days a week for 15 years weighs in at 240 pounds.

Some months ago, Jennifer applied to become a certified instructor for Jazzercise. But this woman leading our class in a “Stompin’ At the Ritz” routine that makes me feel like Fred Astaire before a hip replacement, was rejected. She was turned down, flat-out and in writing, because of her “appearance.”

In most parts of the United States, that would be the end of the story. An overweight applicant for fitness instructor would have no more of a legal case than a purple-haired, tattooed applicant for Disney World tour guide.

But San Francisco is one of the handful of jurisdictions  including Michigan, Washington, D.C., and Santa Cruz, Calif.  that have made it illegal to discriminate against fat people in employment or housing. How “San Francisco” does that sound? Well, this is also a city where a billboard ad for a fitness center read: “When the aliens come they’ll eat the fat ones first.”

San Francisco seems to be an epicenter of both fitness obsession and fat activism. So when Jennifer protested that “I want to be judged on my merits, not my measurements,” her body was placed right at the point of colliding attitudes about our bodies, ourselves.

We are now at two tipping points in our attitudes toward fat. On the one hand we have ratcheted up the talk about an “epidemic” of obesity, adopting a medical model that describes fat as an illness and weight loss as a treatment  even when it doesn’t work. On the other hand, there’s also a growing attempt to reduce the prejudice and misery surrounding weight.

Fat, love it or lose it. Every month, women’s magazines urge us to get rid of ugly fat with surgery, chemistry or grapefruit. But this April’s Vogue, labeled “The Shape Issue,” includes a 170-pound plus-size model, Kate Dillon saying, “Sure, I’d love to have a Britney Spears body. So I’m a little larger  is that so bad?”

In the past few weeks, pediatricians from Yale warn about the risks of diabetes and heart disease among an increasingly weighty generation of children. At the same time, parents in Pennsylvania and Florida who received letters from school warning that their children were fat got outraged at the insensitivity.

The surgeon general’s “Call to Action” last December embodied these conflicts. It sounded the health alarm and pointed wisely to culprits like junk food and inactivity. And it also expressed a hope to eradicate the “social stigmatism associated with overweight and obesity.”

Now, a lot of folks will tell you that the way to eliminate the stigma is to go on a diet. But part of the tension between loving and losing weight is found in research showing that most diets fail, that “cures” may be worse than the “disease” and that we don’t know much about weight, anyway.

This is where Jennifer Portnick comes back in. On one level, this is a simple employment discrimination case. In work-place studies, employers routinely stereotype fat employees as “lazy, lacking in self-discipline, less conscientious, less competent.” Try applying that to this woman waltzing us around the room to the beat of Aretha Franklin.

On another level this case raises the question of whether anyone can be fat and fit. Pat Lyons, a San Franciscan who’s written “Great Shape,” a fitness guide for large women, describes herself as “a healthy fat person.” That’s not, she says, an oxymoron. Maybe the saner medical model is about exercise instead of inches. Maybe Jennifer is a role model and not a reject.

Portnick can’t talk about her case while the lawyers are mediating. But don’t tell me her “appearance” is a turn-off. The large, muscular and graceful woman was the first fitness teacher I ever had who didn’t embody some impossible physical dream.

Fat and fit? Let me put it this way. At 5 feet 8 inches Jennifer Portnick weighs over a hundred pounds more than I do. But later Saturday morning, she went off to a hip-hop class. I went back to bed.

 Ellen Goodman is a columnist for Washington Post Writers Group.