It's dinnertime at the Steinback house in Scottsdale, Arizona, but the hostess isn't eating. "I won¹t be joining you for dinner," Jyl Steinback cheerily announces to a visitor whom she has invited for the evening meal.

She shows her guest to the dining-room table and proceeds to race in and out of the kitchen, serving food so nutritious that just looking at it feels downright healthy. For the visitor and Jyl's husband, Gary, there's homemade chicken soup (lots of vegetables) with dumplings (made with only egg whites, no oil). For nine-year-old Jamie and eighteen-month-old Scott, there are baked potatoes topped with a handful of preshredded cheddar cheese (the low-fat variety). Everyone gets a green salad and a slice of the fat-free bread that Jyl made earlier in the day.

While the group dines, Jyl sips on a glass of cold water and explains why she's not joining in the meal. "I don¹t eat anything after five o'clock," she says. "If I do, the food just sits in my stomach and makes me feel all bloated and heavy."

Meet the Steinbacks, who are fit, firm and trim. While most baby boomers have been caught up in the current health craze, these people have been swept away by it. They down handfuls of antioxidant vitamins (including C, E and beta carotene) in the morning and munch on snacks called "energy bars" in the afternoons. Then, they enthusiastically engage in all forms of exercise, from muscle toning to mountain climbing. They monitor food portions, study labels and count fat grams with such intensity that they practically burn calories while doing so. "We like the way it makes us feel," Jyl says of their fit-as-it-gets lifestyle. "We feel really healthy, really energetic, really awesome."

Not to mention really proud of their hard and lean physiques. "Gary's got five percent body fat," Jyl boasts, referring to the portion of his body that's fat tissue as opposed to lean muscle. (The average healthy male has about 16 percent.) "I think he looks just great," she says, beaming at her husband of fourteen years.

Gary, forty-one, five eight and a half, 142 pounds, smiles and looks somewhat self-conscious.

Jyl's appearance, too, is something to be proud of. At forty, she has the body of every aerobics instructor you¹ve ever envied. "Yes. I would say I'm happy with how I look now," says Jyl, who, at five five, weighs between 105 and 110 pounds, depending on the time of day she steps on the bathroom scale. "But I usually weigh myself only in the morning; I don't want to be a slave to the scale." Besides, Jyl can also monitor any additional ounces by how easily she can zip up her size-four jeans. "If I get bloated, I just make sure to drink more water, and then it's gone in a couple of days," she says earnestly.

Jyl is also relatively happy with her skin, which she coats with sunscreen whenever she leaves the house, and her hair, which she bleached from dark brunette to blond after her fair-haired daughter arrived. ("I wanted us to look like mother and daughter," she explains.) Jyl's only real complaint about her appearance these days is her breasts: They "disappeared after Scott was born." She says she's thought about having a "boob job" to up her bra size from its current 34A status (she had a nose job at fifteen to "fix a bump") but decided against the cosmetic surgery after considering the health risks. "Health is very important to us. If you don't have your health, you don't have anything."

The sun isn't up in Scottsdale, but Jyl is. It's before six, and she's hard at work in the Phoenix home of Larene Greenband, a sixty something woman with the firmness of an average twenty-year-old.

A large boom box blares a disco version of "Don't Cry for Me Argentina," and Jyl, dressed in a skimpy purple leotard, is coaxing Larene through a vigorous series of toning exercises. "C'mon, now, just one more set," she bubbles as she and Larene work their arm muscles. "You're doin' great." At the end of the hour-long session, Jyl dims the lights and softly talks Larene through a relaxation exercise that ends with the admonition to "love your body." Then she packs up her gear and heads home to get Gary off to work, Jamie out to school, and to meet the baby-sitter who will watch Scott while Jyl trains other clients.

"Have a healthy day," she calls out as she leaves.

For virtually all of her adult life, Jyl has worked in the fitness business. Before having children, she was an exercise instructor. Her first "big" job was at Figure Girl, a health club in a strip mall in Phoenix. After that, she was lured to California to teach at the Red Door, an exclusive Elizabeth Arden spa in Beverly Hills. She and Gary had a long-distance relationship at the time, but neither of them liked that, so after a year she moved back to Arizona to work at the iviaine Chance spa, a luxurious Arden retreat (now closed) in Scottsdale. She struck out on her own in 1982, opening a fitness salon called Jump For Life, where she taught aerobics using a mini trampoline. After she injured her neck in a car accident a few years later, she had to abandon trampoline work. She then began working as a personal fitness trainer. (Gary, who works managing commercial real estate, is the main breadwinner in the family.)

Jyl's current schedule includes training two or three clients a day, six days a week. (Her fee is $50 for an hour-long session.) In addition, she spends several hours a day working on her entrepreneurial endeavors: She recently applied for a patent for the "Perfect Body Band," a latex-and-Velcro device to use for weight-resistance exercises. And she¹s self-published two cookbooks of fat-free recipes (Recipes for Fat Free Living and Recipes for Fat Free Living 2). "I feel like I was put on earth to do something, and what I'd like to do is share with the world my knowledge and experience of being the healthiest you can be," she says, with great sincerity.

If remarks like that suggest that Jyl's commitment to fitness borders on almost religious zeal, a look around the Steinbacks¹ home confirms it. The kitchen alone is a shrine to the virtues of healthy living. The double-door refrigerator is crammed with fresh fruits and vegetables, bottles of oil-free salad dressings and assorted fat-free yogurts, cheese and frozen desserts, The pantry (a spacious cabinet of whitewashed wood) is stocked with soups, sauces, cookies and crackers with the labels "lite," "healthy," and "reduced calorie."

On the white Formica counter, there's a juicer and a bread-making machine, which Jyl uses daily. "Most of the bread you buy has fat in it," she explains, as she dumps a few cups of flour, a fistful of N,cast and a half cup of applesauce (instead of shortening) into the machine¹s canister.

A spare room on the second floor of the Steinbacks' house is better equipped than the average YMCA gymnasium. For their workouts, Jyl and Gary have a Stairmaster, two Life-Cycles, a bench, a back machine, a full selection of free weights, even a ballet bar. "I use the bar for Callanetics," she says, referring to a challenging series of exercises that builds and shapes the very deepest of muscle.

There's more fitness gear outside: a pool, where both Jyl and Gary regularly swim laps, and a water-volleyball net that they use when they entertain friends. For Jamie and Scott, there's a basketball hoop, a mini trampoline, a swing set, a balance beam and a paved-over patch of lawn for biking and roller skating. "We want to teach the kids that exercise can be great fun," says Jyl.

As for their diet, she explains that, following the advice of the family pediatrician, they don't restrict the kids' fat intake as much as their own. "We make sure they get what they need, but we want them to learn healthy eating habits," she says.

Scott is still young enough to eat only what his parents feed him: 2 percent milk, bread, rice, small bits of cheese, beans and lots of fruit and vegetables. Jamie is old enough to have preferences of her own. "She likes potato chips," says Jyl, pulling a can of Pringles from the back of the cabinet and holding it up with a faint air of distaste. "I let her take a few of these to school, as long as she takes something healthy too, like carrot sticks."

Scottsdale, Arizona, with its dry air, scant rainfall and average temperature of 76°F, has the kind of climate conducive to healthy living

It was the lure of the weather that drew Jyl and Gary here from the colder and grayer skies of St. Louis, where they both grew up (they didn't know each other) in middle-class suburbs. Gary was a self-described jock; Jyl

At Arizona State University, majoring in elementary education, Jyl took a sabbatical from the sports she played throughout her childhood. "I wasn't in a health mode when I was in college," she says, as she somberly describes her "party-girl" life. "I ate ice cream, I drank beer, I went to bars

"Look how pudgy I was," she says, with a grim tone. "I was about 135 pounds, a size eight."

Jyl and Gary met through a mutual friend. In 1979, he had moved from St. Louis to Arizona because of his allergies and had rented a room in a house that Jyl was sharing with another woman. "We were just friends at first, you know, a Three's Company sort of arrangement," she says, referring to a then-popular TV show about a man whose two house mates were women.

But then they discovered a shared passion for fitness. "Gary tells people I got him into this health thing, but that¹s not true," says Jyl. "He's been into his body for as long as I've known him." In fact, when they first met, he was a jogger

Love bloomed on the jogging trail, and blossomed on the tennis courts. "He asked me to marry him one day after we'd just finished a game of tennis," she says, explaining that they had been to a friend's wedding the night before. Gary's matter-of-fact proposal caught her off-guard. "He said, 'That didn¹t look so bad. Wanna do it?' And I said, 'Do what?' And he said, 'Get married.' I said yes, and that was that."

Jyl has vivid memories of her wedding day

As fitness fads waxed and waned over the years, Jyl and Gary's exercise of choice changed.

Collectively they've favored everything from bouncing on trampolines and doing step aerobics (mid-eighties) to pumping iron and pedaling mountain bikes (early nineties). Healthy eating has always been part and parcel of their fitness fanaticism. First, it was wheat germ and juice fasts. Then, in the mid-eighties, when "low-fat" became a popular craze, the Steinbacks swore off meats and whole-milk products and slowly but surely reduced the fat in their diets. "We've been fat-free for five years now," says Jyl, explaining that the only fat they consume is that which is naturally contained in lean meats, fish and nuts.

Why do they so diligently resist fat? "I'm the kind of person who, when I do things, I go all the way," she says.

Gary goes on to explain that food is simply a fuel that the body needs, not a source of pleasure. "A lot of people live to eat," he says, as if no one ever made the remark before. "We eat to live."

Even though their focus on fitness at times seems overboard, it's hard not to like the Steinbacks. They are friendly, pleasant people, eager to put others at ease. As parents, they seem concerned and loving, constantly building up their children's self-esteem. "You¹re great, Jamie," Jyl tells her daughter one morning as she drops her off at school. "She's such a great big sister," Gary boasts later that night as he cuddles Jamie in one arm while holding little Scott in the other.

Both Jyl and Gary say, without hesitation, that their kids are the most important thing in the world. "We love it when we're together at night as a family," Jyl says.

But, then, Jyl has a remarkably upbeat attitude and seems to love just about everything. "Thank you for saying that," she responds when complimented on a pair of sunglasses she is wearing. "I appreciate you," she tells a guest at the end of a visit.

In fact, her outlook on life is so positive that, when asked what she dislikes, she can't think of anything. Fat people? "No, of course not," she says. "The first thing I think is, Oh, I'd like to help them."

Jyl thinks her consistently positive mood is partly due to the fact that her healthy lifestyle makes her feel so good. But, she is also boosted by the inspirational tapes she listens to daily. "You've got to hear this; you'll love it," she says one afternoon as she pops a cassette by motivational speaker Mike Wickett into her minivan"s tape deck. "The greatest thing that you can do is live enthusiastically," Wickett effuses in an energetic, can-do voice. "You can do anything if you're excited about it. . . . People with excitement attract the breaks, and the luck and the opportunity..."

Jyl says that listening to motivational speakers has helped her set goals

Over the past two years, she's sold more than forty thousand. But she continues to peddle the books more vigorously than she's ever pedaled a LifeCycle. She drives around town, visiting retailers and sales representatives. She calls bookstores around the country, often from the cellular phone she keeps in her car. She goes to trade shows, mall events, health fairs

One evening, she heads out to meet with representatives from QVC, the home-shopping channel, to show off her books for a possible infomercial. She is surprised when Gary shows up, unannounced, to help her unload the things for her display from the car. "Oh, Gary, you're wonderful," she says, pecking him on the lips. She turns to her companion. "He¹s so supportive of me. I couldn't do it without him."

Despite the enthusiastic case Jyl made for her books, they weren't chosen to be sold on QVC. But Jyl seemed undaunted. "That wasn¹t the right time, it wasn¹t meant to be," she says, sounding as cheerful as ever. "We¹ll keep trying."

When asked, Jyl admits that one of the reasons she's so motivated to sell her book is the opportunity to make big money. She's seen people like Susan Powter and Richard Simmons get rich from the fitness craze, and she wants her slice of the fat-free pie. She's been determined to have financial success ever since she worked at the luxury spas. "It made me realize I'd rather go there than work there," she says.

But she insists that, far more important than her own personal ambitions, her reason for writing the cookbooks is the wish to share with the world the fantastic wonders of fat-free living. "Being fit makes you feel terrific," she says.

She repeats that sentiment later in the day as she climbs Squaw Peak, a short but steep mountain trail near her home. While other climbers huff and puff along, Jyl plows ahead with a determined manner and a relaxed smile on her face. "It just makes me feel greeaaatt," she says again and again as she climbs upward.

"I feel really awesome," she repeats when she reaches the summit. She stretches out her arms and gazes at the vista. Her eyes are bright, her skin flushed, her cholesterol low and her adrenaline flowing. She looks as if she's on top of the world.