WALNUT CREEK, Calif. — Nancy Brueheim wishes she could break 100 pounds. Without working at it, Brueheim, who is 71 and stands 5-foot-2, fluctuates between 95 and 98 pounds.

But the skinny life has its challenges. Clothes are difficult to find. And Brueheim has heard a lifetime of hurtful comments. "I'm healthy, I have a lot of energy, and yet people will say such things, like, 'Gosh, you need to eat more.' We'd never say such things to an overweight person," she says.

You know the stereotypes. If someone is very thin or underweight, we assume they are ill or have an eating disorder. Or a hollow leg, right? Maybe they live at the gym. All not true.

While genetics and ethnicity play major roles, there is no one-size-fits-all reason why some people are slim and remain so without really trying, says Kaiser Permanente registered dietitian Nora Norback.

"Even if it comes from a sense of caring, it's not appropriate and can create distress and unhappiness for the person," Ryerson says. "They can feel blamed and criticized. In our society, we associate control, will power and strength with thinness. But for people who are naturally thin, it causes them to feel bad about themselves. They're not trying to be the object of envy. And they don't want to have to explain themselves. Why should they?"

METABOLISM

Many thin folks, including Leary, point to a fast metabolism. Metabolism is influenced by many things, including gender (men have a slightly higher metabolic rate), hormones, sleep, exercise and use of stimulants such as caffeine and nicotine, Norback says. Also, some people simply have mild appetites, and must remind themselves to eat. Bottom line: Don't judge a body by weight, because it's not about numbers.

"It's about health," Norback says. "Healthy people do come in different shapes and sizes, but at the extreme ends, we certainly see more health problems. We make a lot of our judgments on how a person looks. But you shouldn't assume that someone is unhealthy if they're really thin."

To rule out eating disorders, Norback looks at a variety of factors beyond the scale, including body image, diet and malnutrition, hydration status and exercise frequency.

"I have my dad's genes," says Ewart, who wears a size 2 or 4. "I eat whatever I want, I sit on the couch, and I don't gain any weight."

Strangers have felt the need to tell Ewart what they think — that she's too thin — but she feels blessed. She hits the gym to tone up. That's all.

STOMACHS ARE SMALL, TOO

Brueheim does Pilates and walks up to three times a week. Any more cardio and she tends to drop weight. When that happens, she'll add bread to her diet. Fish, pasta and yogurt are her staples.

"What people also don't realize is that little people have small stomachs," Brueheim says. "A doctor once told me that a lean horse runs a long race. I like to remember that."

When Katrina Ritchie hit puberty, she lost 20 pounds and shot up like a bean pole. Her mother was concerned, but, besides a period of anemia in her 20s, Ritchie was and remains healthy.

"They hate me," Ritchie says. "People say, 'You've got to be kidding me.' I can still fit into things I wore when I was 13."

"They're so big, it kind of turns me off," she says, adding that she prefers expensive restaurants, where quality trumps quantity. When people ask her "how she does it," she gives them the skinny.

"I don't diet but I've been exercising most of my life," she says. "I don't eat fast food. I've never had a hot dog. I don't drink soda. I enjoy eating salads and other things people turn up their noses at."

MOST ARE THIN THERE

A few years ago, Gan, who says he was so thin his chest was concave, ate more to gain weight. He put on about 10 pounds, but didn't like the way his face looked as a result, he says.

These days, he swims, lifts weights and takes a protein supplement to build muscle. While he still weighs 120 pounds, he feels strong and toned. And, even though he wears boy's jeans and admits that he's "a little too skinny," he is not without his fat days. Seriously.

"I think I'm fat when I haven't done any physical exercise in a few days, or if I ate and drank too much the night before," Gan explains. "Whenever I complain that I'm fat, my roommate says that if I was any thinner, I'd be two-dimensional."

THIN, NATURALLY

Genetics. If mom or dad are thin, there's a higher chance you will be.

Metabolism. Influenced by factors including gender, age, stress and stimulant use.

Thermic effect of food. An estimate of the energy required to process food. It is usually 10 percent of energy (calorie) intake, but can be higher in some people.

Variations in appetite regulation. Some people have small or mild appetites and have to remind themselves to eat.

Ethnicity. Thinness is more common in some populations, such as Asians.

Eating right. Diets rich in fruits, vegetables, whole grains, lean proteins and low-fat dairy.

Exercising regularly. At least three times a week.