If fat is bad, how good is really skinny?

There’s another side to the obesity story. It’s the skinny story.

Jared Rapier, a 120-pounder at 5-foot-10, is living it. And one day, scientists say, understanding what keeps people like him thin may offer ways to counterbalance the genetics of those who struggle with fat.

Like the obese, the underweight get labeled. In polite conversation, they’re envied for “good metabolism.” In callous moments, they are “skin and bones.”

Rapier was bothered by such comments as a kid. Going to the pool was awkward. He asked doctors how to gain weight, but no luck. Now, at 23, he’s in the acceptance stage. He eats like a Viking.

“And my friends will say, `Where does it all go? I swear you have a tapeworm,’ ” he said.

But are such “constitutionally thin” people destined for better health and longer lives? The answer is maybe, as long as their skinniness is not extreme.

Much is known about obesity. It increases risk of some cancers, diabetes, depression and other health problems. The overweight generally live shorter lives.

The science on thin people is less advanced. Some are skinny due to nongenetic factors; they smoke, have eating disorders or are otherwise ill. But perhaps 70 percent of being thin is genetic, said researcher Cynthia Bulik, director of the eating disorders program at the University of North Carolina.

The hunt is on for the specifics. Researchers say the future may produce weight-management drugs that mimic the genes of skinny people.

“We’re not doing a good job preventing obesity and treating it,” Bulik said. “And we have to go after it from every angle.”

A thin person may have a metabolic system that turns food into energy very efficiently. In some cases, genes may dictate that he gets full faster, eats less frequently or doesn’t crave fatty foods.

Recent research showed the thin are often active – not just exercisers, but people who fidget or “can’t stand still.”

“That is probably part genetic,” said Dr. Susan Yanovski, director of the obesity and eating disorders program at the National Institute of Diabetes and Digestive and Kidney Diseases. “If you get overfed, you may feel uncomfortable and want to move around a lot.”

A muted appetite is not a factor for Rapier, a public relations consultant who lives in Norwood, Ohio. He eats fast food instead of cooking. A typical lunch is two Big Macs, large fries and a Coke. He often wolfs down a full pizza for dinner.

“It’s not like I really like unhealthy food,” he said. “When I’m comparing – do I want a small salad or a Big Mac and fries? – I go for the Big Mac because I don’t have to worry about gaining weight.”

Bad idea, nutrition experts say. While constitutionally thin people can eat tons of junk without adding blubber, long-term consequences are risky.

Fatty foods bestow few health benefits. Too few fruits and vegetables rob the body of vitamins and minerals and could lead to serious illness.

“You can definitely be thin and unhealthy,” Bulik said.

To categorize body sizes, federal officials use a “body mass index.” Normal BMIs range from 18.5 to 24.9. Under 18.5 is underweight. For someone 5 feet 3 inches tall, that’s 104 pounds or less; for someone 5-foot-10, it’s 128 pounds or less. (To calculate your BMI, go to www.nhlbisupport.com/bmi.)

People who naturally fall in the normal range, or even toward the light end of the scale, should not worry about their weight, experts say.

In fact, some evidence suggests an especially lean body conveys advantages. The naturally thin are less likely to have eating disorders.

Thin women have higher self-esteem than their heftier counterparts (not true for men, who favor muscular bodies).

And experiments on animals associate thinness with a longer life.

Stephen Spindler, a biochemistry professor at the University of California at Riverside, said mice switched to calorie-restricted diets have experienced slowed growth of tumors and lived about six months longer than mice fed normally. The studies have not been duplicated in humans for ethical reasons.

“There are things that we know that put you in a position to have a longer life: eating more fruits and vegetables, regular exercise, quitting smoking, eating less meat products and saturated fats,” Spindler said. “What we don’t know is whether caloric restrictions will work on free-living humans.”

But even if this did lengthen lives, there are risks in pushing yourself to extremes of thinness.

Someone with a BMI under 18.5 may be courting malnutrition, said Dr. Sattar Hadi, an assistant professor of internal medicine at Vanderbilt University’s Center for Human Nutrition.

“With malnutrition, you can have different problems,” Hadi said. “You can have decreased immunity. You can get pneumonias and sepsis. … There’s a whole slew of diseases that people can get. And they die earlier.”

Thinness also increases the risk of osteoporosis, a progressive disease that makes bones less dense and prone to breaking. That’s because people with minimal fat produce inadequate estrogens, hormones essential to bone development.

Hormonal imbalances in very thin women may also impair their ability to menstruate and reproduce.

Dr. John Morley, director of geriatrics at St. Louis University School of Medicine, said being underweight is especially problematic for older people. They are prone to brittle bones, and some padding is useful in a tumble.

“If I don’t have fat, I can’t protect my hip when I fall,” Morley said.

But the biggest risk for older people is losing weight so that they fall beneath the ideal range. Toxins accumulated over a lifetime are stored in fat. When fat is shed, allowing the poisons to seep into the bloodstream, serious troubles can occur.

And, when an older person loses weight and then gains it back, the new heft is only 15 percent muscle. As a general rule, Morley said, people should be at an ideal weight at age 50, then keep it there.