HUDSON, Mass. - Inside the trim colonial house, the lock on the kitchen door is protection from mortal danger.

The house's five residents have Prader-Willi syndrome, a chromosomal disorder that is the most common known genetic cause of obesity, affecting about 1 in 14,000 people. People with the syndrome can literally eat themselves to death, either rupturing their stomachs or growing fatally obese by early adulthood.

Punishments, rewards, diet pills - nothing works against the deep drive to eat, fueled by a flaw in the hypothalamus, the part of the brain that controls appetite. People with Prader-Willi (pronounced PRAHDER-WILLEE) tend never to feel full; some spend their whole lives ravenous and can gain 20 to 30 pounds in a single weekend.

"Telling them to learn to control themselves is like saying, 'If I had the right reinforcements, I could learn to be 6 feet tall' - I can't learn to do it, it's not possible for me, it's not a choice," said Dr. Chris Gordon, medical director of Advocates, the large mental health agency that runs the Hudson house.

The Prader-Willi appetite is extreme, but the growing scientific understanding of the disorder and tactics developed for dealing with it could hold lessons even for garden-variety dieters, said Dr. Tony Holland, professor of psychiatry at Cambridge University in England, one among a small cadre of researchers worldwide trying to understand the syndrome.

If scientists determine exactly which gene or genes go awry in the syndrome, Holland said, "you can then say, to what extent does that mechanism apply to general obesity? Does it help to explain it in 50 percent of the cases or 10 percent of the cases?"

Also, he said, strictly controlling access to food, as is done at the five group homes run by Advocates, tends to make people with Prader-Willi happier.

"The general message for dieters is that you've got to try and live in an environment where you're not constantly tempted by food, however difficult that may be," he said.

Prader-Willi syndrome involves a variety of possible problems beyond appetite, including poor muscle tone, short stature, mild intellectual impairment, abnormal regulation of body temperature, and compulsive tendencies.

A consensus has evolved that people with the disorder do best when in settings where all food is strictly controlled, out of sight and out of reach, except during low-calorie, low-fat meals prepared by others. Locks on refrigerators may seem cruel, specialists say, but they actually provide a welcome sense of safety.

At Advocates, which runs the largest Prader-Willi residence program in New England, the policy is to keep the kitchen door locked. Residents do not help prepare food. Field trips try to avoid places where food is available.

Before Jennifer Mars, 34, moved into an Advocates home in May 2005, her weight had reached nearly 300 pounds. Now she is down to 184. Her diabetes has abated, and best of all, she said, she can fit into the front seat of her father's pickup.

"I'm sticking to my diet and stuff," she said, pride adding extra sparkle to her eyes. "It gives me more energy."

The homes offer regular exercise programs and tailor meals to each resident's allotted number of calories. Because the disorder involves an unusually slow metabolism, some, like Mars, must expect to spend their lives eating just 1,200 a day, about half a typical person's daily calories.

Albert DeSa, 22, said he has lost about 100 pounds since his peak weight of 250. Control is easy while in the Hudson house, he said, but in other settings, the struggle remains. "If there's food all around you, we can eat every two seconds," he said. He hates the endless Papa Gino's and Subway ads on TV. And for him, the treat of a Red Sox game also carries a downside: Afterward, "the smell of Fenway franks is all over our clothes, and it's hard."

Finding food-free places to take residents for outings seems to be getting ever harder, said Patrice Carroll, administrative director of the Advocates Prader-Willi program. Movies have popcorn, and even libraries tend to offer coffee and cookies.

The need to control food access also gets in the way of some residents' desire to work. Carroll is hoping to put together a soap-making co-op run by residents that would allow them to earn money while still being supervised by trained staff.

In recent months, researchers like Holland have begun using high-tech brain scans to try to pinpoint what goes wrong in the Prader-Willi appetite. He theorizes that the Prader-Willi brain is getting false signals that the body is starving. Other scientists have been probing the possible role of ghrelin, a hormone related to hunger.

For now, the main medical treatment for Prader-Willi is growth hormone, which offsets the syndrome's typical short stature. Children with Prader-Willi have begun to receive growth hormones routinely.

The extreme appetite does not tend to kick in until children are toddlers, so the Prader-Willi Syndrome Association also emphasizes early diagnosis through chromosome testing.

The most important lesson for parents of Prader-Willi children is that "with our kids, loving your kids means not feeding them," said Janalee Heinemann, the executive director of the association's American branch. "It's the one syndrome where you can love your child to death, and in the past, almost all parents did."

When children with Prader-Willi grow old enough to move out of their parents' house, the challenge then becomes finding a place for them with enough understanding of the syndrome to keep them healthy, Heinemann said.

Specialized programs like the Advocates houses are rare, she said. And current placement practices tend to emphasize putting people in the least restricted environment possible, which can mean dangerous weight gain for people with Prader-Willi.

Fifty-year-old Mary Tsoules used to weigh nearly 400 pounds and once needed a chunk of her stomach removed after it overstretched and the tissue began to die. Now weighing about 130 pounds, she has this advice for run-of-the-mill dieters:

"Keep going and doing a good job," she said. "And don't get discouraged."