Transcript for Rare Condition Makes Kids Intensely Hungry 24/7

Good evening. Bizarre med mysteries that stumped the experts at first. We start with the new tape from the restaurant. Our cameras to catch a screaming fit. A young girl who won't stop eating. Doctors say her mind tells her one thing. She thinks she's starving but she weigh 350 pounds. Why can't she stop eating? Reporter: Tonya Wilkinson, a single mom, is very concerned about her daughter's weight. Let me have this. No it's not going to fill me up if you take a piece away! I'm not letting you touch my food! Reporter: But for 14-year-old Hannah from Phoenix Arizona, this isn't your typical weight problem. She suffers from a mysterious condition. I am hungry. You're hungry? When we get back I can fix lunch for you. No, I want it now. Reporter: Every waking minute of her day is filled with intense hunger. She is starving 24/7. Reporter: Even if she's just eaten. We can have dinner, finish -- I can take her plate away and she'll look at me and say, "Mom, I'm hungry." Reporter: Hannah was born with a rare genetic disorder called prader-willi syndrome, which causes her insatiable appetite and dramatically slows her metabolism. Hannah is gaining weight on only 900 calories a day, less than half of what an average 14 year old needs. She now weighs 350 pounds. The hypothalamus, the part of our brain that controls our hunger, is pretty much shut off. So, they don't know that they're not hungry. Reporter: Do you ever feel full? No. Reporter: Never. No. Reporter: Are you hungry right now? Yes. Reporter: The Wilkinson's lives are ruled by Hannah's hunger. Tonya keeps the kitchen on full lockdown, literally. A padlock on the refrigerator. No food in the cabinets. The pantry door always locked. I was cleaning the kitchen after dinner. We had just had a pot roast. She grabbed a huge piece of that pot roast and shoved it in her mouth. And I turned around and she was choking. I have caught her with the dog food, eating dog food. Reporter: If she were left unrestricted in a room with food -- Uh-huh. Reporter: -- Would she literally eat until she could not eat any more? She would eat herself to death. Reporter: That kind of extreme behavior is not uncommon with those suffering from prader-willi syndrome. Like mirabel Rivera, featured in this documentary. Locked behind this fence for her protection, mirabel throws herself into a frenzy when she can't get to an ice cream truck. Mommy, mommy. Reporter: In a public park she wanders off to a hot dog stand and when she thinks no one is looking starts begging from strangers. Can you buy me a hot dog, please? One. Will you buy me one hot dog, please? Will you buy me one hot dog, please? I am so hungry. Reporter: We saw the same desperation as we watched Hannah's mom try to limit her portion at a nearby restaurant. Hannah, it's a lot honey. No! Mommy didn't realize how big they were. No, this is all I get for the night, and I don't have anything else on this plate. Reporter: If you think you have to be born with the intense desire to eat like Hannah, think again. 12-year-old Alexis Shapiro of cibolo, Texas also suffers from an insatiable appetite. But her affliction is something entirely different. Something she developed when she was nine. Look at the striking difference only a few years earlier. She was only, you know, 52 pounds at nine-years-old. She loved to dance around the living room, put on little shows, make her sister and brother dance with her. I miss you. I love you. Reporter: But in 2011 Alexis was diagnosed with a rare brain tumor. Soon after surgery her parents noticed an immediate change in their daughter's behavior. She immediately started asking for food. Reporter: Even before she was released from the hospital, Alexis packed on eight pounds. And once they got home, things onlyot worse. When it came to food at the house, it was all that mattered to her. Reporter: That joyful little girl who once danced around their living room was now transforming before their eyes, both physically and emotionally. It turns out Alexis' surgery had altered that part of her brain which regulates appetite. The result? Hypothalamic obesity, giving her strikingly similar symptoms to Hannah. She was 52 pounds at surgery, and then it went up to 75, and 100. She'd wear something twice and then it -- she would outgrow it. Reporter: And though Alexis was eating normal-sized portions, she grew to 203 pounds. There's been a long mourning process, because we lost the old child that we had. Reporter: But could these two girls unlock the secrets of obesity for the rest of us? Dr. Robert Lustig thinks it can. In the hypothalamus, the part of the brain that, when damaged, releases insulin causing an insatiable appetite. Controlling that insulin, not calories, might be solution to fighting obesity. The same thing occurs with garden variety obesity. When we get the insulin down they feel better. They lose weight, and their lives turn around. Reporter: But that's all still in the experimental phase. The shapiros were desperate to stop Alexis' weight gain right away. Once again they turned to surgery, this time to reduce her stomach size. After doing lots and lots of research, I found that some patients have had success with gastric bypass surgery. And I felt like I had hope. Reporter: Since the surgery, Alexis' insatiable appetite has returned to Normal, she is more active and has lost 50 pounds. She could not get down from there a month ago by herself. It's the first time I've never helped her. It's just amazing. The medication, we've been on it for two months, and it seems to be working. Reporter: For Hannah and others born with abnormal chromosomes causing prader-willi, these kinds of surgeries are not an option because they do little to suppress appetite. Heart enlargement, we have high blood pressure. Reporter: Tonya says her only hope now is to get Hannah to a specialized facility in Pittsburgh that offers round the clock monitoring and meal planning. She's been fighting with her insurance company for almost a year for them to cover it, but so far they've refused. The problem still to this day is that people consider obesity to be a behavior. They consider it to be your own fault. They consider it to be a matter of gluttony and sloth. Nothing could be further from the truth. And prader-willi patients are the proof that that is not true. So because insurers still view obesity as a behavior, prader-willis sometimes get swept under the rug. I think the thing that affects me the most is just knowing I -- there is someplace I can get her help, and I can't get her there. And if I don't get help, I will lose her. Reporter: For now, they fight on. Hannah has a competitive spirit. Proudly showing medals she won for swimming at the special olympics, saying she wants to lose enough weight to compete again. A determined team vowing to beat this disease. She's just very, very loving. She's very happy. For her to have this syndrome and for it to be such a hard syndrome to deal with, and to live with, she's a really, really amazing kid. Next, from getting selfie

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