(NBC) - What if you could eat whatever you wanted, and never gain a pound? It's Dwanna Swan's reality, but she would trade the extra calories for good health.
Swan, 41, has lipodystrophy. She was born without fat cells, so any fat she takes in doesn't stick to her body.
"I was born without any fat on my body," she said. "It may not look good as a child, but as an adult it's like, 'Wow. It's working for me.'"
She is 115 pounds, a size zero and her arm and leg muscles are cut. Most would say she is naturally fit.
"It's really nice because I don't have to work out for it," Swan said.
She was Dr. Abhimanyu Garg's first lipodystrophy case 25 years ago. Now he is one of the world's leading lipodystrophy researchers and the chief of metabolic diseases at University of Texas Southwestern in Dallas.
"If we eat about 2,000 calories a day, this child may be eating 4,000 to 5,000 calories a day," he said.
In people who have the disorder, fat goes to the liver and can take a serious toll on their bodies, Garg said.
"They develop diabetes very early in their life; most of them during the teenage years," he said. "They have a large liver, because their liver is filled with fat, and it damages them as they grow."
Stephanie Lyon, 42, another of Garg's patients, has partial lipodystrophy. Her body has the signature characteristics: lean muscular arms and legs. Unlike Swan, Lyon gains weight in her face and stomach.
"You can't judge a book by its cover," she said. "They don't understand all of the medical complications that are going on when they look at me."
Under Garg's direction, Swan has been taking Leptin shots to regulate her appetite, and her diabetes is under control. Even though she can eat whatever she wants without gaining an inch, she chooses an extremely low-fat, vegetarian diet for her health.
It's unclear how long she will live, but she is planning a full life.
"I'm just happy that I'm still alive and that I beat the odds," she said.
About one in 1 million people have lipodystrophy. Garg said it's difficult to pinpoint exactly how many people have lipodystrophy, because so many people go undiagnosed or misdiagnosed. Lipodystrophy can be acquired in people with autoimmune diseases, and women are four times more likely to develop it.