The Biology of . . . Appetite  Science zooms in on why people
eat too much  By Kat McGowan|Sunday, September 01, 2002  For
most of us, snacking is a bad habit. For Melissa Moss, a
Californian with a rare genetic disorder, it's a matter of life
and death. A chromosomal abnormality known as Prader-Willi
syndrome has cursed her with both an insatiable appetite and a
metabolism so slow that she gains weight eating just 1,600
calories a day.  Moss, an articulate 28-year-old, used to steal
leftovers from cafeteria trays when she was in elementary
school, or even pick scraps off the floor. Now that she's an
adult, food is never far from her mind. She spends hours every
day counting calories, weighing and reweighing foods, and
figuring out, down to the last half pretzel, how much she can
eat. Her 1,300-calorie-a-day diet of yogurt, fruit, fat-free
turkey sandwiches, and diet TV dinners is spartan and
monotonous, but it has kept her slender, and alive. Untreated,
most people with the syndrome become obese by their tee