Killer Workout  The virtues of dieting and physical fitness
pervade our consciousness. But either can go too far, leading to
self-starvation or compulsive exercising--or both. In fact, one
may actually cause the other, warns W. David Pierce, Ph.D., of
the Sociology and Neuroscience faculties at the University of
Alberta. Here, he discusses a dangerous and increasingly
pervasive phenomenon called "activity anorexia."  Nancy K. Dess:
What is activity anorexia?   NKD: How have you studied this in
the laboratory?   Find a Therapist  Search for a mental health
professional near you.  NKD: Why does this happen?  WDP:
Consider Darwin's theory of evolution through natural selection.
Animals would have gained a survival advantage by migrating when
food was scarce, and by staying on the move until an adequate
supply was found. A trek moved them away from famine and
increased the odds of finding food--and surviving to pass on
this trait.    WDP: Our culture brings dieting and exercise
together. Current cultural values o