Some female athletes believe being extra thin helps in improving athletic performance (see "Female athletes trade thin for results - more discovering that thinner doesn't mean stronger"). It turns out that some girls go on sub-1000 calorie per day diets to try to lose weight. Since this starves the girls from adequate energy and nutrition, performance will suffer. From the article:
About a year ago, Pudwill and several sports medicine doctors and therapists at the University of Colorado opened the Active Women's Health program after realizing many female athletes didn't understand how to meet their body's nutritional needs. Adding to the challenge is society's pressure to lose weight.
"What happens with females is they don't realize how much it takes to fuel their body to do the kind of activity they're doing," she said. "Sometimes, typical of females, they're not eating enough."