Taking calorie cutting too far not only slows your metabolism, it also may weaken your bones, according to a study from the University of Missouri. After putting middle-aged women on a very low-calorie diet for three months and then on a weight-maintenance diet for nine more months, researchers found that the dieters' "bone turnover" (the rate at which bone is broken down and replaced) was significantly higher when they were losing weight too fast. That effect could up their risk for fractures and osteoporosis. "Even after the women were in the weight-maintenance phase, turnover remained elevated," says Pam Hinton, Ph.D., lead study author and associate professor of nutritional sciences at Missouri's College of Human Environmental Sciences. If you're planning to lose 5 percent or more of your body weight, incorporate high-impact, weight-bearing activities into your exercise routine to keep bones strong, says Hinton. And aim to lose extra pounds gradually (about 1 to 2 per week) by keeping your daily calorie intake above 1,200.