Exercise is essential to good health, but so is keeping your body in an energy balance. Skimp too much on key nutrients and you could be doing more harm than good, messing with your menstrual cycle and putting yourself at risk for osteoporosis. “It’s not just women with eating disorders who could potentially develop these issues,” notes Elizabeth Matzkin, M.D., an assistant professor at Harvard Medical School’s Department of Orthopaedic Surgery. “If you’re not taking in adequate nutrition for the level of activity you’re doing, it can seriously affect hormones and menstrual cycles.” And too many missed cycles can lead to low bone density. Warning signs include not getting your period, feeling constantly fatigued, and injuries like stress fractures. To keep bones strong, take in at least 1,000mg of calcium a day and 600 IU of vitamin D through foods like dairy products and/or supplements.