I had to look it up
" Relative Energy Deficiency in Sport (RED-S)
Some physically active people are at risk for Relative Energy Deficiency in Sport or RED-S. This often unrecognized disorder can include low energy availability (inadequate caloric intake); with or without disordered eating; amenorrhea (lack of menstrual periods); and low bone mineral density."
"RED-S can impair health and sport performance.
Lowered energy availability: an underfueled athlete is a slowed and weakened athlete. No matter what the sport, if your muscles lack sufficient and proper fuel, performance is impaired. At first there might just be some early fatigue. As the fuel deficit worsens, actual loss of strength and muscle size can occur as the body uses skeletal muscle in order to fuel essential body functions, like heart function and breathing. Lack of fuel can also lead to your inability to concentrate, not a quality befitting an athlete. If you are an athlete with strength losses and poor concentration, you can be more easily injured. Injuries then are slow to heal in a poorly fueled body.
Amenorrhea: Loss of menstrual periods may signal a change in your body’s intricate and complicated hormone system. Hormone imbalance from underfueling your body can result in lowered estrogen production. There are also other causes of lowered estrogen levels. A diminished estrogen level can have many effects; the most immediately apparent one can be bone loss. Amenorrhea can often go unreported to medical providers because of the common belief it is “just part of the training effect.” We do know that the bone loss that occurs as a result of this is NOT “just part of the training effect” and can start to occur after just a few months with no period.
Low bone mineral density: Loss of bone, especially if you are an athlete, can be an unfortunate setup for an injury. Stress fractures can sideline sports activity and be slow to repair if you are underfueled. Repeated stress fractures and unexplained injuries should be a red flag to further evaluate your eating and exercise patterns. Bone loss that occurs because of amenorrhea can be permanent. "