Complex sentence: When female athletes'don't intake enough calcium (approximately 1,500 mg per day) and experience low estrogen levels this results in weakened bones which leads to higher risk of injures, such as stress fractures.
Split into simpler sentences: When female athletes'don't ingest enough calcium (approximately 1,500 mg per day) and experience low estrogen levels this results in weakened bones. The weakening of bones then leads to higher risk of injuries, such as stress fractures.