Ionizing radiation is a health threat to many, including warfighters, radiological emergency responders, radiotherapy patients, and astronauts. Despite this, no FDA-approved prophylactic medical countermeasures exist to attenuate the symptoms that occur from radiation exposure. Manganese has recently been shown to be critical for radioresistance in a wide range of organisms. In this study, we designed a stringent feeding method to test the prophylactic effects of dietary manganese on