Katie Ledecky who just won another gold medal and is one of the GOATs, has POTS (Postural Orthostatic Tachycardia Syndrome), which causes a fast heart rate, dizziness, and fatigue. For years she didn't speak about the condition as she didn’t want it to shape the media narrative.
https://www.self.com/story/katie-ledecky-health-condition-pots