Written by Robby Berman on April 4, 2021 — Fact checked by Jessica Beake, Ph.D. New research examines what happens to the human heart when in space. Stocktrek Images/Getty Images American astronaut Mark Kelly spent 340 days in space, and changes to his body help researchers understand the effects of living in low gravity. Over the course of his mission, Kelly lost heart mass despite a rigorous exercise regimen. A new research paper says studying extreme swimmers can help understand the impact low gravity has on human bodies. The study reports on a similar loss in heart size in a man who attempted to swim across the Pacific Ocean in 2018.