The Impact of Zero Gravity on Human Bone Density - A Literature Review of Spaceflight Data

Abstract

Spending months in space without gravity takes a serious toll on the human skeleton because without the constant pull of gravity, bones especially in the spine, hips, and legs, lose strength fast, breaking down at a rate of 1–2% per month. This review looks at decades of spaceflight research, from Skylab and Mir to the International Space Station and NASA’s Twins Study, to understand why this happens, how fast it occurs, and what can be done about it. The problem starts with the lack of mechanical loading, which signals bone cells to stop building new tissue while continuing to break down old bone. Changes in calcium balance, hormones, and other body systems make things worse, and recovery after returning to Earth is often incomplete. Astronauts currently rely on intense exercise, bone-protecting drugs, and carefully planned diets, but these are known to only slow down the losses. New ideas like creating artificial gravity, redesigning spacecraft for better loading, using personalized health plans, and monitoring bone health in real time could help protect future crews on missions to Mars or the Moon, and even space tourists. Ultimately, solving this problem is the key for space exploration and improving bone health here on Earth.