Microgravity and Bone in Space

Author: Dr Arun Sah  MBBS, MS-ortho

Tianjin Medical University, China

Bone plays an important role as a structure that supports the body and stores calcium. It retains fracture resistance by remodelling through a balance of bone resorption and formation.
Bones are usually dense and strong enough to support your weight and absorb most kinds of impact. As you age, bones naturally lose some of their density and their ability to regrow/remodel themselves.

Bone Remodelling Cycle. Source: Author

​In a microgravity environment, because of reduced loading stimuli, there is increased bone resorption and no change in or possibly decreased bone formation, leading to bone mass loss at a rate of about ten times that of osteoporosis. Life in the microgravity environment of space brings many changes. Loss of bone mass is particularly noticeable because it affects an astronaut’s ability to move and walk upon return to Earth’s gravity.
Human spaceflight was once a fantasy only to be found in between the pages of a novel or on movie screens, however, now it is almost a tangible reality. Humans are going to spend more time in space. The human body is intrinsically adapted to Earth’s gravity, so exposure to conditions of reduced gravity, or microgravity can cause complications in many normal bodily functions. Microgravity decreases the effort required for movement.The length of space missions—and consequently the amount of time astronauts spend in orbit—has increased since humans began exploring space. Space travellers are exposed to numerous stressors while in space.
The reduced mechanical loading of weight-bearing bones caused by microgravity (μg) leads to bone loss in humans, especially in long-term space missions. As previously mentioned, this bone loss results from increased bone resorption and either unchanged or decreased bone formation, as observed in various human studies conducted both in space and during bed rest. Microgravity causes calcium to be released from bones, which suppresses parathyroid hormone (PTH) and lowers circulating levels of 1,25-dihydroxyvitamin D, although concentrations of 25-dihydroxyvitamin D remain adequate. This process reduces calcium absorption in the body.

Source: InnovaSpace

The decrease in bone formation is associated with impaired osteoblast function and increased osteocyte apoptosis. Physical exercise using devices such as treadmills and resistive exercise equipment can help reduce the negative impact of microgravity on bones and muscles. Weight training and aerobic exercise are designed to simulate the mechanical loads normally exerted by gravity on Earth.
Proper nutrition and the use of supplements—such as vitamin D and calcium—are important to support bone health during and after a space mission. Rehabilitation programs include structured physical exercise, physical therapy, and nutritional monitoring to ensure optimal recovery. Together, these countermeasures aim to preserve musculoskeletal health in space and promote a successful transition back to Earth's gravity. Continued research is essential to refine these strategies for longer missions, such as those to the Moon or Mars.​