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Former Canadian astronaut Robert Thirsk enjoys cycling on board the ISS

Former Canadian astronaut Robert Thirsk enjoys cycling on board the ISS (NASA, CSA)

STEM in Context

Spaceflight and Bone Loss

Subin Rajendran

Summary

Astronauts can experience bone loss in space. To understand why, you need to know a bit about how bones are formed and maintained in your body.

Space travel has often been a big theme in the movies. But long-distance space travel is a cool and exciting possibility for ordinary people in the not-so-distant future. For example, NASA has, since 2009, been developing the Orion spacecraft. One day, this spacecraft might send humans beyond Earth’s orbit. This has not happened since the final Apollo missions to the Moon in 1972.

But don’t start planning your trip to Mars just yet!  Medical researchers are still trying to sort some things out. Specifically, there are some pretty serious health issues associated with long-term travel in outer space. One of these issues is bone loss. Astronauts have suffered bone loss even on relatively short trips closer to Earth.

Did you know?

As of 2019, a total of 562 people have reached outer space. Only 24 of them have left Earth's orbit.

On Earth, you experience a constant gravitational pull towards the centre of the planet. But when astronauts travel into outer space, they experience a condition called weightlessness or microgravity. This is what causes bone loss. To better understand why, you need to know a bit about how bones are formed and maintained in your body.

What are bones made of?

Bones are a type of dense connective tissue that helps to hold your body together. So are ligaments and tendons. Like any other part of the human body, bones are made up of cells. The cells in bone tissue are reinforced by collagen networks. These are clumps of collagen proteins. In fact, your bone tissue is very similar to your hair. The difference is that bones also contain a lot of inorganic minerals like calcium and phosphate. These proteins and minerals are deposited on the outside of bone cells to create something called the bone matrix. The bone matrix gives bone tissue its strength.

Parts of a bone including blood vessels, bone marrow, compact bone, and spongy bone
Parts of a bone including blood vessels, bone marrow, compact bone, and spongy bone (© 2019 Let’s Talk Science based on an image by Pbroks13 [CC BY 3.0] via Wikimedia Commons).

 

As you grow, your bone tissue is continuously breaking down and being rebuilt. This process is called remodeling. It is a very important process for your general health. The two main types of cells that maintain and regulate bone structure are osteoblasts and osteoclasts.

  1. Osteoblasts (Bs) build bones using the same kinds of cells that otherwise become fat, muscle, and cartilage. To do this, they lay down layers of collagen. Then they lay down layers of minerals.
  2. Osteoclasts (Cs) are responsible for breaking up bones and absorbing them back into the body. They use the same cells as white blood cells, which play a key role in your immune system. With the help of these cells, osteoclasts dissolve the minerals on the outside of the bone and break down the bone matrix. This causes the release of a lot of calcium. The body then reabsorbs the calcium. This happens in a process aptly called resorption.

Bs and Cs need to be in balance and be able to communicate properly for bone remodelling to occur and for your bones to remain healthy.

 

Osteoblasts build build bones. Osteoclasts break down bone.
Osteoblasts build build bones. During this step, calcium is absorbed. Osteoclasts break down bone. During this step, calcium is released back into the body (© 2019 Let’s Talk Science based on an image by Shandristhe azylean [CC BY-SA 3.0] via Wikimedia Commons).

There is a third type of bone cells called osteocytes. Osteocytes are mature bone cells that are trapped inside the bone by the osteoblasts. They occupy spaces in the bone called lacunae. Osteocytes play many different roles in bone development and maintenance. For example, they signal to osteoblasts and help with the mechanical functions of the bones. They also play a role in recognizing cracks in the bone and relaying this information to other bone cells.

What happens to bones in space?

Remodelling is key to understanding bone diseases such as osteoporosis. It is also the key to why astronauts experience bone loss in space. In space, astronauts experience spaceflight osteopenia. This condition can cause astronauts to lose, on average, one to two percent of their bone mass every month. This bone loss typically happens in the legs, hips, and spines of astronauts. Once astronauts return to Earth, it can take three or four years for those bones to recover!

The rectangle on the left shows what healthy spongy bone looks like and the rectangle on the right shows what weakened spongy bone looks like
The rectangle on the left shows what healthy spongy bone looks like and the rectangle on the right shows what weakened spongy bone looks like (Source: cropped image by Partynia [CC BY-SA 4.0] via Wikimedia Commons).

Did you know?

Thinning bone mass also triggers a rise in calcium levels in the blood. This increases the risk of kidney stones.

We already know that because of remodelling, your bones are always changing. Remember, astronauts experience microgravity while in space. This means their bones do not undergo the stress of supporting their bodies against gravity. Scientists also know that exercise is important for bone maintenance and regulating the activity of Bs and Cs. Exercise helps to keep both your bones and muscles strong. That makes it key to helping astronauts reduce the amount of bone loss they experience in space. To reduce bone loss, astronauts must exercise for a minimum of 2 hours per day.

On Earth, the best way to build bone mass is by taking part in load-bearing activities, such as walking, running, and playing basketball. In space, it is difficult to perform these activities due to microgravity. Astronauts must strap themselves down to treadmills in order to get the same benefits of walking and running!

How are Scientists Trying to Help?

A number of studies have been conducted, and others are now underway to better understand bone loss in space. A study conducted in 2013 compared the bone loss of astronauts that only exercised while in space and astronauts that exercised in combination with taking bisphosphonates, a class of drugs already used to treat osteoporosis in older adults. The study concluded that taking bisphosphonates along with exercise may be useful in protecting astronauts from severe bone loss in long-duration space flights.

Chris Hadfield undergoes a quantitative CT scan to obtain detailed images of the bones in his ankle, shortly after his return to Earth in 2013
Chris Hadfield undergoes a quantitative CT scan to obtain detailed images of the bones in his ankle, shortly after his return to Earth in 2013 (Source: Canadian Space Agency).

Another study sent a group of six mice into space. A genetic therapy was used to cause three of them to produce extra pleiotrophin (PTN), a protein that is known to help bone development. The results were very positive: the mice with extra PTN lost only 3% of their spine volume, compared to a 41.5% decrease in the untreated mice.

Research on the International Space Station (ISS) is currently underway that will help scientists better understand bone loss in space and on Earth. NASA’s Early Detection of Osteoporosis in Space project is investigating how factors, such as exercise and diet in space, are working to prevent bone loss in astronauts. The ISS is also equipped with a Bioculture System that will allow scientists to conduct stem cell research and explore its applications to bone loss in space. Most recently, scientists have been working on ways to grow new bones!

Did you know?

NASA has established a Microgravity University. It allows university students and K-12 educators to learn and conduct experiments in a microgravity environment!  

So even if space travel comes with some health risks, researchers are actively exploring ways of reducing or even eliminating the risk of bone density loss among astronauts. And who knows? They might even find a cure for osteoporosis along the way! 

Starting Points

Connecting and Relating
  • Would you be interested in conducting an extended space mission, knowing it could cause physical deterioration of your bones? Why or why not? 
  • Have you ever broken a bone? How long did it take to heal? What was involved in treating the fracture and recovering from the break?
Connecting and Relating
  • Would you be interested in conducting an extended space mission, knowing it could cause physical deterioration of your bones? Why or why not? 
  • Have you ever broken a bone? How long did it take to heal? What was involved in treating the fracture and recovering from the break?
Relating Science and Technology to Society and the Environment
  • How is space research changing as the world prepares to send astronauts further into space? 
  • How is space research contributing to our understanding of human health? In what ways can space research be applied to improving human health on Earth?
Relating Science and Technology to Society and the Environment
  • How is space research changing as the world prepares to send astronauts further into space? 
  • How is space research contributing to our understanding of human health? In what ways can space research be applied to improving human health on Earth?
Exploring Concepts
  • What cells are responsible for regulating bone structure and maintenance in the human body? What does each type of cell do? 
  • How does microgravity affect bone density? What specific changes happen at the cellular level to bones exposed to microgravity over extended periods of time? 
  • What things can help counteract the loss of bone density during an extended space mission? 
  • Do you think it would be easier or harder to recover from a broken bone in space? Explain. 
Exploring Concepts
  • What cells are responsible for regulating bone structure and maintenance in the human body? What does each type of cell do? 
  • How does microgravity affect bone density? What specific changes happen at the cellular level to bones exposed to microgravity over extended periods of time? 
  • What things can help counteract the loss of bone density during an extended space mission? 
  • Do you think it would be easier or harder to recover from a broken bone in space? Explain. 
Media Literacy
  • What movies have you watched that include an extended human mission in space?  Do these movies address or allude to the health issues associated with microgravity in any way? Provide specific examples from the films.
  • Have you heard media reports on health studies done on astronauts who have had extended missions to space? What studies have you heard about? What was interesting about the report or the findings? 
Media Literacy
  • What movies have you watched that include an extended human mission in space?  Do these movies address or allude to the health issues associated with microgravity in any way? Provide specific examples from the films.
  • Have you heard media reports on health studies done on astronauts who have had extended missions to space? What studies have you heard about? What was interesting about the report or the findings? 
Teaching Suggestions
  • This article and video can be used for Grades 9 -12 Biology and Space teaching and learning related to health & the human Body, nature of science and space. Concepts introduced include gravitational pull, weightlessness, microgravity, ligaments, tendons, connective tissue, collagen, protein, calcium, phosphate, bone matrix, remodeling, white blood cells, immune system, resorption, osteoporosis, spaceflight osteopenia and International Space Station. 
  • Before reading the article and viewing the video, teachers could provide students with a Vocabulary Preview to help students access prior knowledge and to introduce new terminology. Ready-to-use Vocabulary Preview learning strategy reproducibles are available in [Google doc] and [PDF] formats.
  • After reading the article the teacher could have students use a Concept Definition Web learning strategy to explore the concept of space bone loss. Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats.
  • To extend learning, students could also read the STEM in Context article Why a Trip to Outer Space Could Be Bad for Your Bones and explore the Learn More resources.
Teaching Suggestions
  • This article and video can be used for Grades 9 -12 Biology and Space teaching and learning related to health & the human Body, nature of science and space. Concepts introduced include gravitational pull, weightlessness, microgravity, ligaments, tendons, connective tissue, collagen, protein, calcium, phosphate, bone matrix, remodeling, white blood cells, immune system, resorption, osteoporosis, spaceflight osteopenia and International Space Station. 
  • Before reading the article and viewing the video, teachers could provide students with a Vocabulary Preview to help students access prior knowledge and to introduce new terminology. Ready-to-use Vocabulary Preview learning strategy reproducibles are available in [Google doc] and [PDF] formats.
  • After reading the article the teacher could have students use a Concept Definition Web learning strategy to explore the concept of space bone loss. Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats.
  • To extend learning, students could also read the STEM in Context article Why a Trip to Outer Space Could Be Bad for Your Bones and explore the Learn More resources.

Learn more

What is Osteoporosis and What Causes It? (2019)

Information page about osteoporosis from the National Osteoporosis Foundation

How Astronauts Exercise in Space (2016)

Video (3:50 min.) from Seeker discussing why and how astronauts conduct exercise in space

Growing Bone in Space (2017)

Video (1:52 min.) by NASA Johnson Space Center withChia Soo, a researcher at UCLA, explains recent success testing a new drug on Earth to grow bone, and discusses what she and her colleagues hope to learn through research in microgravity.

Lying in Bed for the Sake of Science: NASA Co-Sponsors Bed Rest Study in Germany (2017)

Article by Monica Edwards & Laurie Abadie discussing a study to observe and analyze the effects of fluid pressure on astronauts’ eyes and optic nerves.

NASA Kelly twins study shows harsh effects of space flight and a brutal return to Earth (2019)

Article from the Washington Post about NASA’s celebrated “twins study,” that was designed to see what spaceflight does to the human body.

References

Bradamante, S., Barenghi, L., & Maier, J. (2014). Stem cells toward the future: The space challenge. Life, 4(2), 267-280. DOI: 10.3390/life4020267

Canadian Space Agency. (2006, August 18). What happens to bones in space?

Holmes, O. (2018, November 19). Space: How far have we gone – And where are we going?. The Guardian.

International Osteoporosis Foundation. (2016, February 18). The link between space flight and osteoporosis

Kirsten, J. (2018). Types of bone cells. Bio Explorer.

NIH Osteoporosis and Related Bone Diseases National Resource Center. (2018). What is bone?

World Space Flight. (2017). Astronaut/cosmonaut statistics.