Spaceflight and Bone Loss

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.

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.

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!

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.

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!