Laurence Vico-Pouget: Space and the human skeleton

When returning from a flight, some astronauts will have lost up to 20% of their bone mass. While we know that the low gravity within the International Space Station (ISS) is the cause, we do not yet know how this bone resorption process takes place. This is what Laurence Vico-Pouget, Inserm Research Director at Saint-Étienne, is studying. With an interest in terrestrial medicine: improving the prevention of bone loss to avoid osteoporosis and the associated fractures.

What are the consequences of microgravity on the human body?

L.V.-P.: The whole body and particularly the bones are adapted to withstand the force of gravity that draws it towards the ground. But in the ISS, such gravity is much lower. As a result, the mechanical stress applied to the bone is greatly reduced. However, the skeleton has a strong capacity for adaptation: if it is not stressed mechanically, it thins. That is why astronauts usually lose mass in weight-bearing bones such as the tibia. On the other hand, some impact sports such as football strengthen the skeleton. But it is all about getting the right dose: too many shocks can cause a build-up of microfissures and a stress fracture.

How do you study this phenomenon?

L.V.-P.: We have a cohort of astronauts that we monitor for the year and a half following their six-month flights. Thanks to this monitoring, we were the first to show that astronauts do not recover the bone mass lost in the lower limbs, and that the bones of the upper limbs, not immediately affected after returning to Earth, gradually deteriorate, without our knowing why. We can compare this to a phenomenon of accelerated ageing. The problem is that some damage may not be reversible.

Can microgravity be mimicked on Earth?

L. V‑P.: Yes, we have developed tools to replicate it. At the Toulouse Space Clinic (Medes), we conducted a study on twenty-four volunteers. They remained bedridden for two months with their heads slightly lower than their feet, a position that mimics some of the effects of space flights. The objective was to measure the effects of this position on the health of these volunteers. Then we used a technique that reproduces weightlessness even better: dry immersion. This involves having a volunteer lie down on a canvas, which is placed on a bathtub filled with warm water. The body finds itself floating with a complete absence of support, which is comparable to a situation of microgravity, with physical inactivity. We observed the beginnings of bone resorption on the first day! But that is not all. We also saw stiffening of the arterial walls, an increase in inflammation, and the onset of insulin resistance [which leads to the development of type 2 diabetes, ed.].

What does your research into bone loss prevention involve?

L. V.-P.: We are currently testing the exact opposite of microgravity, which is hypergravity. To do this, we asked the 24 bedridden volunteers to spend half an hour a day in a centrifuge suitable for humans. Inside this machine, they had to do stationary bike exercises. The aim was to increase the mechanical stress on the body, to see if it is effective in restoring bone. Ultimately, we would like to determine the duration and frequency of these exercises, as well as the intensity of gravity to optimise skeletal repair. In the future, this tool could be used to repair the skeleton of osteoporosis patients.

You also suspect the involvement of poor vascularisation...

L. V.-P.: Indeed, in the absence of the Earth’s gravity, the blood no longer flows normally and no longer reaches the inside of the bone correctly. We have already demonstrated this in rodents. We are now in the process of validating a tool that works using ultrasound to visualise not only the changes in bone structure in humans, but also the internal vascularisation of the bones, with colleagues specialising in ultrasound in the Netherlands, and in the cardiovascular system in Angers. We should be using this device on the first flight of French astronaut Sophie Adenot, in 2026. If validated, this non-invasive method could be used for medicine on Earth. This applies to patients who suffer from vascular problems in the legs, as it is believed that these disorders may lead to bone loss. It may also be useful for patients with osteoarticular diseases associated with blood flow problems. In fact, astronauts offer us a unique model for exploring ageing because, with age, the bones weaken, just like after a journey into space.

How can space advance medicine on Earth beyond skeletal diseases?

L. V‑P.: Overall, conditions in space raise awareness of the harmful effects on the human body of sedentary lifestyle and lack of physical exercise. They make it possible to determine predictive markers of chronic diseases such as type 2 diabetes, obesity or metabolic syndrome. Excessive and prolonged inactivity may result in insulin resistance, vascular dysfunction, decreased cardiorespiratory capacity, and bone and muscle loss. It may also cause an increase in total fat mass, lipid levels in the blood and inflammation. Space medicine therefore directly serves terrestrial medicine.

Laurence Vico-Pouget is an Inserm Research Director at Sainbiose in Saint-Étienne (unit 1059 Inserm/Mines Saint-Étienne/Jean-Monnet – Saint-Étienne University). The French National Centre for Space Studies (CNES) funds all the work carried out at Sainbiose related to space research.