Spaceflight osteopenia refers to the characteristic bone loss that occurs during

spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such as satellites in ...

Astronaut An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a human spaceflight program to serve as a commander or crew member aboard a spacecraft. Although generally r ...

s lose an average of more than 1% bone mass per month spent in

space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...

. There is concern that during long-duration flights, excessive bone loss and the associated increase in serum

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

ion levels will interfere with execution of mission tasks and result in irreversible

skeletal A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

damage.

History

Bone loss has been observed during spaceflight since at least as early as Gemini in the 1960s. Although most early measurements of the amount of bone loss were not reliable, they did show bone loss in Gemini,

Soyuz 9 Soyuz 9 (russian: Союз 9, ''Union 9'') was a June, 1970, Soviet crewed space flight. The two-man crew of Andriyan Nikolayev and Vitaly Sevastyanov broke the five-year-old space endurance record held by Gemini 7, with their nearly 18-day f ...

Apollo Apollo, grc, Ἀπόλλωνος, Apóllōnos, label=genitive , ; , grc-dor, Ἀπέλλων, Apéllōn, ; grc, Ἀπείλων, Apeílōn, label=Arcadocypriot Greek, ; grc-aeo, Ἄπλουν, Áploun, la, Apollō, la, Apollinis, label= ...

Skylab Skylab was the first United States space station, launched by NASA, occupied for about 24 weeks between May 1973 and February 1974. It was operated by three separate three-astronaut crews: Skylab 2, Skylab 3, and Skylab 4. Major operations ...

Salyut 7 Salyut 7 (russian: Салют-7; en, Salute 7) (a.k.a. DOS-6, short for Durable Orbital Station) was a space station in low Earth orbit from April 1982 to February 1991. It was first crewed in May 1982 with two crew via Soyuz T-5, and last ...

Mir ''Mir'' (russian: Мир, ; ) was a space station that operated in low Earth orbit from 1986 to 2001, operated by the Soviet Union and later by Russia. ''Mir'' was the first modular space station and was assembled in orbit from 1986 to&n ...

, and the

International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...

William E. Thornton William Edgar Thornton (April 14, 1929 – January 11, 2021) was an American NASA astronaut. He received a Bachelor of Science degree in physics from University of North Carolina and a doctorate in medicine, also from UNC. He flew on ''Challe ...

, an astronaut and physician, was one of the biggest proponents of exercise as a way of preventing bone loss.

Cause

Bone remodels in response to stress in order to maintain constant strain energy per bone mass throughout. To do this, it grows more dense in areas experiencing high stress, while resorbing density in areas experiencing low stress. On Mars, where gravity is about one-third that of earth, the gravitational forces acting on astronauts' bodies would be much lower, causing bones to decrease in mass and density. Average bone loss of 1–2% was recorded in astronauts on Mir each month. This is in comparison to 1–1.5% bone loss in the elderly ''per year'', and 2–3% in postmenopausal women.

Countermeasures

Since Gemini, exercise has been tried as a way of preventing bone loss, but it has not been shown to be successful. This may be in part due to lack of adequately designed studies (no controlled study had been done as of 2005, either in space or using bedrest as an attempt to simulate conditions which lead to bone loss). It is not known whether a different exercise regiment (perhaps including larger loads than past ones) would be effective. Increasing dietary

and

vitamin D Vitamin D is a group of Lipophilicity, fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group ar ...

is a standard countermeasure for

osteoporosis Osteoporosis is a systemic skeletal disorder characterized by low bone mass, micro-architectural deterioration of bone tissue leading to bone fragility, and consequent increase in fracture risk. It is the most common reason for a broken bone a ...

Clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

is reportedly used by NASA for retaining calcium.Ubick, Suzanne; ''Mud, Mud, Glorious Mud,'' The Magazine of the California Academy of Sciences, Apr. 3, 2008
/ref> A variety of drug remedies currently used or proposed for osteoporosis may work for spaceflight, including

hormone therapy Hormone therapy or hormonal therapy is the use of hormones in medical treatment. Treatment with hormone antagonists may also be referred to as hormonal therapy or antihormone therapy. The most general classes of hormone therapy are oncologic ho ...

(estrogen or progestin),

selective estrogen receptor modulator Selective estrogen receptor modulators (SERMs), also known as estrogen receptor agonist/antagonists (ERAAs), are a class of drugs that act on the estrogen receptor (ER). A characteristic that distinguishes these substances from pure ER agonists ...

bisphosphonate Bisphosphonates are a class of drugs that prevent the loss of bone density, used to treat osteoporosis and similar diseases. They are the most commonly prescribed drugs used to treat osteoporosis. They are called bisphosphonates because they ...

s, teriparatide, and others. Whether they can provide the same benefits for spaceflight as they do for osteoporosis is not yet known.

References

{{Space medicine Space hazards Human spaceflight Space medicine

History

Cause

Countermeasures

See also

References