Spaceflight osteopenia refers to the characteristic bone loss that occurs during

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

Astronaut An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a commander or crew member of a spa ...

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

space Space is a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dimensions. Modern physicists usually consider it, with time, to be part of a boundless ...

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

calcium Calcium is a chemical element; it has 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 to it ...

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 most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fram ...

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 (, ''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 flight. The mission ...

Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

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

, Salyut 7,

Mir ''Mir'' (, ; ) was a space station operated in low Earth orbit from 1986 to 2001, first by the Soviet Union and later by the Russia, Russian Federation. ''Mir'' was the first modular space station and was assembled in orbit from 1986 to ...

, and the

International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...

. William E. Thornton, 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. For example, on

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, 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

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. Bone is difficult to regain once it is lost. Data from immobilization studies and from patients with spinal cord injuries support this. Spaceflight data suggest this as well. This suggests that bone loss prevention over postflight bone recovery is an important factor in countermeasure success. Increasing dietary

and

vitamin D Vitamin D is a group of structurally related, fat-soluble compounds responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, along with numerous other biological functions. In humans, the most important compo ...

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 more porous bone, and consequent increase in Bone fracture, fracture risk. It is the most common reason f ...

Clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

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 hormonal therap ...

(estrogen or progestin), selective estrogen receptor modulators,

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 to treat osteoporosis. Evidence shows that they reduce the risk of fracture in ...

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