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News Navigator: What happens to your body in low gravity?

Japanese astronaut Norishige Kanai speaks from the International Space Station during a Jan. 5, 2018 news conference. (Mainichi)

Japanese astronaut Norishige Kanai, 41, recently revealed on Twitter that he had "grown" 2 centimeters taller since arriving on the International Space Station (ISS) on Dec. 19 last year. The Mainichi Shimbun answers some common questions readers may have about physical changes astronauts undergo during prolonged stays in space.

Question: So why did Kanai get taller?

Answer: There is almost no gravity on the ISS, and so if someone spends a significant amount of time there, the intervertebral discs expand. According to the Japan Aerospace Exploration Agency (JAXA), ISS astronauts typically get 1-2 centimeters taller, though height increases of 7-plus centimeters have been recorded. The astronauts return to their original height after returning to Earth.

Q: Is the effect of gravity really that great?

A: Yes, and not just on height. On the ISS, blood and other bodily fluids tend to collect in the upper body. That can swell the astronauts' features a bit, giving them so-called "moon faces." Kanai's face, too, looked rounder just after he arrived on the ISS than it had on the ground. However, the brain soon sends out signals to regulate fluid intake, and the moon faces quickly subside.

Q: What happens to the lower body?

A: Astronauts get what is called "bird legs," as their legs grow thinner due to the drop in bodily fluid volumes in the lower body. Unlike the moon faces, the ISS crew are apparently stuck with the bird legs for the duration of their stay on the station.

Furthermore, the astronauts' sense of balance weakens, and their appetites diminish as their stomachs come to rest higher in their abdomens due to the low G. They also tend not to feel the need to urinate, as bladders don't get heavy even when full.

Q: So we could say that human bodies have adapted to life in Earth's gravity, yes?

A: If someone gets used to life in zero-G, then their muscle strength deteriorates just like that of a person confined to bed. That's why every astronaut on the ISS does two hours of muscle training daily, using special machines. Furthermore, in low gravity, strong bones become unnecessary to support the body, so astronauts' bones gradually weaken as they lose calcium. The ISS crew take dietary precautions and use medication to combat this loss of calcium, and the resulting risks of broken bones and urinary tract stones.

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