How does microgravity change the bones?

On Earth the human skeleton must safeguard the body day and night from gravitational collapse. In space, without gravity to hold the spinal discs together, the spine grows by up to one and a half inches. Since a strong skeleton is unnecessary in space, the body gradually reduces its minerals. Bones slowly become brittle and shrink. Lack of sunlight intensifies the process. After a maximum of four years of microgravity, the bones would break under even the slightest stress.