I remember hearing in a universe documentary that people who went into space experienced severe bone issues when they returned home.
This is correct; a microgravity environment absolutely strips the calcium right out of your bones unless you can stress them sufficiently.
That's why there's all those videos of astronauts running against bungee cords and such, to try to maintain not just muscle tone but to strain the bones enough to encourage them to retain calcium.
This only applies to microgravity; once you land on the Moon, Mars or anyplace with substantial gravity, these effects lessen dramatically... according to current space medicine, anyway. The fact is that no one has spent years on the Moon to find out... Yet.