According to Einstein's Special Theory of Relativity, moving objects measure shorter in their direction of motion as their velocity increases until, at the speed of light, they disappear. It also tells us that moving clocks run more slowly as their velocity increases until, at the speed of light, they stop running altogether.
Hence, astronauts traveling at great speeds for long periods of time return to earth younger than had they stayed on earth.
Technology, however, limits the velocities of astronauts, the differences are hence minuscule: after 6 months on the International Space Station (ISS), the astronaut crew had indeed aged less than those on Earth, but only by about 0.005 seconds. Hence, your notion that there are drastic changes do not find solid ground in the light of current technology. The effects would be greater if the astronauts were traveling nearer to the speed of light (299,792,458 m/s).
It is the mere physics behind it. Everything slows down relative to a slower moving observer. Not just metabolism, but time itself.