The Risks of a Manned Mission to Mars, Part II

Another health-related risk of long-term spaceflight is the effects of prolonged stay in zero-to-low gravity. Under zero-gravity conditions, the astronauts will experience a decrease in muscle tone, strength, reflexes, and tolerance for physical work. Since Mars has one-third the gravitational force of Earth, the astronauts will be subjected to low-gravity conditions on the surface of Mars. The Martian gravity is low enough that it can lead to a decrease in bone size and volume, which includes a loss of calcium, nitrogen, and phosphorus. The bone loss could prove to be fatal as the astronauts return to a higher gravity environment. Their brittle bones would crumble under the gravitational force of Earth.

http://aerospacescholars.jsc.nasa.gov/HAS/cirr/em/9/6.cfm