Microgravity causes loss of bone tissue due to decreased rates of bone formation and increased rates of bone resorption, most notably in weight-bearring bones. Moderate weight loss of 10% typically results in 1-2% loss of bone mineral density. Both microgravity and weight loss contribute the phenomenon of space osteoporosis. After a 6-month mission, astronauts suffer from severe space osteoporosis resulting in a 12% loss of bone mineral density in the femur. If microgravity-induced changes in bone are not taken into account, a deficient calcium and vitamin D intake leads to calcium resorption from bone to maintain a constant serum calcium level.