In 2008 the Advanced Resistive Exercise Device (ARED), was delivered to the ISS. A new study in the Journal of Bone and Mineral Research compared data from 2006 to 2007 before the ARED was installed and after. They found that a diet with enough calories and Vitamin D, in addition to resistance training was able to maintain bone mineral density.
"After 51 years of human spaceflight, these data mark the first significant progress in protecting bone through diet and exercise," said Scott M. Smith, NASA nutritionist at the agency's Johnson Space Center in Houston and lead author of the publication. It's been thought that resistance training would be key to help remodel bone (bone is constantly broken down and the rebuilding of the bone is called remodeling). "The increase in both bone breakdown and formation suggests that the bone is being remodeled, but a key question remains as to whether this remodeled bone is as strong as the bone before flight," said Dr. Jean Sibonga, bone discipline lead at Johnson and coauthor of the study. These studies are currently underway, along with studies to find the best combination of diet and exercise for long-term missions.