NASA scientists say they've finally made progress in protecting astronauts' bone strength from the damage caused by space flight.
An evaluation of an exercise device introduced in 2008 shows it not only maintains muscle but also bone mineral density.
The Advanced Resistive Exercise Device (ARED) can produce resistance of as much as 600 pounds in microgravity, around twice as much as NASA's previous exercise system.
And, says the team, astronauts using the advanced exercise system returned to Earth with more lean muscle and less fat, and maintained their whole body and regional bone mineral density at the same level as when they launched.
Diet was also a factor, with crew members using ARED also consuming sufficient calories and vitamin D, among other nutrients.
"After 51 years of human spaceflight, these data mark the first significant progress in protecting bone through diet and exercise," says NASA nutritionist Scott M Smith.
Normal, healthy bone constantly breaks down and renews itself in a process called remodeling - and, as long as these processes are in balance, bone mass and density stay the same.
Earlier studies of Russian Mir space station residents found an increased rate of breakdown, but little change in the rate of regrowth, leading to an overall loss in bone density.
But in crew members who used the ARED device during spaceflight, bone breakdown still increased, but bone formation also tended to increase, resulting in the maintenance of whole bone mineral density.
"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," says NASA bone expert Dr Jean Sibonga.
Further tests are in progress at the International Space Station, with one experiment evaluating different ratios of animal protein and potassium in the diet on bone health, and another looking at the benefits of lowering sodium intake.