Once you reach your fifties, you may anticipate some health changes, such as the beginning of bone loss. You may not expect such challenges in your prime-that is, unless you suffered from osteoporosis, limited mobility or were an astronaut. Scientists have known since the early days of space flight that microgravity negatively affects bone density at an advanced rate.

Examining this consequence of living in space provides researchers the opportunity for accelerated study of bone health. At this year's American Society for Bone and Mineral Research (ASBMR) conference in Houston, NASA scientists shared what they've learned from a half century of related space flight research.

Bone research began at the start of human space travel, with experiments both in space and on the ground. This area of study continues today aboard the International Space Station with investigations that build on their predecessors. Crew members stand to lose about as much bone density in one month as a post-menopausal woman loses in the course of a year.

Scientists can turn this negative into a positive, using the microgravity conditions to accomplish more research in less time. Their investigations in space examine the causes of bone loss and identify countermeasures, while contributing to the development of treatments for use on Earth and in space.

"In a six month mission, we can do research on counteracting bone loss that would take an equivalent of roughly a five-year longitudinal study on Earth," said Scott M. Smith, Ph.D., manager for the Nutritional Biochemistry Laboratory at NASA's Johnson Space Center in Houston.

"This helps us to see changes in physiology faster than on Earth, and in a study population very different than the typical [ground] studies. This perspective contributes to overall understanding and provides a valuable addition to the general knowledge base-allowing us all to get to treatments faster."

In a recent publication, "Fifty Years of Human Space Travel: Implications for Bone and Calcium Research," Scott and his colleagues look back on lessons learned.

He points out that in the pursuit to combat bone loss in microgravity, we gain ground on advancing bone health on Earth. For instance, model animal research using rodents in space has already contributed to clinical trials for pharmaceuticals that can mitigate bone loss. Some of these treatments already are available for people to use.

Research continues on orbit, such as with the Pro-K investigation, which will finish sample collection with the current crew. The goal of Pro-K is to develop a way to optimize nutrition by examining how diet impacts bones.

Specifically, researchers are looking at how a decreased ratio of animal protein to potassium that an astronaut consumes may mitigate bone breakdown. The question is if the crew eats more fruits and vegetables with less meat, will this help improve skeletal health?

The findings may have implications for those suffering bone loss on Earth, providing possible mitigation through the right balance in an individual's diet.

"The work done for space travelers contributes uniquely to the overall knowledge base in the fields of nutrition and bone biology," said Smith.

Building better bones is not as simple as drinking your milk, though, regardless of if you drink from a glass or a space coffee cup.

While vitamin D and calcium are among the important components, it's actually a complex quest for scientists to identify the right balance of countermeasures, which fall into three categories: physical, pharmacological and nutritional.

What researchers found is that nutrition is a large part of this equation, but it works in combination with the right type and amount of exercise. These findings were published in the Journal of Bone and Mineral Research.

"We've seen in the past few years that nutrition and exercise can help protect bone mineral density in astronauts," said Smith.

"That's the first time in more than a half century of space flight that we've seen this. We're not done, but we are making progress, and that is incredibly exciting."

Scientists had suspected that, just as on Earth, diet and exercise were a winning team to keep human bones healthy. Application of this knowledge, however, didn't immediately translate into the space environment. Initial attempts to exercise on the Mir space station and early days on the International Space Station showed little impact to mitigating bone loss.

The crew needed resistance-based exercise that provided enough "weight" to productively stress bones and muscles. Enter the Advanced Resistance Exercise Device (ARED), which launched in 2008. The addition of ARED helped researchers prove that the right kind of exercise, combined with nutrition, could maintain bone mineral density.

"What was shown," said Smith in his publication, "is that crew members who had access to the ARED returned from flight with no loss of body mass, an increased percentage of lean mass, a decreased percentage of fat mass, and maintenance of bone mineral density in most regions and in whole body scans."

The role of minerals and diet in bone health came into a different focus during space station habitation.

In 2009, as the crew conducted water reclamation using urine-a necessity for long-duration space exploration-the system had to be shut down because of pressure concerns. It turns out that an excess of minerals, in particular calcium sulfate, was clogging the hardware.

Sulfur, used in the space station toilet, had combined with calcium crew members lost as a result of the accelerated bone breakdown in space. The resulting high concentration of calcium in the astronaut's urine pointed not only to the relationship between bone loss and calcium, but also to the need to increase crew members' water consumption. Similarly, increased water consumption helps keep kidney stone risk low-another concern for astronauts.

Researchers continue to look at the right amount of calcium intake and other dietary factors for bone health, including vitamin D, omega-3 fatty acids, protein and potassium, sodium, iron and phosphorus.

The next study planned for space station seeking the optimal balance of diet and nutrition for bone health is called Integrated Nutrition. The experiments will be a joint effort of NASA's Space Food Systems Laboratory and the Nutritional Biochemistry Laboratory teams at Johnson.

"We're working to optimize intake of a number of dietary factors known to have protective effects on bone in ground-based or flight research to help to protect bone during long-duration spaceflight," said Smith.

"We're very optimistic about the prospects for this for bone and believe this is likely to have beneficial effects on many other body systems as well."

Part of this multifaceted concern is to examine bone strength, not just as a whole, but according to which parts of the bone are building during countermeasures. Researchers hope to find out if the apparent prevention is actually just the buildup of additional bone from resistance-based exercise, rather than a halting of loss overall. The goal is to go beyond just density to understand how spaceflight impacts the strength of the bone.

"Although this mode of bone remodeling, with increases in bone resorption and formation, maintained bone mineral density, it may yield a bone with different strength characteristics than existed before flight," Scott continued.

"Studies to assess bone strength after flight are underway at NASA to better understand this phenomenon."

And in case you wondered, based on the earlier comparison to a post-menopausal woman's bone loss, researchers are looking at gender, too. During the course of studies on orbit, researchers have found similarities between the sexes with regard to processing bone mineral breakdown.

"In July of this year, we published data showing that the response of men and women to space flight-and to diet and exercise-was not different with regard to bone and renal stone risk," said Smith. "The number of astronauts flown is still relatively small...but growing all the time.

"The space station has provided a great platform, and it has already given us many years of long-duration flights. We're starting to be able to see differences in response as countermeasures evolve and differences among sub-groups of astronauts."