Exposure to microgravity (also known as weightlessness) during spaceflight results in rapid bone loss. Researchers recently set out to determine the impact of long-duration space missions on long-term bone health by assessing bone mineral density - a measurement of mineral bone content and an indirect estimator of bone strength.

The researchers studied 28 U.S. crew members (24 men and four women with a pre-flight age range of 36 to 53 years) whose missions in space ranged from 95 to 215 days. All 28 crew members had their BMD measured both before and immediately after (within 33 days) their spaceflight, while 24 had their BMD measured again between six and 18 months following their return from space.

Post-flight BMD changes were compared with what would be predicted if crew members had not experienced long-duration spaceflight. The predicted BMD changes were based on data from 348 men and 351 women who represented a random sample of the adult community population, none of whom had experienced spaceflight.

The men had their BMD measured at the beginning of the study, then again after two and four years, while women had their BMD measured at the beginning and again at one, two and four years.

BMD measurements were performed for the total body, the hip, the lumbar area of the spine, and two sites at the wrist (ultradistal and midshaft radius) in both the U.S. crew members and the community-based adults.

For each BMD site measured, researchers created prediction models based on data from the community-based adults and then used in these created models age, gender, the pre-flight BMD, and the time between the pre-flight, and post-flight/follow-up BMD for each U.S. crew member in order to predict what the follow-up BMD would have been had they not been exposed to long-duration space flight.

The BMD of U.S. crew members immediately after space flight was significantly lower than would have been predicted and consistent with what is known about bone loss during space flight.

However, researchers also noted that, at roughly 12 months after their return from space, BMD at several sites, (but not all) were still lower than would have been predicted had the crew members not experienced long-duration space flight.

These results led researchers to conclude that exposure to microgravity may have a long-term negative impact on the bone health of crew members serving on long-duration missions in space. The variability of BMD changes in crew members, however, underscores the need to understand how multiple risk factors for bone loss may interact and influence skeletal changes during space travel and with recovery.

"Our results complement the work of others and show that even after one year following return from space, the BMD at some sites-particularly the hip-will not have completely recovered for some crew members," says Shreyasee Amin, MD, MPH; associate professor at the Mayo Clinic and lead investigator in the study.

"Ideally, we need to identify better strategies to prevent the bone loss that occurs during space flight so that we can keep the bones of those serving on long-duration space missions healthy and strong for the long-term."

Background: Rapid bone loss in the weight-bearing skeleton is well recognized during microgravity exposure in long-duration spaceflight, but the implications on long-term bone health remain unclear. How bone mineral density [BMD] in US crew members serving on long-duration missions in space [US crew] compare with what would be expected had they not been exposed to microgravity, is unknown.

Methods: We therefore examined the observed changes in BMD (g/cm2) among 28 US crew (immediately post-flight and following ~12 months recovery) relative to comparable age- and gender-expected changes derived from 348 men (age range at baseline: 22-90 yrs) and 351 women (range: 21-93 yrs) representing an age-stratified, random sample of the adult community population. BMD measurements (Hologic QDR 2000) were made at the total hip, lumbar spine, ultradistal and midshaft radius, and total body (sites also measured in US crew).

Men were measured at baseline, 2, and 4 yrs; women were measured at baseline, 1, 2, and 4 yrs. Linear mixed effects models were used to predict follow-up BMD using baseline BMD, age, gender, and follow-up time, adjusting for the fact that most people were measured more than once.

Models including body mass index (BMI) or lean mass were also considered. In US crew (24 men, age range at pre-flight scan: 36-53 yrs; 4 women, age: 41-53 yrs), BMD was measured pre-flight, immediately post flight and at ~12 months post-flight using Hologic QDR 2000, QDR 4500 and Discovery scanners.

The majority had pre- and post-flight BMD on similar machines. Immediate post-flight BMD was performed a median of 6 (range: 3-33) days after return, with a median flight duration of 167 (range: 95-215) days. 22 men and 4 women had a scan within 6-18 months post-flight; median days from landing to ~12 months scan was 376 (range: 184-534) days.

Results: Using their age, gender, pre-flight BMD and follow-up time, post-flight BMD values for US crew were predicted based on the model developed from the community sample. The predicted and observed BMD and rates of change immediately and ~12 month post-flight for US crew are presented (see Table).

Findings were similar using prediction models which included BMI or lean mass. Due to microgravity exposure, the observed immediate post-flight BMD at all sites was significantly lower than predicted. However, at ~12 months post-flight, BMD values at most sites in US crew were still lower than would be expected had they not been exposed to microgravity.

Conclusion: These findings have implications on potential long-term adverse effects to bone health of US crew serving on long-duration spaceflight missions.

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