In this study, greater pre-flight body weight was associated with an increased risk of higher in-flight urinary calcium excretion. Increased body weight also showed a tendency to be associated with a more rapid rise in urinary calcium excretion. These findings suggest that passengers with an increased pre-flight body weight may be at an increased risk of bone loss and kidney stone formation than lower weight counterparts.
To the best of our knowledge, this is the first study that has identified pre-flight weight as a risk factor for increased in-flight urinary calcium. The findings of this study corroborate previous research evaluating factors influencing urinary calcium excretion among healthy terrestrial subjects, including body weight.(3, 14, 16, 23) Weight loss can lead to bone resorption and decreased bone mineral density.(8, 19, 25) Those with higher baseline body weight are especially prone to this phenomenon. (12, 13) Additionally, rapid as opposed to gradual weight loss potentiates this effect.(8, 29) Weightlessness in space unloads the skeleton at a much more rapid rate and degree than weight loss does on Earth. In turn, it is possible that skeletal unloading from weightlessness may explain the relationship between weight and urinary calcium excretion observed in this study. Those with greater pre-flight weight may therefore be more prone to skeletal unloading and have a greater effect from reducing their skeletal load in weightlessness.
Previous research has shown that urinary calcium excretion can serve as a marker of bone loss and kidney stone risk while in space.(10, 22, 33) However, identifying which passengers may benefit from in-flight urinary calcium monitoring has not been well established. Based on the findings of this study, pre-flight medical screening for spaceflight passengers may incorporate body weight to select individuals for whom closer monitoring of urinary calcium levels may be useful while they are in microgravity.
While our findings suggest that higher weight is associated with increased urinary calcium excretion during flight, the effect was not profound. Based on this analysis, a 10 kg increase in body weight would translate into a 28 mg/day increase in urinary calcium excretion, which is an approximate 10% increase based on the average overall urinary calcium excretion. This suggests that other factors may play a more substantial role in influencing calcium excretion. Nevertheless, these results could provide useful information for risk stratification. For the same baseline level of urinary calcium excretion, a heavier individual may be running a higher risk of stone formation.
This study has some limitations. The amount of data available was limited and the number of individuals studied was small and may not reflect the population of commercial spacefarers. Full data on gender was not available. Further research with larger sample sizes is needed to validate the findings of this study since body weight is not the only factor affecting urinary calcium excretion. Additionally, the association between baseline urinary calcium excretion and urinary calcium excretion during flight was tested using measurements for only 9 individuals from the Skylab program because the ISS data did not contain data for pre-flight urinary calcium excretion. While we detected a statistically significant association between baseline urinary calcium excretion and in-flight urinary calcium excretion, the small sample size limited our statistical power to detect an association between pre-flight weight and in-flight urinary calcium excretion during flight using just the Skylab data. To test this association, we therefore combined Skylab and ISS data.
Combining data from the Skylab and ISS programs increased our sample size to 45 participants. This also created another limitation, namely that the two programs differ. There may be differences in diet and the use of exercise as a countermeasure to prevent bone loss in space, which could affect urinary calcium excretion during flight but are difficult to account for. We attempted to adjust for these differences statistically by controlling for mission in our model using the combined data, but residual confounding remains a possibility.
Finally, 19 out of 45 participants had 3 or fewer measurements, and 11 participants had only 1 or 2 measurements. As a result, individuals who could be considered outliers, for example individuals with relatively low weight but very high levels of baseline urinary calcium excretion levels, could have a fairly large impact on the results of our analysis given that we were not able to control for baseline urinary calcium excretion in the combined analysis. Despite these limitations, this study highlights the utility for using pre-flight weight for assessing the risk for increased urinary calcium excretion in space using actual space flight data. Higher weight individuals could be targeted for urinary calcium measurements in space.
The risk of bone loss and kidney stone formation are significant barriers that must be overcome for the future success of long duration spaceflight. This will be especially critical as the general population, including those with endogenous risk factors for increased urinary calcium excretion, will be traveling in space. Loss of bone mineral density leads to an increased risk of fractures as well as kidney stones—either of these events happening in-flight can be problematic or even detrimental in the resource-confined environment of spaceflight. Therefore, targeting individuals at an increased baseline risk for these issues with close monitoring of urinary calcium excretion can mitigate these adverse events. Small, low power, point-of-care devices are being developed that could make these measurements. Further research evaluating the efficacy of using body weight to reduce these risks is needed to validate the utility of using these factors in stratifying bone loss and kidney stone risk.