Study suggests deep space radiation, not weightlessness, is to blame.
A new study shows that astronauts from the Apollo missions have a rate of cardiovascular disease mortality nearly five times that of other astronauts, suggesting that even short missions outside Earth’s protective geomagnetic field have long-term health consequences. Researchers compared cause of death for seven Apollo astronauts to that of 35 astronauts who had flown in low Earth orbit, and 35 who had never flown orbital missions. Published in Nature, the study may have implications for future efforts to send humans into deep space. The authors also conducted experiments with mice to learn more about how weightlessness and deep space radiation affect the cardiovascular system. We speak with lead author Michael Delp.
ResearchGate: What were the results of your assessment of cardiovascular disease in these astronauts?
Michael Delp: The work is a study of mortality rate, so we looked at the causes of death in each of three groups: Apollo astronauts, low Earth orbit astronauts, and non-flight astronauts. The Apollo lunar astronauts are the only humans to have traveled into deep space, beyond the protective magnetosphere surrounding the Earth. We found that death due to cardiovascular disease was higher in the Apollo astronauts than the other groups.
RG: Studying the health of astronauts inherently involves a very small sample size. How should this factor into the interpretation of your results?
Delp: The small sample size is a limitation of the study. One has to use caution in drawing definitive conclusions. Despite this limitation, we thought it was important to publish these results because it is also difficult to ignore the possibility that deep space travel, including deep space radiation, may be having a much more adverse effect on cardiovascular health than previously estimated. With countries and companies like SpaceX stating planning to send humans back into deep space as early as ten years from now, there is not a lot of time to experimentally address the possible implications of these results with Apollo astronauts and answer the many outstanding questions surrounding space radiation effects on the cardiovascular system. For example, it over a year just to complete the experiments in the one mouse study we conducted for this paper. And Brookhaven National Laboratory is the only place in the US where deep space radiation exposures can be simulated.
RG: Can you tell us a bit about the mouse experiments you did to explore what’s behind your findings?
Delp: In a previous study published earlier this year, we examined the early effects of simulated weightlessness, space-relevant radiation, and a combination of the two in mice, similar to what was done in the present study. We found that the weightlessness simulation group produced an impairment of the blood vessels (via the endothelium) of the type that could predispose the arteries to the development of atherosclerotic plaque, also that the radiation produces a similar sort of impairment to the vascular endothelium, and finally that the combination of simulated weightlessness and space radiation produced and even greater impairment than either treatment alone. Please note that these studies were conducted immediately at the conclusion of the treatment period. The results suggested that weightlessness and space radiation may interact to cause greater impairment of the cardiovascular system.
In the present study, we are not looking at the early effects of spaceflight on the cardiovascular system in astronauts, but rather what happens years after their return from space. So we used the same treatment groups and repeated the earlier study, except now instead of doing the vascular experiments immediately after the treatments, we waited 6-7 months, which is the equivalent to about 20 human years. The results were that the weightlessness simulation did not show a sustained impairment of the endothelium, only the radiation treatment did. Thus, it does not appear that the cardiovascular deconditioning that occurs in weightlessness has any prolonged adverse effects. This point is supported by the low Earth orbit astronauts not having any difference in cardiovascular mortality relative to the non-flight astronauts. Only radiation seems to show a sustained impairment.
RG: Do your studies give any indication as to how radiation causes cardiovascular disease in astronauts?
Delp: Radiation damage impairing the endothelial cells lining blood vessels may be a precipitating event to the long-term development of cardiovascular disease. Healthy endothelial cells help prevent atherosclerotic plaque from forming inside the walls of blood vessels—commonly referred to as hardening of the arteries. When the endothelial cells are impaired, like we found with radiation exposure in the mice, then this could predispose these arteries to a prolonged formation of plaque. In the heart this can cause ischemia (low blood flow) and coronary heart disease, or in the brain it could cause ischemic stroke.
RG: What are the prospects for protecting astronauts traveling beyond the magnetosphere from cardiovascular disease in the future?
Delp: Improving shielding effectiveness and anti-oxidant treatment would be a couple of means to help mitigate the risk.
Featured image courtesy of NASA.
