Influence of Space Environments in System Physiologic and Molecular Integrity: Redefining the Concept of Human Health beyond the Boundary Conditions of Earth

Abstract

Space travel since the 1960s has led to a number of physiological alterations to homeostasis in astronauts. Extensive variation in the pattern of responses observed has led a concerted effort to develop countermeasures to overcome such changes and restore homeostasis, and thus "health" is defined as more "Earth-like". These adaptations to a space environment by a species which evolved and normally exists in the 1 g environment, the geomagnetic field, and background radiation of Earth are viewed as threats to health as defined by the conditions of Earth. Exposure to space can lead to alterations in genomic stability and epigenetic signatures, alterations which could redefine "health" and responses to risks for loss of health for those who will return to Earth. In contrast, in the future individuals born in non-Earth space environments will likely develop an integrated metabolic set point defined by those conditions. They will thus be shaped by both the local environments, and space-associated genomic/epigenomic alterations to their parents. Therefore, such an altered set point for those born and raised in non-Earth space environments will potentially have physiological and molecular consequences which may lead to either new evolutionary adaptation, or to compromise of long term health due to drastically altered set points for integrated physiologic function which is at odds with the evolutionary history of humans. The implications of the two options will be critical for defining "health" in altered environments encountered during space ventures, as well as providing insights into the regulation of human integrity at the physiological level. Therefore, the definition of "health" is dependent on the boundary conditions surrounding development and maturation, and is a dynamic concept.

Homo sapiens—A Species Not Designed for Space Flight: Health Risks in Low Earth Orbit and Beyond, Including Potential Risks When Traveling beyond the Geomagnetic Field of Earth

“…That is, particularly for bone and muscle, if the tissues are not subjected to biomechanical loading at a level that is specified by the set point, the tissues will undergo atrophy, even on Earth, as reviewed in [ 13 ]. On Earth, this can be demonstrated by short- and long-duration bed rest via the removal of an individual from the ground reaction forces (GRF), which are required to continually maintain the integrity of the bone and the muscles, as discussed in [ 2 , 11 , 14 ]. If one removes a tissue, such as knee menisci, from its in vivo loading environment, it rapidly leads to the induction of a cassette of catabolic genes within 4 h, which can contribute to the atrophy of the tissue [ 15 ].…”

“…With the exposure to microgravity on the ISS, there is a fairly rapid induction of bone loss and muscle atrophy, as discussed in [ 11 , 12 , 17 ]. The bone loss is more from the lower extremities than the upper extremities, likely indicating that the bones of the lower extremities are more exposed to GRF on Earth, and thus, are more likely to be influenced by its loss.…”

“…As human evolution could not have anticipated exposure to microgravity during space flight or living in LEO, this heterogeneity in response to microgravity arising during evolution must have remained silent as long as humans stayed on Earth, or the heterogeneity could contribute in some manner to disease development and progression during aging, a time when the integrated physiologic systems may be undergoing senescence and a loss of integrity. Actually, some reports have hypothesized that space flight is a model of aging [ 10 , 11 , 12 ]. While this analogy between space flight and aging remains to be confirmed and validated, it is clear from the response of humans to space flight that at least one of the boundary conditions of Earth, namely the 1g gravity, was a factor in the design of the upright mobility of humans.…”

Human Heterogeneity and Survival of the Species: How Did It Arise and Being Sustained?—The Conundrum Facing Researchers

“…Therefore, going into space has revealed considerable heterogeneity in the physiological responses of individuals to microgravity. Thus, there is wide variation in bone loss (0.1% -2% per month), cardiovascular changes, alterations to vision, and others (discussed in [18,19]). Therefore, when individuals as relatively young healthy people go into space, these otherwise silent variants while on Earth, become evident.…”

Potential Impact of Space Environments on Developmental and Maturational Programs Which Evolved to Meet the Boundary Conditions of Earth: Will Maturing Humans Be Able to Establish a Functional Biologic System Set Point under Non-Earth Conditions?

“…Thus, the outcomes of bed rest studies could have relevance not only to space flight, but also better understanding of life on earth. In fact, some have advanced the concept that exposure to microgravity and space flight (and by extension, analogues) are forms of accelerated aging [33,34], forms of aging that can be reversed in some of the individuals affected. This is an intriguing concept that may or may not utilize the exact same mechanisms, but sheds some potential light on possible lines of investigation to pursue regarding the validity of the concept.…”