Although there are minor circadian fluctuations in human body temperature, the core temperature of the body is kept fairly constant. Our bodies maintain our core temperatures within a very narrow range of 36.5-37.5°C [97.7-99.5°F]. This is the temperature range at which our bodies function optimally. Major deviations in core temperature are very dangerous and can even be fatal. Even minor shifts in core temperature cause significant losses to one’s physical and cognitive capabilities. Lots of research has been conducted on how the core body temperature is affected by various factors, such as the environment, clothing, age, sex etc. However, not many studies have been conducted on how human thermoregulation operates in a zero-gravity environment. More specifically, there haven’t been many studies that have investigated the impact of space travel and exercise (in space) on core body temperature.
For many years now, astronauts have anecdotally reported a perception of thermal discomfort during space missions. Astronauts have especially been concerned about the heat stress that occurs during physical exercise in weightless conditions - during strenuous exercise, a huge proportion of the body’s energy expenditure is released as heat. Moreover, earlier research indicates that space travel might have a pro-inflammatory effect on the human body, as shown by the increase in the levels of interleukin-1 receptor antagonist (IL-1ra); IL-1ra has an antagonistic effect on interleukin-1 (which is a pro-inflammatory cellular messenger) and has been known to play a major part in the downregulation of core body temperature.
All of this prompted a more detailed inquiry into the topic.
The study was a collaborative effort by a team of international researchers from various institutions worldwide. Other studies conducted previously have indicated that there is an increase in core temperature during space flights. However, these studies did not include long-duration spaceflights.
This study was focused on studying the impact of long-term spaceflights, such as a potential Mars expedition, on astronauts’ thermoregulation. The study was primarily aimed at studying the effects of long-duration spaceflight on core body temperature, both during rest and intense physical exercise. This type of investigation was sorely needed, given that a vigorous exercise regimen is a vital component of long-distance space missions that are planned in the future.
The participants of the study were a group of 11 astronauts, 7 male and 4 female.
The central hypothesis was that during vigorous physical exercise, core temperature would increase much more rapidly and to a higher point, in space, than on Earth.
A special temperature monitoring device was designed for the purpose of the study. It’s technology combined a skin surface temperature sensor with a heat flux sensor, to create a sensor that was sensitive enough to reflect minute temperature changes, without being invasive. The sensors were attached to the foreheads of the astronauts. The study monitored the body temperatures of the astronauts over a 90 day period, in order to test the validity of their hypothesis that human thermoregulation is impaired in weightless conditions.
The study found that during spaceflights, the astronauts’ resting core body temperature rose steadily over the course of the first 10 weeks, eventually plateauing at 38°C [100.4]. This is 1 degree celsius over the average core temperature of healthy adults, as measured on Earth. A temperature elevation of 1 degree celsius, especially over a long period of time, is a matter that warrants serious concern.
The study also found that, much like they had hypothesised, the astronauts’ core body temperature increased much more rapidly during physical exercise, i.e. The rate of temperature increase was much higher than on Earth. It was found that during exercise in weightless conditions, their core temperatures soared up to a scorching 40°C [104°F].
The researchers also established that this thermoregulatory impairment continues to persist even after the astronauts come back to Earth, and only stabilises slowly, over a period of time.
The human body is designed to operate within a tightly controlled temperature. Our bodies are very meticulous about maintaining our core temperatures within this tight range. This is the temperature range that is most suitable for the various processes involved in our physiology - be it the proper functioning of digestive enzymes or our circulatory systems.
When a person’s core temperature is persistently elevated, it impedes the efficiency with which they can perform physically and mentally demanding tasks. The brain’s performance takes a beating when the body’s core temperature is elevated. We all know this from when we get fevers. We are not at our sharpest during a fever. Our brain finds it more difficult to perform complex and demanding tasks, when we are running a temperature. Diminished cognitive function, needless to say, is not something you want in someone who is on a space mission.
Overheating of the brain can cause brain damage," says Oliver Opatz, who is a researcher at the Charité Medical University. "You don't want somebody who has to land a rocket on Mars to have sub-optimal brain function. You need to be sure the person is fit for the job."
A chronic fever, characterised by a persistent elevation of temperature, can spell a host of other potential health risks to astronauts on extended missions in outer space. It could cause issues across the various systems of the body. When the body’s temperature is high over an extended period of time, there is a significantly increased risk of circulatory disorders, hypertension and even heart attacks.
This is probably a logical question to arrive at, looking at the results of the study. However, physical exercise is extremely important in zero-gravity settings. The human body is extremely parsimonious with its resources. In keeping with this natural tendency towards parsimony, the body’s credo with respect to muscle mass is “if you don’t use it, you lose it”. When a certain muscle/muscle group is not being adequately used, it degenerates, in a process called “disuse atrophy”.
When we are on Earth, many of our skeletal muscles are constantly engaged (even at “rest”), in order to support our body against gravity’s downward pull. In a weightless environment, these muscles are not engaged and moreover, very little muscle contraction is required, generally, to move one’s body around. Astronauts in outer space lose muscle mass to atrophy very rapidly, because of this. An 5-11 day spaceflight can cause a staggering 20% drop in an astronauts’ muscle mass. This loss of muscle mass and the accompanying losses in physical strength can potentially be very dangerous to an astronaut, especially considering that he/she might be required to perform a physically demanding emergency procedure, after re-entering the Earth’s gravitational field.
The only way astronauts can protect themselves from muscle wasting is through a dedicated strength-training regimen. NASA reports that the astronauts on the International Space Station, spend about 2.5 hours each day, working out.
So, this is the predicament that faces the future of space travel. The researchers’ suggestion is to monitor the astronauts’ core temperature during exercise and stop the workout right as it approaches a dangerous point. They have also speculated on the possibility of using a cooling mechanism during exercise, in order to prevent the astronauts’ body from overheating.
The researchers haven’t been able to pinpoint the exact reason why this happens. However, a few possible explanations have been offered.
Firstly, the dynamics of blood circulation are different in space. Because of the reduction in gravitational pull, more blood flows to the head. Dr.Opatz thinks this could affect the core temperature.
Another explanation that has been put forward is that the body finds it difficult to dissipate heat in a weightless environment. The body uses 4 mechanisms to cool itself - convection, evaporation, conduction and radiation.. In space, the amount of heat lost to convection is reduced, because of reduced airflow. On top of this, sweat evaporates at a much lower rate in space, because of reduced gravity. In normal circumstances, most of the heat lost by the body is through evaporation. Especially during intense exercise, the body relies on evaporation to release most of the excess heat that is generated. So, the body is forced to rely on radiation, as a means of heat loss, to a disproportionate extent.This could explain why the body finds it hard to lose heat during exercise, in space.
The researchers also suggest that the temperature increase could be mediated by immune factors. To confirm this hypothesis, the researchers tracked the levels of IL-1ra in the astronauts several times during their spaceflight. They found that much like they had predicted, the IL-1ra levels had increased during the spaceflight. However the increases were only moderate. This led the researchers to consider other possibilities; They claim that another possible explanation is that the inflammatory response could due to the higher levels of radiation that astronauts are exposed to in space or due to a sympathetic response to the psychological stress of being in a novel situation.