What irks me the most about nutrition science is how expensive and cumbersome precise measurements are. Luckily, they don't add much value when it comes to general population. However, when extreme conditions are involved, precision is necessary. There are many things we don't know about a human body's response to extreme environments, complicating experiments. Accuracy can help us understand better what's going on, map out the changes, and ultimately come up with mitigation strategies.
Metabolism is the process of converting calories from the diet into usable energy. It basically determines how many calories we burn, and it's not always constant. It depends on our body's characteristics, and the more exercise we get, the higher our energy expenses are. |
My particular niche, metabolism, is particularly messed up by extreme conditions. Unspecified disruptions, which probably build on each other, make it a challenge to pinpoint the underlying cause of any deficiencies and to apply effective countermeasures. We have a broad idea of what causes these alterations, but the effects are not quantified. Keep reading to find out how complicated it really gets!
Factor #1: Physical Demands
It is common for missions in extreme conditions to involve a great deal of physical exertion, be it prolonged running or climbing while carrying heavy equipment and resources, or performing an extravehicular task in a spacesuit. Especially in the case of a space mission, the lack of gravity makes daily 2-hour-long workout sessions absolutely indispensable. Otherwise, the bones and muscles would not be used enough and would eventually lose functionality.
Of course, any form of exercise is increasing metabolism, which means more calories are needed in order to maintain a healthy body weight. Estimating how many more calories is the real challenge: the physical exercise performed is most of the times difficult to quantify, especially when multiple muscle groups and different types of contractions are being involved. And that's not all...
Factor #2: Mental Demands
A mission in extreme conditions is likely to be mentally demanding as well, and that also affects metabolism. Memory load alone has been shown to increase metabolism... How do you even quantify memory load? And while it is possible to remember many things without being stressed about it, if stress is introduced in the equation, it increases the calories we burn even more. The "fight-or-flight" response that is triggered activates the energy deposits in our body, and in the meantime inhibits other procedures, preparing us to give the fight or the flight our all. If you're keeping the score, that's another parameter that's hard to quantify.
Factor #3: Temperature
Extreme temperatures are also increasing the amount of calories needed for sustenance, whether hot or cold. Specifically, hot environments raise core body temperature, causing a subsequent increase of metabolism. Cold environments, on the other hand, trigger thermogenesis, which means our body is using its energy deposits to increase heat production.
But what qualifies as "hot environment" or "cold environment"? That's the tricky bit. We all have a thermoneutral zone, which does not alter our metabolism and it feels comfortable. The temperatures on the edges of the thermoneutral zones are, however, very versatile: they depend of course on individual characteristics like age, gender, and body composition, but also on other factors, like how much water or food one has had, and how deep last night's sleep was. In other words, as complicated as it gets.
Factor #4: Circadian rhythm
The term "circadian rhythm" refers to processes in our body that repeat every 24 hours. It is basically a type of internal clock that knows when to secrete specific hormones that tell us to sleep, to stay awake, even to eat. How does our body know? The cues include the natural light-dark cycle of the environment, and this is also the reasoning behind the recommendation to not look at screens late at night.
However, it is often that, during a mission in an extreme environment, the habitat is isolated and does not allow for natural sunlight. This disrupts the hormone secretion, altering the central nervous system functions, among the main regulators of metabolism. As if this wasn't enough, sleep is also disrupted, causing further irregularities in the secretion of hormones.
Factor #5: Diet
Diet itself can change metabolism. Short-term, our body spends some calories to digest the food. Everything about a meal can affect metabolism: the amount, its composition, the time it was consumed, maybe even the time it took to be consumed. And let's not forget drinking water - hydration status can affect the efficiency with which our body turns calories into energy.
Diet can also cause long-term metabolic changes. Leaving aside the changes food intake can bring to body weight and body composition, diet influences gut microbiota, the microorganisms that live in our gut. It is common that the diet consumed during a mission in extreme conditions is rich in processed, sterile foods. Usually, fresh fruits and vegetables are rarely eaten, starving gut microbiota from fibre and other elements that can promote their balanced growth, Since gut microbiota take part in digestion processes, any changes in their populations can mean observable changes in metabolism. Gut microbiota can affect metabolism indirectly as well, by exerting control over specific hormones.
So, will we ever be able to put all these metabolic changes into numbers? Chances are that no, we won't. The interindividual variations and the interplay between all the known and unknown factors are rendering the quantification of the effects and their explanation through mathematical models an impossible feat. However, we can elucidate the mechanisms that direct the regulatory effects described. With this knowledge, trained nutrition scientists can monitor the mission crew and any relevant metabolic markers. Thereafter, they will be able to produce effective guidelines and apply precision countermeasures for the prevention and mitigation of a big problem of missions in extreme conditions: malnutrition.
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