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Why do astronauts undereat while in space?

Astronaut Mars

Who in 2021 does not know about Elon Musk? It would probably be weird if so, because Elon, rather than a mind which is changing the world, is a trend. To agree on this, you just need to take a look at the memes that he continuously posts on his Twitter account or to the parties that his close friend Kayne West organises with him.

Elon Musk and Kayne West
Elon Musk and Kayne West. Photo credit: Dazed.

I know, usually, geniuses would not be that cool. Despite I might not coincide with some of his ideas such as the satellite Internet constellation project so-called Starlink (neither the astronomers), I have to admit that he has been a great inspiration for many people worldwide. Elon is convinced that humanity will reach and colonise Mars within the next decades, and has even stated that he would likely die on Mars. But are humans prepared to experience everyday life in other planets? Will food even taste the same out there in space? Let’s see what science tells us after 20 years of feeding astronauts.

Like every form of human exploration, space missions rely strongly on science to achieve success. Although it is evident that engineering is the most notable discipline within a spaceship, food science is as crucial as the latter for crewed missions because the long-term success of the mission strongly depends on the food supply available. Indeed, even for a short mission such as the one performed for only 108 min by Yuri Gargarin in 1961 (the first human into space), there was food limited to squeeze tubes containing pureed meat and chocolate sauce.

Yuri Gagarin
Yuri Gagarin, the first man in space. Photo Credit: RBTH.

Napoleon Bonaparte once said: “An army marches on its stomach”.

After the International Space Station (ISS) establishment in 1998, the food supply from Earth to space improved significantly as a need to feed six astronauts at a time continuously appeared. Since then, the ISS food supply has been further developed to deliver tasty and nutritious food in more than 200 different menu items. An extraplanetary mission, though, will require more complexity and knowledge to solve the food provision problem. According to NASA, a mission to Mars will take around 2.5 to 3 years, and while resupplying the ISS with food at 400 km from Earth is feasible, the 225 million km distance to Mars represents a very different task.

According to estimates, 12 metric tons will be required for a 3‐year trip with six crew members, which means that the launch will be around $15,000/kg, and the overall cost approximately $180 million!

Space food NASA
Space food. Photo credit: NASA.

Additionally, there is also a concern about the unknown radiation levels that will potentially be present during the flight, which consequently could lead to chemical reactions in the food stored, affecting its sensory/nutritional properties. As a result, it has been proposed that astronauts grow part of their food on-board during the journey to supplement the prepackaged items.

Nevertheless, after decades of feeding astronauts, the most significant concern is not about the menu, neither its source nor the supply. Indeed, it is that astronauts typically only consume 80% of their daily calory intake requirements. This calory deficit has a minimum impact because generally an astronaut will not spend more than 12 months on the ISS, but this might change in view of the potential new technology that could bring us to Mars (a mission to Mars could take 30 to 36 months).

On average, an astronaut’s daily intake should be around 2,800 kcal/day, although this strongly depends on the individual resting metabolic rate and the activity factor. Water, muscle, and fat loss are expected within a spacecraft as this is thought to be related to microgravity, but when the fat loss exceeds 2 kg, it indicates that the calory supply is inadequate. Severe weight loss during a space mission has been termed “space anorexia”. But what are the triggers that induce undereating in astronauts?

Consistent calorie deficit in a group of 11 astronauts (on different missions) over a period of 28 weeks. Photo credit: WileyOnlineLibrary.

When we eat, the food transit in our intestine relies on peristalsis (the involuntary movements within the digestive tract) plus a little help from gravity, so it has been thought that the lack of gravity might influence the transit time of food and thus the food intake. Research is also being conducted on the potential microbiome changes that an astronaut might experience during a mission. In any case, there is little evidence that can associate this to the astronaut’s food intake.

Another theory that may relate undereating within a spaceship is that food flavour is different in space, which could be due to: the rehydration of freeze-dried foods with recycled water from the spacecraft (it might concentrate volatile and non-volatile compounds after continuous recycling processes); the high CO2 levels (could activate the sour taste receptors); the constant noise within the aircraft (70 dB on average); and the redistribution of fluids due to microgravity (nasal congestion that decreases the aroma that gets to the odour receptors).

Flavour perception is undoubtedly one of our most multisensory experiences and can be influenced by a wide range of product‐intrinsic and contextual/atmospheric factors experienced in space. Nevertheless, although there is evidence that food flavour can affect food intake, and that astronauts experience a change in flavour during spaceflight, none of the elements thought to affect food flavour in space (displayed in the figure below) are significantly related to the calory deficit that several individuals experience during space missions, according to a recent review.

Factors that have the potential to affect the flavour perception of foods in space. Photo credit: WileyOnlineLibrary.

That being said, future work should be focused on replicate the particular combination of environmental conditions found in space rather than studying a single parameter at a time. This is because it is likely that the various elements influencing food flavour may operate in an unpredictable manner.

Elon Musk recently hoped to have humans on Mars by 2026 if he gets his way. Who would not get excited about thinking that our generation might become extraplanetary at some point? To make it possible, though, we first need to get to the knowledge, and this certainly includes the food science behind future space travels.



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Pedro Rivero

(BSc, PhD)


I am a Food Science PhD candidate wishing to communicate my knowledge in this field through this blog and my social media.

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