#astrodietetics

Previously, we explored the general psychophysiology behind menu fatigue, or Sensory Specific Satiety (SSS). Based on previously conducted research, SSS is not just emotional boredom, but deeply rooted in unconscious physiology – to the extent that amnesiacs grew tired of repeated exposure to a food and rated the taste as increasingly unpleasant, even though they still felt hungry, had previously enjoyed the food, and had zero memory of eating it.

Space mission menu diversity is a challenge that NASA has grappled with over half a century. The challenge is twofold: 1) Keeping up with food technology that allows crew members to eat semi-normal foods in a space-and-resource limited environment, and 2) Providing sufficient variety to avoid boredom and inadvertent drop in consumption of essential nutrients. 

Menu Fatigue on Previous Missions: Underconsumption and Weight Loss

Okay, but… we’re talking about astronauts here. They’re not going to space for a Michelin 3 star experience. Could they just, you know, suck it up and eat stuff they’re bored with for a few months? It’s space, not Hell’s Kitchen.

Well, not really. Based on concerning body composition changes on previous missions, we can’t dismiss this problem as a question of astronauts’ willpower. It’s already proven to be a problem. In a study of meal replacement bars (MRBs) for isolated environments and possible future missions, Sirmons et. al. noted, 

It is often assumed that high-performing, highly-trained individuals such as astronauts will eat whatever is required, regardless of quality or appeal, to maintain nutritional status and successfully complete a mission. On the contrary, food familiarity and quality assume added psychological importance on extended-duration space missions, where it may mitigate stress associated with prolonged isolation and confinement. Limited variety or palatability can be a source of increased stress.

Established Underconsumption

If the food isn’t palatable, it won’t be sufficiently consumed to avoid nutrient deficiencies. On previous missions, astronauts have been found to lose an average of approximately 2-5% of their pre-flight weight. Sometimes, weight loss is as high as 10%. This is on relatively short missions compared with a trip to Mars or beyond, and while such weight loss might level off, the health consequences could be dire. Astronauts have the training and knowledge to fully understand the importance of adequate oral intake. However, they tend to consume an average of 25-30% less than usual during a mission.

Along with psychophysiological reasons for SSS, studies have suggested that the microgravity environment on space missions leads to an increase in two hormones that increase satiety: Leptin and GLP-1. Additionally, the exercise required for maintenance of bone mass and overall conditioning increases astronauts’ energy expenditure, exacerbating the problem.

Commodity Theory

Astronauts aboard the ISS have affirmed the psychological benefits from deliveries of fresh fruits and vegetables several times each year. Could this mean that underconsumption might be just a simple case of “I want what I can’t have?” The previously discussed study examining physiology vs. psychology in SSS asked the question of whether “Commodity Theory” was responsible for menu fatigue. Wilkinson and Burnstrom define Commodity Theory as a description of “the relative increase in value of a commodity when it becomes unavailable.”

In the study, participants were given a test meal (the eaten food). The groups were then told about an alternative food (the uneaten food); some were told they would have access to the uneaten food and some were advised that they would not. The authors hypothesized that satiety for the eaten food would decline more sharply when participants were told that they would not have access to a different food. Surprisingly, this wasn’t the case, leading them to the conclusion that SSS/menu fatigue was more innate physiological rather than the result of higher cognition or pure mental boredom.

Implications for Mars and Beyond

Missions aboard the ISS range from 4-6 months to nearly one year, the record holder being Mark Vande Hei with a stay of 355 consecutive days. Scott Kelly hold the previous record, clocking in at 340 days. Kelly and his twin brother Mark, who is also a NASA astronaut and now a United States senator, underwent a series of “twin tests” to determine how a lengthy space vacation affects overall health, genes, cognition, and body composition. While on the ISS, Kelly lost a total of 15 pounds.

Credit: NASA

A speculated journey to Mars will take a good 6 months in one-way travel. Add in an 18 month stay on the surface and another 6 month journey home, and we’re looking at a mission of at least 2.5 years. Now take away the established food systems and menu diversity on the ISS, and forget about the semi-annual fresh food deliveries. This could put crew members on a vicious downward cycle of increased needs and lack of resources for sufficient intake.

The previously mentioned study of meal replacement bars (MRBs) was conducted to determine if standardized bars would be an appropriate solution for decreasing overall food system mass on upcoming lunar missions. The MRBs were approximately 750 kcals, meant to replace the energy and nutrients of one meal. In a 30-day closed chamber study, reliance on these bars for one meal/day led to an overall reduction of caloric intake, along with decrease in morale.

If we’re going deeper into space, we need to figure menu fatigue out. Food systems are going to be essential to the success of these missions. 

Space tourism

We’re hoping to regularly send non-astronauts to space eventually, possibly even on extended missions. This won’t even be an option if we can’t figure out how to get enough food into not only highly-trained astronauts, but regular people on a lunar joyride. Though perhaps not of immediate concern, there’s going to be a lot of “I’d like to speak to the space-manager, please” if the food isn’t up to par. 

Coming up in Part 3, we’ll look at some possible solutions.

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"I flew to Mars and Lost My Appetite"

When contemplating the excitement of aeronautical advancement, “menu fatigue,” well, isn’t on the menu. It’s not exciting. It’s not pretty. Although it might be preferable to The Phantom Menace, we’re not going to see a Star Wars prequel called Bored Taste Buds of the Old Republic.

That said, menu fatigue in space has already been an issue on previous missions, missions much shorter than our aspirations of Martian journeys and beyond. Whether we’re planning an exploratory mission of unprecedented interplanetary travel or a future ship full of garden variety space tourists, menu fatigue could prove to be the unexpected, boring wrench in our plans. Unless Star Trek-style replicators become a thing anytime soon, it’s a problem we need to figure out.

So, let’s start with the basics. What is menu fatigue, and why could it be a problem for long-term space travel?

What exactly is “Menu Fatigue?” 

The What

In general terms, “menu fatigue” (MF) refers to the boredom associated with repeated consumption of the same foods. It’s a problem restaurants have to consider when designing their menu and planning updates to continue attracting customers. As we’ll see, it causes a drop in the amount of food eaten, whether intentional or unintentional. 

Most importantly, MF is a likely result of the psychophysiological phenomenon Sensory specific satiety (SSS). It’s not just a question of conscious boredom, but of an innate biological response. 

SSS refers to “the decline in pleasantness associated with a food as it is eaten relative to ‘uneaten’ foods that have different sensory qualities." When you’ve eaten a lot of a particular food, it’s SSS that makes you want to stop. However, if a different food becomes available, SSS subsides and you may resume eating. One study compared consumption when participants were offered a one-course meal vs. when they were offered a four-course meal. The participants ate a whopping 60% more when given multiple courses. This shows that the desire to eat isn’t just about hunger but also perceived variety.

The Science

SSS: Habituation vs. Cognitive Response

Is MF/SSS a “low-level” response or a higher cognitive one? Prevailing assumption was the former, that SSS was driven mainly by habituation, which Oxford defines as “the diminishing of a physiological or emotional response to a frequently repeated stimulus.” A 2016 study conducted a series of tests to determine if SSS was mostly a physiological response or one of psychological/emotional boredom. Would perceived availability of a variety of foods affect the SSS of whatever food was being consumed? 

In Round One, participants were given a sample of one lunch food and asked to rate pleasantness and desire to eat. Afterwards, they were given a full plate of the same food, advised that this was their lunch, and that they could have another serving if they wanted. When they’d had enough, they were again asked to rate pleasantness and desire to eat more. Not surprisingly, ratings for both declined significantly once participants were full and had consumed as much as they wanted of this particular lunch food.

*Round One  “No Uneaten Foods” TL;DR: 1) Rate a sample of this food; 2) Here’s a full portion – this is the only food you’ll get but you can have as much as you want; 3) Rate it again.

In Round Two, participants were offered bite-sized samples of four different foods and asked to rate them, again on pleasantness and desire to eat. They were then offered one of the foods and advised they could have as much as they wanted, but that the other three foods would not be available. After they had their fill of the one food, participants were asked to rate all four foods again.

*Round Two “Unavailable Uneaten Foods” TL; DR: 1) Rate four samples of different foods; 2) Here’s an unlimited portion of one, but you can’t have the others; 3) Rate all four foods again.

Round Three followed the same protocol as Round 2, the difference being that participants were told they could switch to one of the other four foods after they consumed as much as they wanted of the first one and already completed the second round of ratings (they couldn’t have their “switch” food until they’d re-rated all four).

*Round Three “Available Uneaten Foods” TL;DR: 1) Rate four samples of different foods; 2) Here’s an unlimited portion of one, and you will be able to eat as much as you want of another one after you rate again; 3) Rate all four again; 4) Now have as much as you like of your selected new food.

As expected according to SSS, Round One participants rated a significant decline in pleasantness and desire to eat more after they’d had their fill of the same food. No surprise there. But what if participants knew that another food would be available to them? Would that make their eaten food or uneaten food more desirable? Less desirable? Surprisingly, there was no significant difference in rating changes for any of the foods regardless of availability. Researchers conducted another round of studies in which they limited the portion of the eaten food rather than giving participants free reign. Results were the same. 

Mean (± SE) of the change in rated pleasantness and desire to eat of the eaten food from baseline to meal termination for the no uneaten foods, unavailable uneaten foods and available uneaten foods conditions. (Wilkinson, LL, Brunstrom, JM, 2016)

What this study shows is that it’s not just a desire to eat another available food, or a longing for an unavailable food, that makes whatever someone is eating less appealing as they continue to eat it. Sensory specific satiety happens to a similar extent whether another food is available or not. Although SSS is generally considered a psychophysiology concept, evidence shows it leans heavily towards physiology. In other words, you can’t talk yourself out of becoming satiated with a particular food. 

SSS in Amnesic Patients

Want some crazier evidence that SSS is physiological? A 2008 study (“Sensory Specific Satiety is Intact in Amnesics who Eat Multiple Meals”) probed the presence of SSS in several patients with amnesia. They intended to test the theory that impaired SSS is what often causes amnesic patients to consume multiple meals in succession, regardless of fullness. 

Following cases of encephalitis, the patients in this study suffered from a form of amnesia related to lesions in the temporal and frontal lobes, impairing their ability to incorporate new memories. They were amnesic to the extent that they could not remember having just consumed a meal, often eating an entire new one if offered. 

The patients and control group were tested with both multiple helpings of the same foods and with one meal of one type of food followed by a different type. Like in the aforementioned study, they were asked to rate their liking of the foods before and after. Predictably, the control subjects rated a decline in liking for foods that they had already eaten to satiety - but so did the patients with amnesia. There was no significant difference between the groups.

One patient, after eating a cheese pastry to satiety, lowered his ranking when presented with it a second time. He described the taste of the pastry as an unpleasant “sour,” even though he had previously enjoyed it and had no memory of eating it in the first place. Like the control group, he opted for a previously uneaten food when offered a choice for his second helping. Researchers concluded that SSS appeared intact, even when memories of recently eating the satiated food were absent.

Clearly, Sensory Specific Satiety has, at the very least, a strong physiological component. It’s not just about being picky or bored with repetitive choices. But if it’s about more than wanting to try something new and exciting, what would be the reason for it? After all, it seems that, from an evolutionary perspective, it would be more beneficial to be content with the limited choices around you. What might account for this innate response to eat less when confronted with lack of variety? 

The Why

Nutrient Diversity

With its physiological stronghold, SSS can’t be without evolutionary reasons. An easy hypothesis is the need for nutrient diversity. If we’re content to eat one particular food for every meal with no satiation, we’re likely to miss out on whatever vitamins and minerals might be missing from that food. This would also explain why we experience a renewed interest in eating when presented with a new food choice (often dessert) even after we’re full (and why aforementioned study participants ate 60% more at a multi-course meal). 

No false advertising from Jell-O, then. There really is “always room.” 

“Jell-O provides relief from sensory habituation and re-invigorates your desire to eat more, regardless of satiation” might have been more accurate, but that one probably got a big thumbs down from the marketing team. 

In an ancestral setting, the thought process was more like, Ugggghhh I’m so full of woolly mammoth I can’t eat another freaking bite or blubber will start pouring out of my ears oooooooh look, foraged cloudberries! Pass ‘em over! Boom. Now we have a dash of Vitamin C along with tasty woolly mammoth protein. No scurvy on this tundra.

Poison Control

There’s also the possibility that we evolved with SSS so that we would never eat too much of one thing, and therefore be spared the worst effects of tainted food. While this might be part of the explanation, it seems less likely since severe food poisoning can happen even with a little bite. And if something in nature is poisonous, it tends to be really poisonous. SSS won’t save you from a disagreeable mushroom.

Absorption Conflicts

When we look at our daily suggested amounts for vitamins and minerals, it’s not as simple as manufacturing the perfect pill that gives us all of them, every day, in the exact portion that we need. Most nutrition experts recommend vitamins to guard against deficiencies, but very few would encourage you to get all your essential nutrients this way. Prevailing advice is to get a good variety in your diet and take multivitamins to round things out. 

Why can’t we just take a super-duper-all-inclusive-multivitamin and be done with it? Absorption conflicts. Some vitamins and minerals interfere with the absorption of others, making it difficult - if not impossible - to get everything you need from one single, repeated source. 

A few absorption conflict examples:

*Zinc and copper – High intake of zinc can cause a copper deficiency. Zinc increases release of the intestinal protein metallothionein, which binds more readily to copper than zinc. Metallothionein binds to and traps copper in intestinal cells, preventing absorption and allowing more zinc to be absorbed instead.

*Vitamin C and B12 – High doses of Vitamin C have been shown to destroy Vitamin B12 when taken close together, potentially causing B12 deficiency if C is repeatedly used in elevated amounts to colds and other bugs.

*Vitamin E and Vitamin K – This one is more an issue of utilization interference than absorption. Vitamin E supplementation has been shown to increase bleeding in some individuals, while Vitamin K is known to assist in blood clotting. So, taking E with K can counteract K’s clotting effects.

*Calcium and everyone else – Okay, not everyone, but a few other minerals like phosphorus, zinc, magnesium, and iron. In They Might Be Giants terms, calcium is Triangle Man.

Additionally, even mere adequate intake of some vitamins and minerals can mask a deficiency of another, such as folate (B9) and B12. Absorption is complicated, and relying solely on a specially tailored multivitamin, though certainly better than nothing, might not be possible to meet all of our needs in the long term. 

The absorption conflict explanation is especially relevant to our discussion of astrodietetics, since over-reliance on supplements could cause these issues. Could we figure out a series of supplements to take, in exactly the right amounts and at exactly the right times, so that they don’t interfere with each other? Possibly, but there’s a lot we still don’t fully understand about absorption and utilization, and at best it would be complicated. 

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