Sustaining Astronauts: Resource Limitations, Technology Needs, and Parallels between Spaceflight Food Systems and those on Earth
Round 1
Reviewer 1 Report
The paper „Sustaining astronauts: Resource limitations, technology needs, and parallels between spaceflight food systems and those on Earth” gives an overview of current research on spaceflight food systems and combines the data with recommendations for a future research strategy basing on NASA mission plans. For technology validation a stepwise testing and integration of bioregenerative modules into the life support structure under space conditions is recommended. The importance of further ground-based testing is stressed. The authors conclude that due to the importance of food security during space missions the necessary shift to increasingly bioregenerative food production with increasing mission duration has to be thoroughly tested in a phased approach of system validation.
With regard to space research the outlined strategy is perfectly reasonable and in accordance with the common approach of building modular life support systems by designing and testing single modules and then combining them depending on mission scenario. Thus, for space researchers the paper certainly is interesting. But I am not sure if the readers of a journal which focuses on sustainability on Earth, most of them surely not being space researchers, are interested in such a detailed description of space research approaches.
In order to increase the benefit for the readers I propose to change the main focus of the paper from the research strategy to the parallels between sustainable spaceflight food systems and sustainable food systems on Earth, and to highlight the impact that findings concerning the food system in space had or can have on developing a sustainable food system on Earth. At some points there has also to be differentiated between system requirements in space and on Earth. For example, the influence of nutrient composition of urine-derived fertilizers varies greatly between Earth and space applications, therefore studies concerned with sustainable horticulture on Earth come to very different conclusions regarding fertilizer quality (see e.g. the differences between Zabel et al. and Halbert-Howard et al., 2020: Evaluating recycling fertilizers for tomato cultivation in hydroponics, and their impact on greenhouse gas emissions, DOI 10.1007/s11356-020-10461-4).
Author Response
We thank the reviewer for the thorough review and insightful comments. However, we respectfully disagree that this will not be of interest to those working towards sustainability solutions on Earth and therefore request to publish with minor clarifications for these reasons:
1 – New technology and a different way of thinking about a problem can come from the need to find solutions in an eminently more challenging environment than our current situation on Earth. There are many examples where spaceflight need has driven technology development that has provided benefits on Earth as well as in space. This is an opportunity to infuse spaceflight bioregenerative development concepts with a different audience that may lead to solutions we cannot predict.
2 - The reviewer has stated that this article is written and researched appropriately as it is presented. Spaceflight has long been known to excite the human mind, and we believe this will be of interest to a broader community.
3 - We do highlight a few areas where findings related to developing a food system in space have had an impact on sustainable solutions on Earth, such as vertical farming and LED lights, which we have further clarified on page 7.
4 - We also added the reviewer’s point that system requirements would need to be differentiated between spaceflight and Earth on page 3. We appreciate the reviewer’s clarification here. However, to the point of the Zabel and Halbert-Howard paper, different urine or urine-synthesized fertilizer solutions were evaluated in each paper and the difference in results could have multiple sources. The Halbert-Howard paper also got different yields depending on the source.
We are also adding a figure to visualize the roadmap strategy, which could be used as a strategy for any bioregenerative system.
Reviewer 2 Report
General comments
This is a very interesting paper; authors present the challenges and opportunities for present and future food systems and associated nutrition considerations. Overall the emphasis is placed on plant-based systems over animal-based systems, with the importance of nutrition from animal sources barely acknowledged, despite the indisputably superior nutrient profile derived from animal sources. The manuscript could be enhanced with some additional acknowledgement of this crucial point including considerations as to how this might be approached (even if it seems impracticable), especially for longer term space travel.
Specific comments
Saturated fat: Pg 4: The authors state: “For instance, although saturated fat is more stable to oxidation it has lower limits for health. Most missions require food to have a multi-year shelf life, which may be a challenge to meet with some unsaturated fats”.
I am unaware as to what extent the authors are familiar with the controversies surrounding nutrition, particularly that of the unfair demonisation of saturated fat in being causally linked with cardiovascular disease. This relationship has recently come under intense criticism. It would be worth adding some text noting that this is being challenged, which would open up the use of shelf-stable saturated fats in sustainable food systems, and less open to criticism from a health perspective.
Meal replacement bars. The authors mention the use of bars with a higher fat content not being consumed despite acceptable scores in sensory evaluations. It would be good to explore further (and perhaps add an explanation) as to why this was the case (satiety?). Some details around the macronutrient content of the bars could be useful in guiding future considerations for macronutrient distribution in food system planning.
Pg 10, 5.3. second line. Spelling error. Change ‘therefor’ to ‘therefore’
Discussion of wheat and soy. The production of these crops in our modern world on Earth was the start of the processed food industry producing highly palatable, processed foods, resulting in poor health. In the context of the paper, which aims to parallel space with Earth food systems, it would be comforting to know if given the chance for the creation of a whole new food supply that the same mistake would not be made again. This is an opportunity for a fresh start, so perhaps the energies should not be invested into wheat production, but rather into other more nutritious food sources. Carbohydrate (i.e., wheat) is not a biologically essential nutrient i.e., the body can create its own glucose as needed. Fat (essential fats, omega 3, 6) and protein (8 amino acids) are essential (i.e., the body cannot produce them) and therefore we are reliant on dietary / exogenous sources for survival. Obtaining these nutrients should be the priority for optimal nutrition in space travel especially for the longer-term options). These essential nutrients are mostly provided from animal sources. Protein sources (beyond that of soy) that provide such nutrients should feature in this discussion more prominently than a focus on the production on wheat, which is deficient in some amino acids and if consumed in large quantities may lead to poor gut health and chronic disease. Given the constraints with food supply outside of the Earth environment, some dialogue about this would enhance the overall nutrition and health context of the paper.
Author Response
This is a very interesting paper; authors present the challenges and opportunities for present and future food systems and associated nutrition considerations. Overall the emphasis is placed on plant-based systems over animal-based systems, with the importance of nutrition from animal sources barely acknowledged, despite the indisputably superior nutrient profile derived from animal sources. The manuscript could be enhanced with some additional acknowledgement of this crucial point including considerations as to how this might be approached (even if it seems impracticable), especially for longer term space travel.
Response: We thank the reviewer for the thorough review and insightful comments.
We offered crops as an example of the path to integration because it is currently at the most advanced stage of different bioregenerative systems in research and development for spaceflight. Crop research has already provided some spaceflight useful solutions that are simultaneously Earth beneficial (LEDs, vertical farming). We added some edits on page 7 and 8 to further clarify this was not suggesting this was the only path forward, but no others are developed to a point where infusion is likely in the near future or where benefits have already been applied to Earth.
Although a very interesting and important topic, the goal of this paper was not to compare nutrition between different sources, but to provide a path for researchers to consider all of the impacts their food system would have on a spaceflight mission as they work to determine how they could achieve integration. We do make the point of the importance of an acceptable food system to health and performance on page 2, which is best achieved by providing high quality variety and choice that will enable astronauts to choose how they meet their nutritional goals.
We elaborated on animal proteins on page 3 as we do expect these will be a required part of a food system. We also make the point that prepackaged foods are likely to be required for a long time, as well as some dependence on Earth. We have a wide variety of animal protein sources in the prepackaged food supply.
Information added: Although there has been recent progress in animal cell culture, substantial infrastructure efficiencies and culture media sustainability solutions would need to be achieved for spaceflight feasibility20. Farming live animals or fish would present an entirely different set of challenges that include feed, waste removal systems, processing, and welfare in novel environments that are not resource realistic for any currently planned mission. Insects have also been proposed21, but present acceptability challenges in addition to resource and processing challenges. Astronauts who want to consume animal protein will continue to rely on prepackaged sources until advances are achieved. Advances in bioregenerative technologies for Earth may eventually enable integration of some of these solutions into spaceflight vehicles, and limited spaceflight resources may drive further advances in efficiency.
Specific comments
Saturated fat: Pg 4: The authors state: “For instance, although saturated fat is more stable to oxidation it has lower limits for health. Most missions require food to have a multi-year shelf life, which may be a challenge to meet with some unsaturated fats”.
I am unaware as to what extent the authors are familiar with the controversies surrounding nutrition, particularly that of the unfair demonisation of saturated fat in being causally linked with cardiovascular disease. This relationship has recently come under intense criticism. It would be worth adding some text noting that this is being challenged, which would open up the use of shelf-stable saturated fats in sustainable food systems, and less open to criticism from a health perspective.
Response: We concede this point, and have added a sentence to page 5, with some references. "However, there is currently debate over the health impacts of saturated fat, based on dietary components providing the saturated fat, and this may impact product development requirements for future high-density food options27, 28"
Meal replacement bars. The authors mention the use of bars with a higher fat content not being consumed despite acceptable scores in sensory evaluations. It would be good to explore further (and perhaps add an explanation) as to why this was the case (satiety?). Some details around the macronutrient content of the bars could be useful in guiding future considerations for macronutrient distribution in food system planning.
Response: There is a full paper on the study with meal replacements, referenced in text, which explores the reviewers questions.
Full reference here: Sirmons, T.A. et al. Meal replacement in isolated and confined mission environments: consumption, acceptability, and implications for physical and behavioral health. Physiology & behavior 219, 112829 (2020).
Pg 10, 5.3. second line. Spelling error. Change ‘therefor’ to ‘therefore’
Response: Done, and thank you!
Discussion of wheat and soy. The production of these crops in our modern world on Earth was the start of the processed food industry producing highly palatable, processed foods, resulting in poor health. In the context of the paper, which aims to parallel space with Earth food systems, it would be comforting to know if given the chance for the creation of a whole new food supply that the same mistake would not be made again. This is an opportunity for a fresh start, so perhaps the energies should not be invested into wheat production, but rather into other more nutritious food sources. Carbohydrate (i.e., wheat) is not a biologically essential nutrient i.e., the body can create its own glucose as needed. Fat (essential fats, omega 3, 6) and protein (8 amino acids) are essential (i.e., the body cannot produce them) and therefore we are reliant on dietary / exogenous sources for survival. Obtaining these nutrients should be the priority for optimal nutrition in space travel especially for the longer-term options). These essential nutrients are mostly provided from animal sources. Protein sources (beyond that of soy) that provide such nutrients should feature in this discussion more prominently than a focus on the production on wheat, which is deficient in some amino acids and if consumed in large quantities may lead to poor gut health and chronic disease. Given the constraints with food supply outside of the Earth environment, some dialogue about this would enhance the overall nutrition and health context of the paper.
Response: Wheat is mentioned twice in this paper, only as an example of a crop that could be processed. As mentioned for meat above, the purpose of this specific paper was not to compare nutritional sources. An acceptable food system that supports health and performance is best achieved by providing highly acceptable variety that will enable astronauts to choose how they meet their nutritional goals. We added in some clarification that crops were used as an example, and any bioregenerative system could use this strategy.
Nutrition is one of the key considerations when selecting all of the foods or food system components. Although wheat is not a complete nutritional source, all input and output factors associated with integrating different ratios of crops would be considered to achieve the balance in nutrition, acceptability, and resource impacts.
Wheat has some resource advantages to where it could still be one of the crops, but not the only crop. Specific crops have not yet been chosen and other advances (such as genome editing) could factor into robustness and resource advantages of the crops ultimately selected for flight. However, wheat is day neutral, meaning you can give short or long photoperiods; it is self-pollinating (anemophilous); agronomic cultivars are typically homozygous and so their seed maintains the desired genetic traits; it is very tolerant of high light intensity, meaning you can push it very hard; and cultivars are available that get remarkable yields from smaller areas (resource savings). Although other crops may offer more types of nutrients, if looking at calories CHO is the primary and most efficient product of photosynthesis. When plants begin to make fat and protein, this comes at a metabolic toll, and the intrinsic yield (edible biomass per unit area) drops. In other words, fat and protein are more expensive metabolically for plants to make. For space systems, that means you get less edible biomass per MJ of energy. Granted, fat is more carbon dense and has a higher calorie content, but that comes at a cost. Salad crops that may be high in some nutrients are typically harvested in the vegetative stages where the leaves, stems, petioles are consumed directly. This gives them a very high harvest index (% edible biomass) but the vegetative tissue is often >90% water and so you need to eat a lot to get calories. Seed and tuber crops store CHO and sometimes fat and protein and hence are more nutrient dense, but they take longer to develop, which typically drops the harvest index. Therefore, wheat may still have resource advantages that puts it in the trade to provide some calories if all other nutrients are available from foods provided within the system. All factors will be in the trade, as greater resource needs are mission limiting.
We are also adding a figure to visualize the roadmap strategy, which could be used as a strategy for any bioregenerative system.
Round 2
Reviewer 1 Report
Dear authors,
I considered your arguments and especially the statement under point 1 saying that this "is an opportunity to infuse spaceflight bioregenerative development concepts with a different audience that may lead to solutions we cannot predict" convinced me of the idea behind the article. This is exactly why I always encourage students from various fields to develop and test their own ideas regarding BLSS technology.
