Nutrition for Long-Term Space Travel: An Overview

Nutrition has many important functions in space travel, from providing enough nutrients and meeting the metabolic needs of a healthy body to enhancing an individual’s emotional well-being. Nutrition also plays a key role in offsetting many negative effects of space travel, such as radiation exposure, immune deficiency, oxidative stress, and bone and muscle loss [30]. Therefore, space nutrition must provide reasonable support for optimal physiological and psychological wellbeing in space in addition to accommodating diverse tastes, variety, and acceptability. Space nutrition should meet the daily human needs for protein, fat, and sugar, as well as inorganic elements, trace elements, fat-soluble vitamins, and various water-soluble vitamins. Space nutrition contains 16 essential nutrients: protein, calcium, iron, vitamin A, vitamin C, thiamine, riboflavin, vitamin b-12, folate, vitamin D, vitamin E, magnesium, potassium, zinc, fiber, and pantothenic acid [31]. In-flight nutrition requirements are set by the World Health Organization (WHO) according to the daily requirements of people on Earth. Therefore, a macronutrient composition with an average of 15% protein, 30% lipids, and 55% carbohydrates is recommended as a bare minimum [31,32].

The evolution of space nutrition has been well documented by nutritional literature and NASA [33,34,35,36]. In the early 1960s, the research on space food systems focused largely on calorie-dense, nutritious, and palatable food without provisions for specific food storage on the spacecraft for short-duration missions. American and Russian astronauts lost weight from consuming primarily aluminum tubes packed with minced meat, jam, and other paste food, as well as bite-sized cubes with high-calorie mixtures of protein, fat, sugar, fruit, or nuts. Although these space foods met the nutritional, sensory, and microbiologic prerequisites in ground-based tests, astronauts experienced menu fatigue [35]. While dehydrated food and ice were developed in collaboration with the U.S. army’s Natick laboratory, rehydrating foods was difficult until hot water became available for the Apollo program (1968 to 1972) to improve the taste of space food [35].

The missions became longer from the mid-1960s to the early 1970s. Consequently, increased variety, improved quality, and longer-term storage shifted the focus of space nutrition towards packaged foods such as cans, food bars, and retort pouches [35]. Retorting enabled food storage at ambient temperature for a long period by thermally sterilizing the food [35]. Eating from open containers with utensils became possible for the first time as rehydrated foods were made more prominent in the space food systems by the abundance of by-product water from the increasing use of hydrogen and oxygen fuel cells to power American spacecraft [34]. The character and flavor of rehydrated food are closer to those in the common diet on the ground. While satisfying the taste of astronauts and increasing food choices [37], dehydration also helped reduce storage space and power needs by minimizing the need for food refrigeration [34,35].

The rapid development of food refrigeration and heating equipment on manned spacecraft facilitated the use of thermal stabilization bags, canned fruit, irradiated meat, and freeze-dried food in subsequent stages of space flight. Today, the types and varieties of space food are quite close to the nutritional choices available terrestrially. Astronauts aboard the ISS are able to eat fresh vegetables, fruits, and heated soup for most meals. The food supply during space flight must be safe, nutritious, convenient, and compact, while meeting the psychological and taste requirements of astronauts under weightlessness or artificial gravity.

The main space food categories are canned food, dehydrated food, medium moisture food, natural food, refrigerated food, fresh food, irradiated food, and functional food [30]. The first five types of space food are relatively mature and widely used, while the last three kinds of space food are popular foods being actively developed to meet the emerging needs of commercial and recreational space flights and long-term space missions. The ISS recently started to test the viability of an on-demand nutrient production system composed of a desiccated yeast strain and edible growth substrate to produce ready-to-consume nutrients for long-duration missions [38]. While on-demand nutrient production system may become a new space food category once it has been validated to be safe and feasible, it is the least familiar type of food for astronauts. Despite its short shelf life, fresh food has been and will still be necessary for improving space food acceptability. The ISS provides astronauts with fresh food, mainly fruits and vegetables for direct consumption or vegetable salads [41,42]. Irradiated food refers to food sterilized by irradiation. Although this method processes food in small quantities most of the time, irradiation-based sterilization can be mass produced, especially by exploiting the harsh radiative atmosphere in outer space. Currently, irradiated food on the ISS mainly includes meat and bread. Korean scholars developed ready-to-eat consumables such as nutrition bars, noodles, and two kinds of traditional Korean food (kimchi and cinnamon beverage) by using high-dose gamma-ray radiation treatment [41,42]. Functional food alludes to special nutrients with supplemental health functions as space food additives to help astronauts better cope with the adverse effects of space living conditions through absorbing the restorative effects of the supplementation in a long-term manner. Anti-radiation functional food is an example of such approach [42,43].

After decades of effort, space food has enjoyed a diversification of variety and taste. To avoid monotony, the diets for the American and Russian astronauts are generally based on a 4-to-6-day cycle, during which the food is different every day except for the drinks. Much of the Russian space food is canned. Lamb with vegetables, beef with barley, sturgeon, and chicken rice are the meal options that typically appear on the Russian menu. These options can be heated in the microwave. There are also many dehydrated foods, such as tvorog, macaroni, tomatoes, fried rice, and shrimp. The general diet of the American astronauts is divided into A, B, and C meals: an A meal has peaches, roast beef, scrambled eggs, pancakes, cocoa, orange drinks, vitamin pills, and coffee; the B meal consists of pork mix, turkey sausage, bread, bananas, almond crackers, and apple drink; the C meal is composed of shrimp, steak, risotto, broccoli, cocktail, pudding, grape juice, and ice cream. Chinese aerospace recipes are mainly made of traditional Chinese dishes, such as eight treasure rice, tangerine beef, beef in soy sauce, lotus seed porridge, green tea, ink fish balls, beef balls, and other Asian delicacies.

At present, astronauts are provided mainly with processed and packaged food. Fresh vegetables and fruits can only be enjoyed in the early stage of a space mission due to limited storage time and high cost. It is estimated that a 3-year Mars mission with a crew of six would have a total energy expenditure of 12 megajoules per person per day, regardless of water requirements, and would carry 22 tons of water-containing food on the spacecraft [44]. Even if the water is completely recycled and the food is partially dehydrated, the transport costs are estimated to be very high (20,000 euros per kilogram) [40,45]. The ISS prioritizes processed food with minimal weight and high nutrient density due to the significant cost associated with transporting fresh food.

Astronauts’ interest in health-promoting food, including fresh vegetables, is on the rise. There is no substitute for a healthy diet related to vegetable intake because fresh vegetables contain many health-promoting properties, including vitamins, minerals, dietary fiber, and secondary compounds [77]. The lack of adequate fresh vegetables and fruits is one of the most significant current challenges of space nutrition.

Fuel cells provide water to astronauts as a by-product from energy generation. The ISS recently started using solar cells to harvest energy from the sun. However, water and electricity remain extremely valuable and scarce resources due to the weight constraint of a space shuttle. These water, power, and weight limitations continue to make it challenging for space missions to accommodate freezers and refrigerators. Until these limitations have been addressed, there is no quality advantage to using refrigerated and frozen food [31].

Mission resources, including power, size, mass, crew time, and waste disposal capacity, must be considered when developing space nutrition systems. Misuse of these resources will affect the success of the mission. While food and resource use may be contradictory, both are critical to the success of the mission [31]. Due to the high resupply cost, it is unrealistic to rely on transporting materials from the Earth to support long-term space missions and human settlements on other planets. There is a need to develop regenerative and self-sufficient water, food, and energy production systems.

A long-term space nutrition system must maintain sensory palatability, nutritional efficacy, and safety over a period of 3 to 5 years. NASA has aimed to develop nutrient-dense and environmentally sustainable food compatible with the cooking processes for microgravity [31]. However, it is challenging to use these same cooking processes to make space food last for more than 3 to 5 years without changing the food quality and nutritional value at the expense of human health. Even with the development of artificial gravity, which will make more food preparation and preservation methods feasible, the adoption of Earth-based systems for long-term space nutrition requires a drastic reduction in the external input of resources and output of wastes.

Food acceptability can be affected by eating habits. Food and mealtimes can help promote solidarity among astronauts, resulting in important psychological and social benefits [31]. Food and mealtimes play a key role in reducing the stress and boredom from prolonged task execution, while delicious food provides pleasure for the eater. The appearance, taste, texture, and smell of food can have a significant psychological impact on astronauts [48]. Despite the great variety of food available today, it is still not enough for long-term space missions that last several years. However, the importance of the space food supply to long-term space missions cannot be understated. During the simulated manned landing on Mars experiment of MARS500 in Russia, many subjects showed “diet menu fatigue” and even became tired of their favorite food [49].

In addition to being nutritious and safe, space food needs to function as a countermeasure to the negative effects of spaceflight by including nutrients that help the human body and mind adapt to weightlessness and the extreme conditions of space [49,50,51]. While the lack of gravity and circadian rhythm are well-known and widely studied aspects of spaceflight travel, there is a need for a more comprehensive nutritional study on other ancillary conditions, such as food taste alteration (due to the changes in atmospheric pressure) and the adaptations of human digestive, olfactory, and perception systems to long-term space habitation.

Preliminary results from terrestrial studies have shown that an increased protein/carbohydrate ratio is correlated with long-term weight maintenance after weight loss [78]. Such weight maintenance strategies have yet to be tested against the averse living conditions in space. It is challenging for astronauts to maintain their energy balance during long space flights [79,80]. This negative energy balance leads to weight loss [79,80,95,96,97]. Astronauts typically lost 2% to 5% of their pre-flight weight [53,99,100,101,102]. In many cases, more than 10% weight loss was observed even though there was plenty of food on board [45]. If sustained, this could result in a weight loss of 5 kg per month [45]. A mission to Mars could result in an initial weight loss of 15% or more, which could have serious health implications [45].

On Earth, a long-term negative energy balance can lead to compromised muscle performance, impaired cardiovascular function, increased muscle fatigue, increased susceptibility to infection, impaired wound healing, altered sleep, and decreased overall well-being [102]. Chronic energy deficiency can exacerbate some harmful physiological adaptations to the space environment, resulting in cardiovascular dysfunction, bone density, muscle mass and strength loss, impaired exercise ability, and immunodeficiency [45,54,55,102]. These physiological changes may jeopardize the health and performance of the crew, as well as the overall success of the mission [52].

Astronauts need to consume more food to offset the decrease in their energy intake due to microgravity, small spaces, insufficient exercise, and shortened circadian rhythm changes. However, the poor palatability of processed and packaged space food causes the astronauts to eat less. Astronauts, on average, eat 25–30% less than they did before flying [30]. Studies have shown that microgravity conditions do not change the amount of metabolic energy (i.e., nutrients that enter the blood through intestinal cells for use by cells) required to stay healthy [99]. Although body water loss occurs during space flight, this can only explain part of the decline in body mass [101]. The decrease of energy intake is the main cause of negative energy balance. A comprehensive review of anorexia in spaceflight suggests that microgravity during spaceflight leads to increases in the two hormones (Leptin [85] and GLP-1 [56,86]) that cause satiety. These changes in appetite-related hormones may cause the decrease in appetite observed during spaceflight. Other biological factors also affect appetite, such as astronauts’ preference for carbohydrates rather than fats [81,82].

During spaceflight, reduced food intake [81,82,83] and impaired anabolic responses [215] may reduce the production of reactive oxygen species (ROS) in mitochondria [57], and this further leads to aging, disease, and cell death [57,58]. Chronic under-intake causes permanent damage to the body [58].

While exercising in space is a popular idea for reducing muscle and bone loss and cardiovascular cleansing, it increases total energy expenditure, necessitating a greater energy intake to maintain energy balance. Exercise may further affect eating behavior, leading to acute anorexia, which can exacerbate anorexia [30]. Fresh, tasty food may stimulate the astronauts’ appetite to make them eat more. Nutrient-dense food can also help to increase energy intake more efficiently through eating.

In the presence of microgravity, the energy cost of daily activities is greatly reduced due to the waste of muscles and the reduction of exercise cost in microgravity [45]. Responses to microgravity include fluid redistribution, reduced plasma volume, rapid loss of muscle mass and strength, cardiovascular deconditioning, impaired aerobic exercise capacity, bone-loss, immune and metabolic alterations, as well as effects on the central nervous system [87,88]. Muscle and bone atrophy and loss of cardiovascular function, which characterize the aging process, occur 10 times faster in space than on Earth due to microgravity-induced physiological changes [89].

Space radiation can cause harmful effects such as DNA damage [90,91] and cell aging [92]. The higher cardiovascular risk among Apollo astronauts is presumed to be associated with exposure to severe deep space radiation [93]. Oxidative stress induced by space radiation and microgravity is an important factor leading to aging and disease [94]. The main measure for astronauts to resist deep space radiation is to rely on space protection facilities. Functional foods rich in anthocyanin and Omega-3 fatty acids have been used to slow down the damage caused by radiation.

Space missions cause metabolic stress among astronauts. Metabolic stress affects major body systems, increases the metabolic rate, and suppresses the immune system. Metabolic stress is also a strong predictor of type 2 diabetes and cardiovascular disease [93]. In addition, associated oxidative stress and inflammation have recently been implicated in the process of muscle atrophy [94] and bone loss [104]. Spaceflight has a short-term impact on the body’s iron metabolism and can lead to iron deficiency anemia as a long-term effect [61].

Astronauts often stay in bed because of their limited mobility and reduced exercise. As a result, astronauts are similar to the general population of sedentary, inactive adults and people who are confined to bed [62]. At rest, the astronauts’ core temperature will be 1 °C higher than on Earth [63]. This can be attributed to an impaired convective heat transfer and evaporation process to cool down the body, low-grade pro-inflammatory responses to weightlessness, psychological stress-induced hyperthermia, and strenuous exercise protocols leading to the so-called “space fever” [63,64,65].

Microgravity leads to decreased beneficial and increased harmful bacteria in the intestinal flora. The gastrointestinal function changes accordingly, affecting the digestion and absorption function of the human intestinal tract over time [66]. In the spacecraft’s sealed living environment, relatively common infections (such as epidemic cerebrospinal meningitis, penicillin resistance staphylococcus genus) may also endanger the life of astronauts and threaten the mission [67,188].

Astronauts’ eyesight changes after landing [103]. More than half of American astronauts have a refractive change in their eyes after a long spaceflight. Findings have also included structural changes in the eye and signs of increased intracranial pressure [68].

Changes in liquid and electrolyte balance due to short-term exposure to microgravity have been observed in the past [216]. Such long-term and sustained changes may adversely affect the health of the crew and thereby jeopardize the success of the mission.

The reduced intake of space food by the astronauts is potentially because their sensory responses to the taste, smell, and texture of food in space [50] reduce the perceived attractiveness of food [45,99,100]. Even though the quality of food has improved since the beginning of the space program, food provided to astronauts is still not as palatable as what is available on Earth. At the same time, the artificial fortification of food can lead to the destruction of nutrients over a long period of time and can also produce an unpleasant taste [31]. Processed food has major limitations. The 3-to-5-year shelf life limits the variety of processed food, and many processed food items lose their original flavor. The traditional focus on the form of food and meeting the crew’s caloric needs is no longer sufficient for long-term missions [31]. There is an urgent need for fresh food with distinctive, original flavors. Future research should investigate ways to maximize fresh food production despite the space, weight, and resource constraints during space flight.

The stress astronauts experience during spaceflight alters their appetite and energy intake [217,218]. Improvements in the variety and quality of food and the emphasis on dietary culture may help boost their energy intake [45]. Dietary culture involves the use of familiar food sources and utensils, cooking methods, and eating methods and places. Meals play an important role in people’s emotional and social wellbeing. Having astronauts participate in the production, harvesting, and cooking of ingredients not only increases the amount of exercise for them but also relieves stress and induces happiness. The happiness induced can be mainly attributed to the sense of fulfillment one obtains from participating in the agricultural process. Further, cooking and sharing food together helps astronauts identify with the space environment and feel a sense of belonging. The resultant place identity and attachment increase the astronauts’ food intake, helping to maintain their physical health. In space exploration missions, the creation of human habitats around food production areas as loci of emotional attachment and social interactions is paramount to the success of long-term missions.

The type of food and the amount of nutrients in the diet can also affect caloric intake [45]. Astronauts often prefer carbohydrates rich in energy and micronutrients [81,118]. However, the supply of such low-density food is limited, because the space environment presents unique challenges for transporting large quantities of food. If carbohydrates are used in large quantities to provide space nutrition, large quantities of drinks and vegetables will also be required [31].

Nuts are by far the most effective food, with relatively high energy and important nutrients in a compact food matrix. However, nuts are rich in fat and protein, both of which can affect shelf life. Future space nutrition research should address significant gaps in supplying more lightweight nutrient-dense foods [31] and developing fresh food production methods to better meet the astronauts’ demand for a large amount of carbohydrates, vegetables, and drinks.

The high concentration of carbon dioxide in spacecrafts and the ISS changes energy intake and reduces food consumption [70,71]. Astronauts’ eating habits are affected by the 90-min diurnal cycle in the Earth’s orbit and the insufficient light level in spacecraft [51,72]. The ambient light cycle variations disrupt the circadian rhythms and affect the hormonal balance in the body [73,74,75]. For example, changes in light and dark cycles have been shown to change the rhythmicity of leptin secretion and affect food intake and body weight in rodents [76]. Thus, continuous light exposure may at least partially explain the higher leptin levels in astronauts [56,72]. To mitigate the effects of excess leptin, space nutrition research should investigate ways to reduce inflammatory foods, such as sugary drinks and trans fats while increasing anti-inflammatory foods, such as olive oil, avocado, coconut, fish, and grass-fed pasture-raised animals. Long-term space habitats should be designed to regulate circadian rhythms and facilitate the production of anti-inflammatory foods. To overcome the payload limitations at launch while providing non-plant-based proteins, future research should investigate ways to use in-space manufacturing to construct large-scale space habitats with artificial gravity to accommodate growing at least fish, if not animals, from embryos.

Astronauts had different immune responses to a decrease in their energy and nutritional intake [69]. For example, a decrease in mitogen proliferation reaction is related to deficiency in Vitamin B6 (VB6), Vitamin B-12 (VB12), biotin, Vitamin E (VE), copper or selenium. Delayed hypersensitivity is associated with VB6, VB12, VC, or iron deficiency. Protein and individual amino acid deficiencies have different effects on various immune functions [84,119]. Providing effective nutrition for astronauts can help to combat immune dysfunction during spaceflight.

Bone loss is one of the important factors affecting astronauts’ health. Although its mechanism is not fully understood, it is clearly multifaceted. Dietary intake resists changes in bone metabolism during space flight. An optimized diet [105,106] helps to mitigate bone loss with little risk of side effects by providing adequate amounts of calcium, vitamin D, and vitamin K in the space diet [107]. Many parallels have been found between nutritional orthopedics and space nutrition because of the similar challenges faced by an aging orthopedic patient on Earth and an astronaut experiencing the degenerative effects of long-duration space missions [108]. A daily balance of fiber, liquids, and bioactive substances, such as coffee, is necessary to prevent hip fracture when transitioning from a low-gravity field to hyper-gravity while landing on a planet.

Most space food has a high sodium content, mainly sodium chloride. This leads to a high dietary sodium intake for astronauts, averaging more than 5000 mg/day and more than 12,000 mg/day for individuals [53,55,82,132,133]. High sodium chloride intake increases urinary calcium excretion [134,135,219,220] and the risk of kidney stones [136], as well as low-grade metabolic acidosis [137,138], which further leads to increased bone resorption [138]. A reduced intake of sulfur-containing amino acids and sodium chloride has been shown to reduce bone loss during bed rest [109]. NASA has made an enormous effort to reformulate 90 foods to reduce sodium intake to 3000 mg/day [109]. As sodium chloride is a food additive used in space food processing and preservation, it is challenging to reduce sodium content without negatively affecting the taste of food. The ultimate solution to excess sodium intake is to replace packaged food with fresh food.

The ratio of protein to potassium in the diet may also affect bone metabolism [109,110]. The type of protein in the diet is also important for bone health [103]: animals have lower potassium (and potassium salts) than plants. Animal proteins are usually high in sulfur-containing amino acids, and animals have lower potassium than plants. The oxidation of sulfur-containing amino acids may lead to low-level metabolic acidosis and corresponding bone resorption which can be compensated by reducing the animal protein–potassium ratio, especially in the final stages of bed rest studies [103]. The arrhythmias in the Apollo 15 astronauts were caused by a lack of potassium due to a lack of nutrients in the space food system [110]. The potassium deficiency in this short-term task was alleviated by potassium supplementation in the subsequent task [31]. Potassium citrate, as a non-sports antagonism scheme for renal calculi, has been transferred from the flying observation stage to the clinical research stage. In addition to vegetables that are rich in plant protein and potassium, supplementation with bisphosphonates may be a countermeasure to bone loss [122,123]: potassium-rich foods include legumes, peanut, mushroom, laver, and kelp. Plant protein mainly comes from various forms of beans.

Adequate energy and vitamin D intake, as well as regular resistance exercise, can significantly improve bone condition [103]. A space nutrition system low in vitamin D, coupled by the lack of ultraviolet radiation, leads to vitamin D deficiency in the astronauts [47]. Inadequate intake of vitamin D supplements can cause body reserves to lose vitamin D during flight [53,111]. While vitamin D may not be the cause of bone loss during space flight, it certainly exacerbates the problem. Foods that contain more vitamin D include fish, milk, liver, eggs, mushrooms, and beef. It is imperative for long-duration space flights to develop effective means to cultivate mushrooms because it is not feasible to produce fresh fish and animal protein in the near future due to weight and space limitations associated with the current space flight capabilities.

Newborns, pregnant women, and astronauts tend to experience deficiencies in vitamin K, which is known as the clotting vitamin. There are two kinds of natural vitamin K: (1) vitamin K1 promotes blood clotting for female astronauts to help reduce period hemorrhage in great quantities and prevent internal hemorrhage and hemorrhoid [124]; (2) vitamin K2 is involved in bone metabolism. Studies have found that loss of vitamin K is associated with low bone mass or increased bone loss [125]. Supplementation with vitamin K has been shown to normalize low carboxylate levels of osteocalcin, counterbalancing the loss of bone formation [126,127] and helping to protect astronauts from bone loss. At the same time, vitamin K can promote the absorption of calcium elements to effectively prevent bone marrow calcium loss.

Vitamin K-rich foods include yogurt, alfalfa, egg yolks, fish eggs, algae, carrots, and green leafy vegetables. The roots of Gynura bicolor (Begonia fimbristipula in Latin) have been used as a functional food in aviation because they contain vitamin K, which is absent in most vegetables. It has been developed as a functional food in aviation [128]: after the human body absorbs the vitamin K1 in Gynura bicolor, the vitamin K1 will be partially converted into the vitamin K2. This plant has a significant application value in addressing calcium and bone loss for the astronauts because it can effectively improve their physiological function through promoting the rapid normal coagulation of blood and increasing the absorption of calcium to minimize bone loss. In addition, Gynura bicolor helps to protect astronauts from accidental injuries while enhancing the wellbeing of female astronauts during menstruation.

The space environment causes bone loss, which in turn, causes calcium loss. The time spent in the adverse space living environment is associated with the amount of bone and calcium loss. In-flight astronauts’ calcium intake was often close to or at the planned intake, similar to the recommended intake on Earth [47,53,127] despite reduced absorption, increased urinary calcium, and a further risk of kidney stones [109]. Calcium-rich foods include milk, legume, fish, shrimp, kelp, black agaric, laver, and sea cucumber. Increasing the intake of fruits and vegetables may prevent changes in bone mineral content when adequate calcium is consumed during spaceflight [107].

Unsaturated fatty acids are superior to saturated fatty acids in preventing metabolic changes, including insulin resistance and inflammation [140,141]. Studies have shown that omega-3 fatty acids [120] and vitamin E [142] help facilitate space adaptation [142]. Omega-3 is a family of polyunsaturated fatty acids, including three types of fatty acids: namely, α-linolenic acid, DHA, and EPA, which are found in linseed oil, vegetable oil, fish oil, seaweed, and leafy greens. Omega-3 contains essential nutrients that cannot be synthesized by the body and must be obtained from food. omega-3 intake has been shown to reduce bone resorption during bed rest [120]: to obtain plentiful omega-3, fish is the best animal resource, while flax and peony seed oil are the best plant sources. Fruits and vegetables also play an important role in providing omega-3. Providing a diet rich in fruits and vegetables, such as a Mediterranean diet, over the long term will have a positive impact on bones in long-term tasks, while adding omega-3 and fish can help counteract bone loss. Fish and omega-3 have also been found to support bone health protection [143,144], mitigate muscle atrophy, and enhance immune function [132]. Thus, an increase in the intake of fish may help strengthen the skeletal muscle to support a strong immune system in addition to reducing the risk of cancer [145,146,147,148,149,150]. Micronutrients in nutrient-rich foods, such as fruits or vegetables, can also help to combat most physiological disorders caused by microgravity [189].

Exposure to microgravity reduces muscle mass, volume, and performance [47]. Weightlessness can lead to changes in skeletal muscle function and structure, such as decreased muscle mass, fatigue, motor nerve stiffness, and autonomic motor ability. Therefore, it is particularly important for astronauts to maintain at least their normal energy supply and protein intake for short-term space missions.

Studies of astronauts on long flights that last more than 100 days suggest that a decrease in protein synthesis may be related to a lack of energy intake [129]: the data showed that the protein macronutrient compositions at an average of 15%, as recommended by the World Health Organization (WHO), were underestimated by 41%. Therefore, astronauts should intake more protein for long-term space missions. In addition to protecting astronauts’ health, protein is also crucial to their ability to move when they return to earth.

Supplementation with essential branched chain amino acids enhances protein synthesis to help fight muscle atrophy [130,131]: branched chain amino acids stimulate the production of insulin, which also promotes the absorption of amino acids by muscles. Branched chain amino acids also help prevent protein breakdown and muscle loss. Foods containing branched chain amino acids are fish, shrimp, milk, soy, corn, glutinous rice, cauliflower, and so on.

Supplemental animal protein has been shown to reduce the pH of the blood through the oxidation of sulfur amino acids, thus exacerbating bone loss [117]. Astronauts should limit the amount of animal protein they eat to minimize bone loss while increasing other sources of protein to stay above their normal protein intake level for long-term space missions.

Astronauts often suffer from a variety of infectious diseases caused by fungal hypersensitivity or the degradation of protective intestinal flora [188,216]. To combat the microgravity-induced degradation of intestinal flora, Italian space agency researchers developed calcium- and probiotics-rich space food from freeze-dried plain yogurt and uniquely flavored yogurt with blueberries [131].

The packaging forms and basic materials of food used for consumption in space mainly include the following four types [166,171,172,222]: (1) frozen food packaging using basic materials, such as plastic (OPP, NY, LLDPE, PET, PE), paper, and composite aluminum foil; (2) semi-solid food packaging and hose packaging, including aluminum foil, plastic, and resin; (3) canned packaging using aluminum foil, plastic film, and kraft board; and (4) packaging in bags, aluminum foil, and plastic. In summary, the main food packaging materials are aluminum foil, plastic, resin, and glass, among which aluminum foil and plastic are the most widely used materials.

In recent years, processed and packaged foods have been identified as some of the main sources of human exposure to plasticizers and bisphenols, which have migrated from plastic packaging [154]. Phthalates are used as plasticizers in many consumer products, mainly in food packaging, flooring, household, and other consumer products [223]. Bisphenol A (BPA) is used primarily in the production of polycarbonate plastics and epoxy resins, as a protective coating in food cans, and in water and feeding bottles. Neither phthalates nor BPA chemically bind to products. In addition, they penetrate, migrate, or evaporate into indoor air, dust, the atmosphere, and food. Phthalates have attracted public attention over the past few decades because of their potential health risks [154]. Dibutyl phthalate (DBP), Benzyl butyl phthalate (BBP) and Diethyl p-nitrophenyl phosphate (DENP) are listed as compounds with suspected endocrine disrupting functions. The serious effect of bisphenol compounds on liver and kidney function was confirmed in mice. BPA can affect liver and kidney function even at very low doses [155,156,157,158]. Surprisingly, plasticizers, especially BBP and bisphenols, were detected in high concentrations in foods. Their contamination is widely involved in packaged foods including grains and cereal products, milk and dairy products, fats and oils, fish, and candy. Foods that are high in fat are most affected by BBP [159,160,161].

Aluminum foil packaging is a composite material made of various materials and aluminum foil, which is a thin metal film made directly from aluminum. With the extension of temperature and time, acid food and alcohol can cause significant aluminum dissolution in packaging foods. Aluminum dissolves and causes heavy metal pollution to food, harming the human body [162]. The toxicity of aluminum is manifested in brain, liver, bone, hematopoietic (multipotent) stem cells, and cells [163]. The health of space residents can only be achieved by eating raw, natural fresh foods without the long-term, slow toxicity from packaging materials.

Long-term space missions require a shelf life of up to 3 to 5 years for food, especially for ingredients that are high in oil and easily oxidized. Packaged food contains food additives, which are (1) synthetic or natural substances added to food for the purpose of improving the quality of food such as color, aroma, and taste, as well as (2) anti-corrosion and processing technology. It remains challenging to preserve food for long-duration missions without a wide range and a significant number of preservative-active substances or carriers. The U.S. space agency recently reduced sodium in food by 90% [103]. Many food additives are bound to threaten the health of astronauts.

There has been a growing demand for unprocessed or minimally processed natural high-quality foods that are preservative-free but have an acceptable shelf life [164,165]. The protective functions of packaging have been refined and improved through the development of new packaging technologies as follows: (1) modified air packaging (MAP) [167,168,169,170], (2) active packaging (AP) [173,174,175], (3) intelligent packaging (SP/IP) [174,176,177,178,179], and (4) the use of nanomaterials for packaging [182,183,184]. However, these packaging systems can only extend the shelf life of packaged foods by a few days to a year [185]. This is far from the 3–5-year shelf life necessitated by long-term missions. In addition, these packaging systems have several disadvantages, such as risk of accidental breakage, the need for additional packaging procedures, and moisture sensitivity, making them unsuitable for use with beverages or wet food [166,186,187]. Above all, space missions cannot rely entirely on packaged food.

To effectively use space nutrition as a countermeasure for microgravity-related health risks, the production of fresh foods is necessary to enhance nutrient diversity and dietary acceptability for the astronauts [42,188,191,192,193,224]. Since major crops can be dried and preserved for a long time, the production of fresh fruits and vegetables with hydroponic, aeroponic, and substrate culture techniques helps to enhance the food quality and the variety of the menu to ensure that astronauts can acquire sufficient quantities and types of natural vitamins [194,197].

Microgravity requires different approaches to store and circulate nutrient solutions to the root zones of plants in small chambers, motivating the development of many closed-looped small-scale food production systems: the NASA Ames research center has developed a “salad machine” that provides astronauts with fresh salad vegetables such as lettuce, cucumbers, and carrots [201]. In April 2014, a vegetable production chamber called “VEGGIE” arrived at the ISS. It became the first system to grow plants on the space station. The VEGGIE project is the first step towards building a “self-sufficient” space station. VEGGIE also provides “gardening therapy” for astronauts to relieve stress and depression [202]. NASA introduced LED growth lights for growing tomatoes and fresh salad greens. The team adjusted lighting conditions to optimize plant growth under a variety of conditions and then replicated these scenes in Advanced Plant Habitats on the ISS to meet the complex requirements of producing food in space [203]. In 2016, the U.S. applied the cultivation method of the “Plant Pillow” (substrate culture technology) to grow crops in space [205]. In 2017, the ISS started to produce fresh food with a self-watering hydroponic system called the Passive Orbital Nutrient Delivery System (PONDS); the system uses capillary mat materials to connect the substrate around plant roots with a reservoir containing oxygen-infused nutrient solutions [207]. Tupperware and Techshot have subsequently developed a semi-hydroponic system that requires less maintenance.

There are many challenges associated with food production and preparation in microgravity. For example, if fresh fruits and vegetables are eaten without proper cleaning protocols, they can be vulnerable to microbial contamination. Eating non-sterile food presents astronauts with some level of risks because the sterilization procedures that work on Earth may not be as effective in space, where microbes can mutate more quickly and exhibit more robust behaviors in microgravity [208]. A proven method for monitoring the microbiome of fresh space foods is needed to reduce potential health risks associated with potentially harmful food pathogens. Microgravity and the lack of atmospheric pressure also affect the transfer of heat and mass, making it challenging to use common cooking methods to prepare food in space [31].

Many crops have been successfully grown in microgravity [209,210]: onions, cucumbers, and radishes were cultivated on Salyut 7. The vegetables currently grown by astronauts include tomatoes, strawberries, and lettuce. These crops were included because they have a short growth cycle in space and ripen quickly enough to be edible after 28 days. The astronauts ultimately expanded their selection of edible plants to include a wide range of fruits and vegetables, including wheat, peanuts, soybeans, mizuna, pea, and other food crops. Space nutrition became increasingly more closed-looped as the astronauts’ metabolic wastes were converted into sources of nutrients for food production or even processed food through biotechnology [211].

With perchlorate at levels commonly toxic to plants, the Martian regolith cannot become a viable growing substrate without addressing restricted metal availability and nutrient enrichment [212]. However, even with nutrient supplementation, none of the three currently available Martian Regolith Simulants (MRSs) are capable of supporting plant growth [213]. These studies suggest that closed-loop life-support systems will be necessary for supporting food production on Mars. Similarly, the lunar regolith cannot support plant growth effectively without addressing issues associated with potentially toxic elements, pH, nutrient availability, air and fluid movement parameters, and its cation exchange capacity [214]. Compared to sand and sungro, pioneer plants, such as Sporobolus airoides, have been found to germinate equally well in a new agricultural grade lunar regolith simulant (General Lunar Nearside Highland Non-Agglutinate Regolith Simulant) developed by Off planet Research, LLC, and Saint Martin’s University [198]. Supplementing closed-loop life support systems with mechanisms for resource separation will help maximize in-situ resource utilization without introducing toxic elements into closed-loop life support systems.

The German aerospace center has developed an advanced closed-loop life support system that uses algae-powered photobioreactors to provide continuous and breathable air in space by converting astronauts’ breath and sunlight into oxygen and food. The system could provide about 30% of the food that an astronaut needs. The concept of the human habitat is based on a symbiotic relationship between humans and plants. Edible plants consume carbon dioxide and release the oxygen that humans need. In return, human waste and non-biodegradable plant matter energy (after decomposition in a microbial tank called a bioreactor) provide nutrients for plant growth. These plants can also provide medicine. However, the effects of gravity, light, atmosphere, soil, and radiation on the growth capacity of plants in this regenerative life support system remain largely unknown [197].

On the surface of the Moon and Mars, higher plants from regenerative life support systems (controlled ecological life support systems) will be the main food source for astronauts. They include wheat, potatoes, sweet potatoes, peanuts, soybeans, rice, and dried beans. Salad ingredients include tomatoes, onions, spinach, beets, cabbage, carrots, lettuce, and radishes. Food processing equipment suitable for the space environment should take into account safety, power, volume, water consumption, air pollution, waste production, cleanliness, and noise. One such piece of equipment is a multifunctional fruit and vegetable processing system developed by NASA for cutting, heating, separation, and concentration [199,200]: for example, the system can process tomatoes into tomato juice, tomato sauce, tomato slice, and tomato soup.