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.

2. Limitations of Existing Space Nutrition

The following limitations have been the major drivers for innovations that contribute to the advancement in space nutrition.

2.1. Dominance of Processed over Fresh Food

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.

2.2. No Quality Advantage for Resource-Intensive Refrigerated and Frozen Food

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].

2.3. Space Food Supply Is Restricted by Limited Transportation and Storage Space

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.

2.4. Long-Term Space Nutrition Requirements for Food Storage and Cooking Methods

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.

2.5. Diet Menu Fatigue

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].

2.6. Lack of Nutrients to Cope with Extreme Conditions of Space

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.

3. The Influence of Adverse Space Environment on Astronauts’ Diet and Health

The space environment is quite different from the terrestrial environment on Earth. Astronauts are faced with several unfavorable conditions for human survival: microgravity, radiation, confined space, motion sickness, and circadian rhythm changes, as well as a low-pressure atmosphere that is low in oxygen and high in carbon dioxide. Human spaceflight data show that space environments characterized by microgravity and 90 min light/dark cycles trigger countless adaptive responses from almost all physiological systems. These adaptive responses can lead to a loss of body mass, fluid transfer, electrolyte imbalance, dehydration, constipation, loss of potassium, loss of calcium, loss of red blood cell mass, intestinal microecological disorder, and space motion sickness [188]. The diet and health of astronauts can be negatively impacted by the adverse living conditions during manned flight [52].

3.1. Less Energy Intake and Weight Loss

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.

3.2. Effect of Microgravity

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].

3.3. Long-Term Radiation

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.

3.4. Metabolic Stress

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].

3.5. Changes in Physical Condition

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].

3.6. Intestinal Microecology Disorder

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].

3.7. Vision Damage

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].

3.8. Fluid and Electrolyte Imbalance

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.