Nutrition has played a critical role throughout the history of exploration, and space exploration is no exception. While a one- to two-week flight aboard the Space Shuttle might be analogous to a camping trip, adequate nutrition is absolutely critical when spending several months aboard the International Space Station or several years on a mission to another planet. To ensure adequate nutrition, space-nutrition specialists must know how much of various individual nutrients astronauts need, and these nutrients must be available in the spaceflight food system. To complicate matters, spaceflight nutritional requirements are influenced by many of the physiological changes that occur during spaceflight.
Space Physiology
Spacecraft, the space environment, and weightlessness itself all impact human physiology. Clean air, drinkable water, and effective waste collection systems are required for maintaining a habitable environment. Without the Earth's atmosphere to protect them, astronauts are exposed to a much higher level of radiation than individuals on the Earth. Weightlessness impacts almost every system in the body, including those of the bones, muscles, heart and blood vessels, and nerves.
Bone.
Bone loss, especially in the legs, is significant during spaceflight. This is most important on flights longer than thirty days, because the amount of bone lost increases as the length of time in space increases. Weightlessness also increases excretion of calcium in the urine and the risk of forming kidney stones. Both of these conditions are related to bone loss.
Many nutrients are important for healthy bone, particularly calcium and vitamin D. When a food containing calcium is eaten, the calcium is absorbed by the intestines and goes into the bloodstream. Absorption of calcium from the intestines decreases during spaceflight. Even when astronauts take extra calcium as a supplement, they still lose bone.
On Earth, the body can produce vitamin D after the skin is exposed to the sun's ultraviolet light. In space, astronauts could receive too much ultraviolet light, so spacecraft are shielded to prevent this exposure. Because of this, all of the astronauts' vitamin D has to be provided by their diet. However, it is very common for vitamin D levels to decrease during spaceflight.
Sodium intake is also a concern during spaceflight, because space diets tend to have relatively high amounts of sodium. Increased dietary sodium is associated with increased amounts of calcium in the urine and may relate to the increased risk of kidney stones. The potential effect of these and other nutrients on the maintenance of bone health during spaceflight highlights the importance of optimal dietary intake.
Bone is a living tissue, and is constantly being remodeled. This remodeling is achieved through breakdown of existing bone tissue (a process called resorption) and formation of new bone tissue. Chemicals in the blood and urine can be measured to determine the relative amounts of bone resorption and formation. During spaceflight, bone resorption increases significantly, and formation either remains unchanged or decreases slightly. The net effect of this imbalance is a loss of bone mass.
It is not clear whether bone mass lost in space is fully replaced after returning to Earth. It is also unclear whether the quality (or strength) of the replaced bone is the same as the bone that was there before a spaceflight. Preliminary data seem to show that some crew members do indeed regain their preflight bone mass, but this process takes about two or three times as long as their flight. The ability to understand and counteract weightlessness-induced bone loss remains a critical issue for astronaut health and safety.
The changes in bone during spaceflight are very similar to those seen in certain situations on the ground. There are similarities to osteoporosis, and even paralysis. While osteoporosis has many causes, the end result seems to be similar to spaceflight bone loss. Paralyzed individuals have biochemical changes very similar to those of astronauts. This is because in both cases the bones are not being used for support. In fact, one of the ways spaceflight bone loss is studied is to have people lie in bed for several weeks. Using this approach, scientists attempt to understand the mechanisms of bone loss and to test ways to counteract it. If they can find ways to successfully counteract spaceflight bone loss, doctors may be able to use similar methods to treat people with osteoporosis or paralysis.
Muscle.
Loss of body weight (mass) is a consistent finding throughout the history of spaceflight. Typically, these losses are small (1 percent to 5 percent of body mass), but they can reach 10 percent to 15 percent of preflight body mass. Although a 1 percent body-weight loss can be explained by loss of body water, most of the observed loss of body weight is accounted for by loss of muscle and adipose (fat) tissue. Weightlessness leads to loss of muscle mass and muscle volume, weakening muscle performance, especially in the legs. The loss is believed to be related to a metabolic stress associated with spaceflight. These findings are similar to those found in patients with serious diseases or trauma, such as burn patients.
Exercise routines have not succeeded in maintaining muscle mass or strength of astronauts during spaceflight. Most of the exercises performed have been aerobic (e.g., treadmill, stationary bicycle). Use of resistance
exercise, in which a weight (or another person) provides resistance to exercise against, has been proposed to aid in the maintenance of both muscle and bone during flight. Ground-based studies (not done in space) of resistance exercise show that it may be helpful, not only for muscle but also for bone. Studies being conducted on the International Space Station are testing the effectiveness of this type of exercise for astronauts.
Blood.
A decrease in the mass of red blood cells (i.e., the total amount of blood in the body) is also a consistent finding after short- and long-term spaceflight. The actual composition of the blood changes little, because the amount of fluid (blood plasma) decreases as well. The net result is that the total volume of blood in the circulatory system decreases. While this loss is significant (about 10 percent to 15 percent below preflight levels), it seems to be simply an adaptation to spaceflight, with no reported effect on body function during flight.
The initial loss of red blood cells seems to happen because newly synthesized cells (which are not needed in a smaller blood volume) are destroyed until a new steady state is reached. One consequence of the increased destruction of red blood cells is that the iron released when they are destroyed is processed for storage in the body. Too much iron may be harmful, and is thus a concern for long space missions.
Space Food Systems
Historically, space food systems have evolved as U.S. space programs have developed. The early Mercury program (1961–1963) included food packaged in bite-sized cubes, freeze-dried powders, and semiliquid foods (such as ham salad) stuffed into aluminum tubes.
The Gemini program (1965–1966) continued using bite-sized cubes, which were coated with plain gelatin to reduce crumbs that might clog the air-handling system. Freeze-dried foods were put into a special plastic container to make rehydrating easier.
The Apollo program (1968–1972) was the first to have hot water. This made rehydrating foods easier, and also improved taste and quality. Apollo astronauts were the first crew members to use the spoonbowl, a utensil that eliminated having to consume food into the mouth directly from the package.
The quality, taste, and variety of foods improved even more during the Skylab program (1973–1974), the only program to have refrigerators and freezers for storage of fresh foods. The menu contained seventy-two different food items.
The Shuttle program, which began in 1981, includes food prepared on Earth from grocery store shelves. With the help of a dietitian, crew members plan individual three-meal-per-day menus that contain a balanced supply of the nutrients needed for living and working in space. Crew members are allowed to add a few of their own personal favorite foods (which may require special packaging to withstand the rigors of spaceflight). Freezedried foods are rehydrated using water that is generated by the Shuttle's fuel cells. Foods are eaten right from the package (on individual food trays), or they may be heated in a convection oven in the Shuttle galley.
During the Shuttle-Mir program (1995–1998), a joint menu was used that contained half Russian and half U.S. Shuttle foods. These had to meet the nutritional needs established by technical committees representing both space programs. The Russian four-meal-per-day menu was used, with each space program providing two of the meals. Three larger meals were designed to be eaten as scheduled meals; the fourth meal was composed of foods that could be eaten at any time throughout the day.
The current food system for the International Space Station, which started in 2000, is similar to the system used in the Shuttle-Mir program. The four-meal-per-day menu plan is used, with equal provision of foods by the U.S. and Russian space programs. The menu is composed mainly of packaged foods that are freeze-dried and thermostabilized (canned), with very few fresh foods. The crew members plan their own menus with the assistance of a dietitian, and an effort is made to include all of the nutrients needed for working in the space environment. After the habitation module galley is equipped with refrigerators, freezers, and a microwaveconvection oven, a more extensive menu, including a variety of fresh foods, will be available.
Dietary Intake during Spaceflight
Dietary intake has been monitored on select Apollo, Skylab, Shuttle, and Shuttle-Mir flights as a part of scientific studies. Preflight and postflight intakes are determined using conventional methods for dietary assessment. Crew members are provided a diet-record logbook and digital scale, or the foods are weighed by the research dietitian and provided during each of the five- to eighteen-day data collection sessions. A variety of nutrient-analysis software programs are used. Crew members record their intake during space-flight by writing it in a log or, more frequently, they use a barcode reader that scans the food package label and then record the amount consumed. The amounts of certain nutrients in each meal are calculated from the record of how much of each type of food was eaten, plus knowledge of the amount of each nutrient in each type of food. Nutrient calculations using chemical analysis data for each spaceflight food item are performed after the flight. On the International Space Station, crew members complete a food-frequency questionnaire each week, and the data is down-linked for analysis. Dietary intake can thus be assessed in real time. Changes in diet may then be suggested to the crew members to prevent nutrient deficiencies.
A primary concern is that astronauts consume enough energy (calories) for optimal work performance and good health. Of the flight crews that have been monitored, only the Skylab crew members consumed enough energy—99 percent of their predicted intake. Most of the crew members in other flight programs consumed about 70 percent of what was planned. On the Skylab flights, much time and attention was given to eating and food preparation, and the crew members' extensive exercise program may have stimulated their appetite. On all other flights, the crew members have had a very busy schedule, with little time and attention devoted to eating.
Crew members' dietary intakes on Skylab, Shuttle, and Shuttle-Mir flights have tended to be higher in carbohydrate and lower in fat than their pre-flight intakes. This change may have been related to an abundance of foods high in carbohydrates, especially sugar-sweetened beverages, or perhaps these items are more easily prepared during a busy work schedule. Ample fat sources are available in the Shuttle food inventory—more than half of the main dish items contain greater than 30 percent of their calories as fat.
Intake of fluid should be about 2,000 milliliters (2 liters) per day, which is sufficient to prevent dehydration and kidney stone formation. Fluid intakes have varied from 1,000 to 4,000 milliliters per day, indicating that some crew members are getting less than the recommended amount.
Inflight sodium intakes of all crew members have exceeded the recommendation of less than 3,500 milligrams per day. Sodium intake is high because many of the "off-the-shelf" food items used have a high sodium content.
Calcium intakes have been below the recommended range of 1,000 to 1,200 milligrams per day. This level is estimated to minimize the bone mineral loss that occurs during spaceflight.
Iron intakes have been 50 to 60 percent greater than the recommendation of ten milligrams per day. As with sodium, iron intakes are high because the food items have already been iron-fortified. Too much iron in the body may cause tissue damage.
Nutrition is critical for health, both on Earth and during spaceflight. Specific nutrition concerns for spaceflight include adequate consumption of calories for energy, adequate fluid intake to prevent dehydration and renal stones, adequate calcium to minimize bone loss.
There seems to be an excess of both sodium and iron in the inflight diet, compared to predicted requirements. A food delivery system needs to be designed to include foods that will provide nutrients at the recommended levels, while providing variety and palatability to make eating more pleasant.
The International Space Station represents the beginning of an era of humans living and working in space, with the potential for a permanent human presence in space. Nutrition will play a vital role in ensuring the health and safety of spacefaring individuals, whether they are in low Earth orbit or on journeys to the moon, Mars, or beyond. A more complete understanding of the effects of spaceflight will not only help humans to explore the universe, but will provide information needed to maintain human health and treat diseases here on Earth.
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