Nutrition and Metabolism
As it pertains to space travelers, the term “nutrition” encompasses many things, including the body’s basic need for nutrients. The amounts of nutrients required for optimal health can be affected by aspects of the environment, including gravity, radiation, temperature, and humidity. Nutritional requirements can be further altered by effects of microgravity on the body’s absorption, processing (metabolism), and excretion of nutrients. “Nutrition and metabolism” also encompasses the interaction of nutrients with the biochemical pathways (metabolism) by which physiological systems, such as bone, muscle, and cardiovascular systems, and even behavior and performance, accomplish their functions.
For exploration, having enough food is important, but having the right nutrition is critical. History provides many examples of how insufficient knowledge of or planning for nutrition led to disastrous results. Scurvy among sailors is a common example, and while many people have a vague notion of the fact that scurvy, aka vitamin C deficiency, was important in long-distance exploration hundreds of years ago, they do not realize the magnitude of this problem. In fact, scurvy killed more sailors in the so-called “Age of Sail” than all other causes of death combined, including shipwreck. It is estimated that more than 2 million sailors perished from scurvy, which often claimed more than 90% of a ship’s crew.
For space exploration, nutrition is even more critical, in part because food will not be found during the journey. The nutrient requirements need to be known and provided in palatable foods, the nutrients need to be stable over long durations, and the food needs to be consumed. We need to understand the human body’s need for nutrients, that is, its nutritional requirements, and how these change during spaceflight. We need to understand what countermeasures will be applied to body systems to prevent harmful effects of spaceflight, and account for effects of those countermeasures on nutritional status. As one such example, if an intense exercise regimen is undertaken to mitigate muscle loss, but enough calories are not provided to support this level of exercise, then the exercise will ultimately fail to protect muscle.
Furthermore, we need to understand the role of nutrition in human adaptation to spaceflight so that crewmembers’ health can be maintained in the best possible state on missions to other planetary surfaces or “exploration-class” missions. Nutrition provides fuel, structure, and the ability to function properly to virtually every system in the body. We know that astronauts who do not consume enough energy during spaceflight have less than optimal bone, muscle, and cardiovascular health. They also have increased tissue damage caused by oxidation, and changes in the function of their immune and endocrine systems, among other issues. We know that vitamin deficiencies can cause problems during spaceflight, and conversely, nutrient excess is also a concern. Beyond the metabolism of nutrients, the role of food and nutrition in crew behavior and performance is also substantial.
Understanding and balancing nutrition is required for success on exploration missions. This problem has many facets, some of which we are only now beginning to understand.
The role of nutrition in space travel has been reviewed in many publications and in various degrees of detail (Smith et al. 2009, 2014a, b, 2015; Zwart et al. 2012); a brief overview is provided herein.
Historically, crewmembers often did not consume adequate energy for many reasons. On International Space Station (ISS) missions, especially on those that followed the deployment of the advanced resistance exercise device (ARED) in late 2008, energy intakes have generally increased, and many crewmembers have maintained their body mass over the course of their missions. Although maintaining body mass might seem a rather pedestrian achievement, we know that inadequate energy intake and loss of body mass affect many physiological systems and can affect behavior and performance. As evidence of this, since the deployment of the ARED, many crewmembers have consumed adequate energy, had good vitamin D status, maintained their body mass, and even maintained density of their bones.
Suggestions have been made that during spaceflight abnormalities occur in homeostasis of body fuel, including alterations in metabolism of glucose and lipids, but additional research is required before this can be confirmed. Dietary intake and supplementation of protein (and amino acids) has long been studied as potential countermeasures for muscle loss during spaceflight. Unfortunately, these studies have often been conducted in individuals consuming less than adequate energy and/or protein, in either the treatment or the control group, and thus the literature on this topic is inconsistent and therefore inconclusive. Conversely, it is known that high protein intakes can be challenging for kidneys, bones, and other tissues, in part as a result of the acid load associated with increased amino acid oxidation.
Fresh fruits and vegetables are the best sources of vitamins, and they are key ingredients missing from the nominal space food system. Some fresh foods are flown on cargo vehicles heading to the ISS, but deliveries occur only a few (Smith et al. 2009; Smith et al. 2014b) times during a crewmember’s 6-month mission. While this is helpful from a nutrient intake point of view, as well as for morale, on longer exploration missions resupply will be much more limited.
Most vitamins are complex organic structures, highly susceptible to breakdown and inactivation. These processes can occur over time and during food preparation. During spaceflight, another concern is exposure to radiation, especially when one considers the duration and deep space location of exploration-class missions.
Vitamin D is a unique vitamin, because of the body’s ability to synthesize it when the skin is exposed to ultraviolet light. Obtaining an adequate amount of vitamin D becomes an issue for inhabitants of spacecraft, which are shielded to protect crews from ultraviolet light and where the food system has few sources of vitamin D. ISS crews are provided with 800 international units per day of vitamin D supplements, which maintain their vitamin D levels in what is considered by many to be optimal status.
For other vitamins, widespread deficiencies have not been observed on ISS missions. However, assessment of astronaut nutritional status has revealed that many nutrient issues have arisen in individuals before, during, and after flight, most of them reflecting inadequate dietary intake. These findings highlight the need to ensure adequate nutritional status.
As with virtually all nutrients, excess intake can pose health risks and problems, as can deficiency. Monitoring of all sources of nutrient intake, food and supplements alike, is required.
Adequate (and not excessive) mineral intake is required for general health. Many specific examples of cellular processes and functions exist that require the likes of calcium, iron, zinc, copper, magnesium, and more. Many of these are described in more detail below, in the “Nutrition and the Physiology of Body Systems” section.
Calcium is a key nutrient for bone health, but during spaceflight, the effects of microgravity lead to the release of calcium from bone (and thus bone loss), a reduction in the absorption of calcium from the diet, and an increase in excretion of calcium in the urine. The latter effect contributes to concerns about the risk of developing kidney stones, and astronauts are often reminded about the need to stay well hydrated to minimize this risk as much as possible. Attempts to mitigate bone loss with supplemental calcium in bed rest studies (an analog of spaceflight) were unsuccessful, but nonetheless, maintaining adequate calcium intake is important to keep from exacerbating bone loss.
Maintaining iron homeostasis is important because an excess or a deficiency of this mineral is problematic. Humans have little capacity to excrete iron, so regulation of iron homeostasis occurs mostly by changes in iron absorption. During spaceflight, iron status increases early, but by the end of the flight in most crewmembers it drops back to preflight levels. The lack of gravity during flight results in a smaller circulatory volume, and newly formed red blood cells are lysed in a process called neocytolysis. As a result of this process, the red blood cell mass in the body decreases by about 10% in the first few weeks of flight. The decrease in red blood cell mass is accompanied by an increase in tissue stores of iron, as evidenced by an increase in ferritin. These changes in iron status during flight are associated with oxidative damage markers and changes in bone mineral density after flight. The participation of iron in Fenton reactions yields hydroxyl radicals, which are one of the most reactive types of free radicals known.
Nutrition and the Physiology of Body Systems
The interrelationships of nutrition (and nutrients) with physiological systems represent a key facet of understanding and optimizing human adaptation to microgravity. That is, although deficiency or toxicity of any given nutrient (or worse, group of nutrients) can be disastrous, the effects of inappropriate nutritional or nutrient status on physiological systems can be damaging as well.
Nutrition is intertwined with bone health in many ways, from the basics of calcium and vitamin D, to effects of specific nutrients, to secondary interrelationships of dietary intake with acid-base balance. In more than a half-century of human spaceflight, recently for the first time bone mineral density has been maintained by astronauts with nothing more than diet and exercise. Recent data from the ISS document that crewmembers who had good nutritional status (energy intake, vitamin D status) and who exercised with the advanced resistance exercise device (ARED) maintained bone mineral density after missions of 4–6 months. These data are also supported by ground-based bed rest studies. While questions remain about bone strength and quality after a period of weightlessness during which bone remodeling takes place, these results represent a marked improvement over essentially all earlier data from spaceflight.
Ground-based studies have provided evidence that dietary patterns known to affect acid-base balance may be useful as countermeasures to bone loss. Spaceflight studies have also been conducted, with final analyses pending at time of this writing. High sodium intakes and diets with a high ratio of animal protein to potassium have both been suggested to be deleterious to bone health. Both of these examples lead to increased endogenous acid production, which the body neutralizes through bone resorption and calcium carbonate release. Additionally, atmospheric carbon dioxide concentrations on board the ISS, which are often 10 times those on Earth, can also contribute to bone demineralization.
Specific nutrients can also have effects on bone. Increased iron stores during spaceflight are associated with increased oxidative stress and regional bone loss (as described earlier). Fish intake, and specifically intake of omega-3 fatty acids, has been shown to mitigate bone loss in both flight and ground analog studies.
Bone serves as a sink for many minerals, including copper, zinc, and even heavy metals like lead. Bone resorption can lead to the release of these minerals into the bloodstream, raising concerns about increased exposure to them during spaceflight, which could be particularly concerning if, for example, an astronaut’s lifetime lead exposure was increased for any reason.
Protein intake and metabolism have long been studied in an effort to mitigate spaceflight-induced muscle loss. Given the inherent contribution of protein to energy balance, use of protein to mitigate muscle loss is difficult or impossible to evaluate without considering both protein and energy. ISS crewmembers who maintain their total body mass and exercise with the ARED actually return from flight with a higher percentage of lean body mass than those who do not.
Given the intensity of the on-orbit exercise protocols, many crewmembers have chosen to fly protein supplements of one form or another. Since the body cannot store protein, excess protein consumption leads to oxidation of amino acids for energy when total calories are insufficient to meet requirements, or to conversion into carbohydrate or fat for storage when energy consumption exceeds requirements. Amino acid oxidation and/or conversion leads to release of nitrogen and sulfur, which present additional challenges to kidney and bone.
The relationship between nutrition and cardiovascular health has many facets, but unfortunately, not many of these have been well characterized during spaceflight. As with other systems, intake of fruits and vegetables (and the nutrients they contain) is associated with improved cardiovascular health in the terrestrial population, and it is assumed there is no difference with astronauts. The benefits of healthier food intake have yet to be studied on orbit, but with missions of 6 months’ duration, and now with the initiation of 1-year missions, diet will no doubt have a greater effect on cardiovascular health during and after flight. Specific nutrients, including potassium, magnesium, omega-3 fatty acids, and others, have specific effects on cardiovascular health.
Energy intake during spaceflight has been correlated with cardiovascular performance after flight, and more specifically, inadequate energy consumption leads to greater plasma volume loss. The degree of plasma volume loss in turn is associated with cardiovascular performance at landing. This has been documented in short-duration spaceflight, as well as in controlled bed rest studies where energy intake was restricted. The implications of this link between energy intake and cardiovascular performance have not been fully explored. That is, some of the data detailing cardiovascular decrements during flight may not represent microgravity-induced deconditioning alone, and perhaps adequate energy intake would be a simple countermeasure to some of these effects.
Nutrition is an important factor in eye health. Some eye conditions affected by specific nutrients are cataract, age-related macular degeneration, dry eye, optic neuropathy, and overall vision function (Zwart et al. 2014). In recent years, it has been noted that some crewmembers coming home from 4–6 month missions on the ISS experience ophthalmic changes such as choroidal folds, optic disc edema, optic nerve sheath distension, visual acuity changes, and cotton wool spots. The etiology of the ophthalmic changes after long-duration spaceflight is not known and is being researched. Nutritional biochemistry data from crewmembers before, during, and after flight indicate that the folate- and vitamin B12-dependent one-carbon transfer pathway may be involved.
The immune system is impaired by malnutrition. A micronutrient deficiency can affect T-cell mediated immune response and adaptive antibody response. Nutrients involved in immune system regulation include arginine; glutamine; vitamins A, C, E, and D; and micronutrients including copper, zinc, iron, and polyphenols.
During spaceflight, dysregulation of immune response and increased reactivation of latent viruses occur. Some of the immune changes include altered distribution of circulating leukocytes, altered production of cytokines and chemokines, decreased activity of natural killer cells, decreased function of granulocytes, decreased activation of T cells, altered levels of immunoglobulins, and altered virus-specific immunity (Heer et al. 2012). No direct studies have been performed during spaceflight to investigate whether altering nutrition affects immune response during flight, but there is evidence from ground-based studies that particular nutrients may provide protection. For example, in individuals with high serum cortisol, higher vitamin D status was related to a lower probability of viral reactivation and shedding in saliva.
Behavior and Human Performance
Nutrition and food in general play a role in behavior and human performance on Earth, and perhaps even more so in space. Undernutrition or fasting is associated with risks such as fatigue, irritability, and headache in the short term and more serious risks in the long term, including neurological changes as well as musculoskeletal changes. Nutrition also plays an important role in maintaining circadian rhythms. Disruption of circadian rhythms with shift work or sleep deprivation has been associated with symptoms of metabolic syndrome including increased adiposity. Single nutrients capable of altering circadian rhythms include glucose, amino acids, sodium, thiamine, cholesterol, and retinoic acid.
Nutrition is a cross-cutting discipline that affects many, if not all, systems of the human body. Although humans have been flying in space for more than a half-century, detailed study of the role of nutrition has been sporadic. Efforts in recent years to characterize changes in nutritional biomarkers during ISS missions and the relationship of dietary intake to these changes has opened many doors to areas where optimizing nutrition might provide valuable countermeasures to detrimental effects of spaceflight.
- Smith SM, Zwart SR, Kloeris V, Heer M (2009) Nutritional biochemistry of space flight. Nova Science, New YorkGoogle Scholar
- Smith SM, Zwart SR, Heer M (2014a) Human adaptation to spaceflight: the role of nutrition (NP-2014-10-018-JSC). National Aeronautics and Space Administration Lyndon B. Johnson Space Center, HoustonGoogle Scholar
- Smith SM, Heer M, Zwart SR (2015) Nutrition and bone health in space. In: Holick M, Nieves J (eds) Nutrition and bone health, 2nd edn. Springer, New York, pp 687–705Google Scholar