DEPRESSED METABOLISM
In anticipation of prolonged manned space flights, NASA has sponsored research related to metabolism depression. The daily food requirements, for example, of astronauts during a voyage of several months can constitute a major portion of the weight and storage capacity of the spacecraft. A somewhat promising and fundamental approach to this problem is the reduction of the astronauts' daily metabolic requirements. It has been suggested that astronauts on prolonged space missions be put in a state of suspended animation until their destination is reached. Though this sounds fantastic, 10 years ago no cell had been frozen to cryogenic temperatures and survived. Today it is commonplace for tissues to be frozen, stored at low temperatures, and thawed and then to maintain their viability and function.
Animal metabolism may be depressed by reducing body temperature, as in hibernation and hypothermia. Other means by which metabolism can be lowered include drugs and electronarcosis. Hibernation is a nonstressful state and results in a great decrease in metabolism. However, human beings are not hibernators, and much research is needed before the mechanism of hibernation is understood, and the possibility of inducing it in humans evaluated. Hypothermia is the direct cooling of the body to temperatures where metabolism is substantially depressed. Extracorporeal circulation systems combined with cooling are in routine use in most medical centers throughout the world. Hypothermia is not an ideal solution, however, since general body hypothermia is a stressful condition. Pharmacologic induction of hypothermia can be accomplished by such drugs as chlorpromazine and harbamil. Other drugs can be used to depress metabolism, but all have some disadvantage.
In recent years there has been a growing interest in electronarcosis, the induction of sleep by an electric current. Although potentially valuable, this method is far from routine application.
Outstanding advances have been made in metabolism suppression. Recent progress in the biochemistry and physiology of hibernation and hypothermia have shown that the oxygen requirements of individual mammals, organs, and tissues can be reduced. When the chemical composition of the blood and the cardiac output are sufficient to meet cellular requirements, regulatory mechanisms remain effective and animal survival is assured. In contrast, when oxygen transport is interrupted, a reduction in cellular activity occurs and regulation is impaired. In induced hypothermia, the low temperature slows the rates of all processes and modifies the action of metabolites and other substances. This in itself is not harmful, as shown by the true hibernating animal (e.g., ground squirrel), but will become disastrous as soon as anoxia and chemical imbalance begin to develop.
The phenomenon of natural hibernation is being investigated in the laboratory in the hope that the unusual tolerance of hibernating animals to reduced metabolism and low body temperature may some day be produced artificially in ordinary laboratory animals and man. Experiments with the ground squirrel, a typical hibernator, show that the artificially cooled ground squirrel does not tolerate such long periods of low body temperature as does a naturally hibernating animal.
Other studies of the brown adipose tissue (fat), which is present in most hibernating mammals, show it to be essential to hibernation. Indications that brown fat has a thermogenic role in rats exposed to low temperatures suggest that this may be the case in true hibernators ([ref.199]). Arousal of the hibernating animal by cold is triggered by sympathetically activated thermogenesis in areas of brown fat so located, relative to the vasculature, that the heat is transferred to areas of the body concerned with normal metabolic and nervous activity.
Soviet work comparing various depressed metabolic states and resistances to acceleration shows deep winter hibernation to be most effective, followed by deep hypothermia, and drug narcosis as the least effective.
Experimental evidence is being accumulated to show that hibernation and hypothermia somewhat protect animals against radiation. Clinical studies on irradiation of cancer patients indicate that lowering the body temperature reduces cellular metabolism and thus decreases tissue sensitivity to gamma radiation ([ref.200]).
The use of prolonged hypothermia, hibernation, drugs, and electronarcosis appears to hold some potential for reducing astronauts' metabolic requirements. If one or mote of these methods become practical, human requirements for food and oxygen could be drastically reduced. Simultaneously, these methods may afford radiation protection and acceleration tolerance.
NUTRITION IN SPACE[10]
The human body can use food stores so that the nutritional requirements can be reduced for a short time. This will vary widely among individuals and each individual may exhibit characteristic patterns of nutritional behavior. During reduced food intake, muscular efficiency may not change significantly over a period of 4 to 6 days; unfortunately, however, mental activity begins to decline after 24 hours. Feeding requirements can be divided into two categories: short term (for missions of less than 21 days) and long term. Since dehydration can occur in a matter of hours under adverse conditions, water requirements must be considered as a special case.
Water Requirements
Water requirements are extremely critical and the amount supplied should not under any circumstances be kept to a minimum. Rather, a large margin of safety should be allowed.
Present data on water requirements show a very strong dependence upon suit inlet temperatures. In the absence of an accurately controlled suit temperature, water requirements can easily double. If this should occur, the mission would probably have to be aborted, since it is doubtful if electrolyte balance would be maintained at such high rates of water loss. Normal or even extreme conditions of the terrestrial environment usually include diurnal variation in temperature which may modify water needs. These conditions will not be obtained in the spacecraft.
In addition to ground-based experiments, measurements of water intake should be made under actual flight conditions. Data from short-term flights should be used for extrapolation to longer missions.