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This paper reviews the experimental findings describing human physiological adaptations induced by chronic or repeated cold exposure. The adaptations are classified as acclimatization when they occur naturally as a result of climatic or seasonal changes in temperature, or as acclimation when they occur in response to artificial or experimental manipulations of the ambient thermal environment. Three different patterns of cold adaptation can he identified. Habituation, the most common pattern observed in both acclimatization and acclimation studies, is characterized by a blunted shivering and vasoconstrictor response to cold exposure. Habituation appears to require only brief, intermittent cold exposures to he induced, and can develop when only small body regions are exposed unprotected to cold. It allows extremity skin temperatures to be maintained higher during cold exposure. The higher skin temperatures coupled with the absence of shivering are advantageous in that manual dexterity and comfort are enhanced. In one acclimation study in which subjects were exposed to moderate cold conditions for a prolonged period, a metabolic form of cold acclimation appeared to develop. This adaptation was characterized by an enhanced shivering thermogenesis during cold exposure. When individuals acclimatize or acclimate to cold conditions severe enough to repeatedly cause a significantly body temperature fall, an insulative pattern of adaptation develops, characterized by enhanced mechanisms for body heat conservation. The mechanisms determining the pattern of adaptation to chronic cold exposure appear related to type of cold exposure conditions, the amount of body heat lost and the degree to which shivering thermogenesis compensates for heat loss and defends body temperature.
Environmental Stress: Individual Human Adaptations is the result of a symposium where scientists addressed questions about individual variability in response to different environments. The symposium aimed to create more interest in the roles of age, gender, genetic heritage, and other individual differences in response to various environmental stressors. The book is divided into five sections, each dealing with one aspect of environmental stress. These are: heat stress, air pollution, work physiology (exercise), cold stress, and altitude. Circulatory adaptations to heat and exercise are discussed in the heat section while studies of sleeping patterns associated with high altitude hypoxia are tackled in the section of altitude. In the section of air pollution, the different effects of pollutants such as carbon monoxide and sulfuric acid are tackled. This text will be very useful to students and scientists in many fields such as medicine, physiological sciences, biophysics, and environmental health.
Heat acclimation consists of adaptations that mitigate physiological strain of heat stress, which improve thermal comfort and exercise capabilities. Adaptations are induced by repeated heat exposures that are sufficiently stressful to elevate core and skin temperatures and elicit perfuse sweating. Most adaptations to daily heat exposure occur during the first four days, and the remainders are complete by three weeks. Heat acclimation mediated adaptations include: lower core temperature, improved sweating and skin blood flow, lowered metabolic rate, reduced cardiovascular strain, improved fluid balance, and increased thermal tolerance (i.e., cellular stress protein adaptations). These adaptations vary somewhat depending if exposed to dry or humid heat. Adaptations to chronic cold exposure can be categorized into three basic patterns: habituation, metabolic adaptations and insulative adaptations. The exact determinant of which pattern will be induced by chronic cold exposure is unclear, but the magnitude and extent of body cooling, frequency and duration of exposure, and individual factors all influence the adaptive process. Habituation is characterized by blunted shivering and cutaneous vasoconstriction; body temperature may decline more in the acclimatized than unacclimatized state. It is the most common cold adaptation and results from periodic short-term cold exposures. Metabolic adaptations are characterized by enhanced thermogenesis that develops when cold exposures are more pronounced, but not severe enough to induce significant declines in core temperature. Insulative adaptations are characterized by enhanced vasoconstriction and redistribution of body heat toward the shell that develops from repeated cold exposures severe enough to induce marked declines in core temperature.
This review focuses on the responses and mechanisms of both natural and artificial acclimatization to a cold environment in mammals, with specific reference to human beings. The purpose is to provide basic information for designers of thermal protection systems for astronauts during intra- and extravehicular activities. Hibernation, heat production, heat loss, vascular responses, body insulation, shivering thermogenesis, water immersion, exercise responses, and clinical symptoms and hypothermia in the elderly are discussed.
The activities of the Food and Nutrition Board's Committee on Military Nutrition Research (CMNR, the committee) have been supported since 1994 by grant DAMD17-94-J-4046 from the U.S. Army Medical Research and Materiel Command (USAMRMC). This report fulfills the final reporting requirement of the grant, and presents a summary of activities for the grant period from December 1, 1994 through May 31, 1999. During this grant period, the CMNR has met from three to six times each year in response to issues that are brought to the committee through the Military Nutrition and Biochemistry Division of the U.S. Army Research Institute of Environmental Medicine at Natick, Massachusetts, and the Military Operational Medicine Program of USAMRMC at Fort Detrick, Maryland. The CMNR has submitted five workshop reports (plus two preliminary reports), including one that is a joint project with the Subcommittee on Body Composition, Nutrition, and Health of Military Women; three letter reports, and one brief report, all with recommendations, to the Commander, U.S. Army Medical Research and Materiel Command, since September 1995 and has a brief report currently in preparation. These reports are summarized in the following activity report with synopses of additional topics for which reports were deferred pending completion of military research in progress. This activity report includes as appendixes the conclusions and recommendations from the nine reports and has been prepared in a fashion to allow rapid access to committee recommendations on the topics covered over the time period.
This book reviews the research pertaining to nutrient requirements for working in cold or in high-altitude environments and states recommendations regarding the application of this information to military operational rations. It addresses whether, aside from increased energy demands, cold or high-altitude environments elicit an increased demand or requirement for specific nutrients, and whether performance in cold or high-altitude environments can be enhanced by the provision of increased amounts of specific nutrients.
A text that explores how humans adapt to conditions of physical stress
The widespread interest in "stressful" aspects of contemporary society which contribute to its burden of illness and diseases (e.g. gastro intestinal, cardiovascular) has led to a large number of state ments and reports which relate the manifestations to a maladaptation of the individual. Furthermore, recent research suggests that under some condi tions stress may have a more generalized effect of decreasing the body's ability to combat destructive forces and expose it to a variety of diseases. Breakdown in adaptation occurs when an individual cannot cope with demands inherent in his environment. These may be due to an excessive mental or physical load, including factors of a social or psychological nature and task performance requirements ranging from those which are monotonous, simple and repetitive to complex, fast, decision-taking ones. Experience shows however that not all people placed under the same condi tions suffer similarly, and it follows that to the social and psychological environment should be added a genetic factor influencing, through the brain, the responses of individuals. It is clear that, besides human suffering, this "breakdown in adaptation" causes massive losses of revenue to industry and national health authorities. Thus a reduction in "stress", before "breakdown" occurs, or an improvement in coping with it would be very valuable.