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OCR for page 125
--> III THE COLD ENVIRONMENT CHANGES IN PHYSIOLOGY AND NUTRIENT requirements due to the cold environment are considered in Part III. The mechanisms to maintain core body temperature and to reduce and restore heat loss during cold exposure are collectively termed thermoregulation. Chapter 7 discusses physiological thermoregulation, or the way in which dry heat loss is reduced through vasomotor responses and replaced through metabolic responses, in relation to physical performance. While involuntary shivering contributes to body-temperature regulation, voluntary physical activity can do more to increase heat production. Individual characteristics, such as body composition and physical fitness, also contribute to the regulation of the rise and fall of body temperature. Biological clocks control various physiological processes and often conflict with military schedules. Chapter 8 explains the behavioral and physiological responses that the body utilizes to manage these changes and focuses on the physiological process of sleep and how it is disrupted by military operations. To enhance physical and cognitive performance in extreme environments, it may be possible to develop biological and pharmacological agents that may alter circadian rhythms and other biological clocks. As Chapter 9 explains, environmental extremes can cause disruption in fluid balance, and dehydration is a possibility in cold environments as well as hot environments. The most significant factors associated with dehydration during cold exposure include cold-induced diuresis, respiratory water losses, the metabolic cost of movement, and reduced fluid intake, with the most
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--> the metabolic cost of movement, and reduced fluid intake, with the most significant being the increased fluid loss associated with high metabolic work rates. Dehydration negatively influences physical and cognitive performance and thermoregulation, and increases possible susceptibility to injury. Chapter 10 briefly summarizes some recent research on how skeletal muscle fuels shivering. Understanding shivering can be useful in a survival situation to enhance thermogenesis and delay the onset of life-threatening hypothermia. Chapters 11 and 12 focus on energy requirements in the cold. To determine the optimal macronutrient ratio, several factors must be considered, including the caloric density of fat as compared to carbohydrate or protein, the enhancement of thermogenesis associated with the thermic effect of food, and the preference for a particular nutrient in the diet. As an example of environmental influence on appetite, cold exposure increases energy expenditure which may stimulate appetite to allow an enhanced intake of energy. The thermic effects of food, of cold, and of exercise are important considerations in understanding appetite and weight maintenance in the cold. Using the Military Recommended Dietary Allowances (MRDAs) as a frame of reference, the influence of cold exposure on the need for vitamins and minerals is reviewed in Chapter 13. There is little scientific justification for supplementation above the MRDAs to cope with cold stress and exposure. In Chapter 14, the possible effects of iron, copper, and zinc deficiencies on thermoregulation are discussed. Major emphasis is placed on iron's role in maintaining core body temperature, although the MRDA for iron appears to be sufficient to prevent deficiency. For the most part, any short-term deficiency that may occur during a military operation is not likely to lead to micronutrient deprivation unless there is a preexisting reserve depletion. Finally, this section on the cold environment concludes with a discussion in Chapter 15 of the possibility of a drug-mediated delay of hypothermia during cold exposure. The author is unable to verify the claims of a commercial sports bar and its ability to delay hypothermia, but does find that ephedrine-xanthine mixtures represent a safe agent to enhance cold tolerance in humans.
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