Nutritional Needs in Cold and High-Altitude Environments Applications for Military Personnel in Field Operations (1996) / Chapter Skim
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7 The Physiology of Cold Exposure
7 The Physiology of Cold Exposure
Pages 127-148
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From page 127... ...
That is, they wear clothing, remain in shelters, and use various heat-generating devices. However, when behavioral strategies are inadequate to defend body temperature homeostasis, physiological responses are elicited.
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From page 128... ...
Thus, environmental characteristics besides temperature influence the potential for heat loss and the resulting physiological strain of defending body temperature. 2 During concentric work, the muscle shortens as it develops tension; during eccentric work, the muscle lengthens as it develops tension.
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From page 129... ...
The decrease in peripheral blood flow reduces convective heat transfer between the body's core and shell (skin, subcutaneous fat, and skeletal muscle) and increases insulation.
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From page 130... ...
Thus, during cold exposure, central core temperature defense occurs at the expense of a decline in skin temperature. The reduction in blood flow and consequent fall in skin temperature contribute to the etiology of cold injuries (Purdue and Hunt, 1986~.
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From page 131... ...
. Thus, voluntary physical activity during work or exercise increases metabolic heat production (exercise in the cold will be considered later in the chapter)
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From page 132... ...
The electromyographic measurement in individual shivering muscles can be analyzed to quantify shivering activity (Muza et al., 19861. More commonly, however, shivering thermogenesis is quantified by measuring the increase in whole-body oxygen uptake (VOLT By assuming that the respiratory exchange ratio represents a nonprotein respiratory quotient, calculation of the thermal equivalent (i.e., metabolic heat production)
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From page 133... ...
While it is obvious that the increment in nutritional energy requirement will be proportional to the duration and severity of cold exposure, accurate predictions of individual requirements are difficult. Attempts have been made to determine whether the increased metabolic rate of shivering muscle causes preferential use of a particular substrate.
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From page 134... ...
Furthermore, muscle glycogen depletion does not compromise metabolic heat production or core temperature defense during cold exposure. Using a similar experimental design, also using eight young male subjects, but with shorter immersions, Martineau and Jacobs (1989)
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From page 135... ...
Thus, muscle glycogen is probably not an obligatory substrate for shivering, at least at sea level. When alternate substrates, such as blood glucose, are available, muscle glycogen can be spared or resynthesized at a rate equal to its use.
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From page 136... ...
In contrast, increased heat loss during exercise in cold water can be so great that metabolic heat production, even during intense exercise, is insufficient to defend core temperature (Toner and McArdle, 19881. During submaximal exercise in the cold, VO2 can be higher than, or the same as in temperate conditions, depending on the exercise intensity (Young, 19901.
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From page 137... ...
137 is higher in cold than in temperate conditions, since metabolic heat production during low-intensity exercise is insufficient to maintain core and skin temperatures high enough to prevent the afferent stimulus for shivering. Thus, the increased VO2 represents the added oxygen requirement for shivering activity.
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From page 138... ...
Heart rate is usually lower and stroke volume higher during exercise in cold air or cold water compared to exercise at the same VO2 in temperate conditions (Doubt, 1991; McArdle et al., 19761. This phenomenon probably reflects the effect of increased cardiac preload due to the increased central blood volume that is associated with cold-induced peripheral vasoconstriction.
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From page 139... ...
(1984) observed that heat loss and the decline in body temperature in cold water were greater during arm than during leg exercise at the same metabolic rate because of the greater surface area-to-mass ratio of the arms.
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From page 140... ...
Heat loss was measured under conditions in which peripheral blood flow was minimal (immersion in water cool enough to induce maximal vasoconstriction without eliciting shivering)
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From page 141... ...
Nevertheless, controlled laboratory comparisons show that older men may be less able than younger men to defend core temperature during cold exposures. The cutaneous vasoconstrictor response to cold may be slower and coldinduced vasodilation may be blunted (see Figure 7-3)
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From page 142... ...
When women and men of equivalent subcutaneous fat thickness are compared, the women have a greater surface area and smaller total body mass. Although insulation is equivalent, women's total heat loss is greater due to the larger surface area for convective heat flux.
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From page 143... ...
There are nutritional implications of the physiological responses, particularly the thermogenic response. Increasing metabolic heat production requires increased energy intake.
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From page 144... ...
YOUNG ET AL. Some general recommendations can be made: · Research should resolve discrepant findings concerning effects of muscle glycogen depletion on thermogenesis and heat balance in the cold, with emphasis on effects of body fat differences.
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From page 145... ...
Romet, and D Kerrigan-Brown 1985 Muscle glycogen depletion during exercise at 9 degrees C and 21 degrees C
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From page 146... ...
Pandolf 1988 Respiratory and cardiovascular responses to cold stress following repeated cold water immersion. Undersea Biomed.
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From page 147... ...
Pandolf 1989 Thennoregulation during cold water immersion is unimpaired by low muscle glycogen levels.
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