Nutritional Needs in Cold and High-Altitude Environments Applications for Military Personnel in Field Operations (1996) / Chapter Skim
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16 The Physiology of High-Altitude Exposure
16 The Physiology of High-Altitude Exposure
Pages 295-318
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From page 295... ...
National Academy Press The Physiology of High-Altitude Exposure Allen Cymerman INTRODUCTION The problems inherent in traversing, fighting, and surviving at altitude are as true today as they were almost 2,000 years ago. This is evident by the quotation Tom Tookim, a Chinese official addressing the Generalissmo Wang Fung in 35 B.C.
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In a relatively short time, a soldier could easily go from a sweating situation with several layers of SLEEP I EXERCISE | I HYPOXIA I | HEAT | 1 /| COLD | WELLBEING DETERIORATION ~ EFFECT 1 ~1 ON | FIGURE 16-1 Interaction of various factors impinging on the well-being of individuals at higl1 terrestrial elevations.
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Because relative humidity is generally lower and ventilatory rates higher with increasing altitude, the amount of insensible water loss can be large enough to cause dehydration and weight loss. Estimates of insensible water loss at high altitudes have been derived from sea-level measurements since no direct measurements have been made.
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and no decrease in expired air temperature, it is estimated that at the same ventilatory rate of 100 liter/min, the respiratory heat loss would be 1.6 times greater (Gonzalez et al., 19851. It is generally believed that an altitude-induced diuresis normally occurs during the initial phases of altitude acclimatization when acute mountain sickness, high-altitude pulmonary edema, and high-altitude cerebral edema are not present.
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Figure 16-2 displays several significant elevations relative to the summit of Mount Everest and illustrates the curvilinear relationship between barometric pressure and terrestrial elevation. Assuming that cold is not a contributing factor, the problems of highaltitude exposure can be initially attributed to the reduction in the partial pressure of oxygen in the ambient atmosphere and the body's subsequent responses.
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, 760 600 400 200 O PB (mmHg) FIGURE 16-2 Relationship between altitude and barometric pressure (PB)
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The reduction in barometric pressure, however, does cause a concomitant, obligatory decrease in the partial pressure of oxygen (Po2) , which is defined as the specific fractional composition of the mixed gas expressed as a pressure.
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Figure 16-3 illustrates the relative changes in ventilation, end-tidal CO2 partial pressure, and blood oxygen saturation over the course of 20 days at the summit of Pikes Peak (4,300 m t14,110 fly. Resting minute ventilation of approximately 5 to 7 liter/min at sea level can increase to almost 15 liter/min at this elevation.
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. 96S Stimulation of the sympathetic nervous system, principally secretion of norepinephrine, as a response to altitude exposure has been confinned in many studies (Brooks et al., 1991a; Cruz et al., 1976; Cunningham et al.,~ 1965; Hoon et al., 1976; Kotchen et al., 1973; Mazzeo et al., 1991; Moncloa et al., 1965; Reeves et al., 1992a; Young et al., 1989~.
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These illnesses are termed acute mountain sickness (AMS) and high-altitude cerebral edema (HACE)
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A factor not TABLE 16-2 Symptoms of Acute Mountain Sickness (AMS) and HighAltitude Cerebral Edema (HACE)
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From a nutritional standpoint, it should not be a concern for troops at high terrestrial elevations. WEIGHT LOSS AT ALTITUDE Altitude exposure usually leads to significant weight losses in nonacclimatized individuals.
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307 V, ._ 3 C~ ct c~ ._ ce o Q ct c~ q)
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Headache, the most prominent symptom of AMS, can be sufficiently severe to have a negative impact on food intake. It is estimated that food intakes are decreased by as much as 25 to 50 percent during the first 3 days of acute exposures to 4,300 m (14,110 ft)
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Imbalance Between Energy Intake and Expenditure The discordance in energy intake and expenditure is evident in military field situations irrespective of the terrestrial elevation. It is not unusual for daily energy expenditures to be on the order of 4,000 to 6,000 kcal, with contributions of approximately 1,200 to 2,800 kcal/d from body energy stores.
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This level of energy expenditure resulted in body weight losses of 2.5 and 1.3 kg, respectively. It appears that body weight losses, especially at altitude, are inevitable in field studies involving high levels of strenuous exertion, regardless of the availability of food.
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Most climbers prefer a high-carbohydrate, low-fat diet, with fatty foods actually becoming distasteful. Studies bearing on the composition of a "mountain" diet indicate that rations used at sea level would be adequate at moderate altitudes.
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. AUTHOR'S CONCLUSIONS AND RECOMMENDATIONS The following conclusions are made regarding the physiology of highaltitude exposure: Initial weight loss with altitude exposure may be unavoidable due to increased water losses and decreased energy intake relative to expenditure.
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Gnadinger 1988 Atrial natriuretic peptide in acute mountain sickness.
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Reeves 1982 Fluid retention and relative hypoventilation in acute mountain sickness.
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Askew, and A Cymerman 1994 Doubly labeled water measurement for human energy expenditure during exercise at high altitude.
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Roy, and C.S.V. Subramanyam 1969 Acute mountain sickness.
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Houston 1989 Operation Everest II: Plasma lipid and hormonal responses during a simulated ascent of Mt. Everest.
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