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13
Effects of Cold and Altitude on Vitamin and Mineral Requirements

Robert D. Reynolds1

INTRODUCTION

The importance of adequate caloric and fluid intake must be rated at least as highly as that of oxygen

(Pugh, 1965, p. 314).

Although Pugh limited his statement about the importance of nutrition at high altitudes to that of energy and fluids, the same concept can be extended to all of the nutrients. However, a superficial perusal of the published literature on the effects of altitude demonstrates that the preponderance of research efforts has been directed to understanding the impact of a limited oxygen supply on human performance. With respect to nutrition, there has been only a modest effort to determine energy intake and to determine the preferred mix of energy derived from carbohydrate, fat, and protein. Unfortunately, a paucity of efforts has been expended to determine the effects of cold and altitude on vitamin and mineral requirements even though these micronutrients are absolutely essential in converting the food consumed into the energy that is

1  

Robert D. Reynolds, Department of Human Nutrition and Dietetics, The University of Illinois at Chicago, Chicago, IL 60612-7256



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--> 13 Effects of Cold and Altitude on Vitamin and Mineral Requirements Robert D. Reynolds1 INTRODUCTION The importance of adequate caloric and fluid intake must be rated at least as highly as that of oxygen (Pugh, 1965, p. 314). Although Pugh limited his statement about the importance of nutrition at high altitudes to that of energy and fluids, the same concept can be extended to all of the nutrients. However, a superficial perusal of the published literature on the effects of altitude demonstrates that the preponderance of research efforts has been directed to understanding the impact of a limited oxygen supply on human performance. With respect to nutrition, there has been only a modest effort to determine energy intake and to determine the preferred mix of energy derived from carbohydrate, fat, and protein. Unfortunately, a paucity of efforts has been expended to determine the effects of cold and altitude on vitamin and mineral requirements even though these micronutrients are absolutely essential in converting the food consumed into the energy that is 1   Robert D. Reynolds, Department of Human Nutrition and Dietetics, The University of Illinois at Chicago, Chicago, IL 60612-7256

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--> required to function in these extreme conditions. These micronutrients are no less important than is oxygen. It just takes longer to become deficient in them than it does for oxygen. In the past, U.S. Army recommendations regarding micronutrient intake for work in cold and high-altitude environments have been limited to three areas of consideration. They have suggested the need for increased requirements for vitamins and minerals to accommodate the caloric requirement of cold and high-altitude operations; a high altitude-induced increase in the requirement for vitamins A, E, and C, specifically; and the need for caution in the use of vitamin and mineral supplements to attempt to prevent cold stress (Thomas et al., 1993a, b; Askew, 1989). However, increased intakes have not been recommended. Under any environmental condition, it is necessary to measure accurately intake, excretion, and several indices of status to determine nutrient requirements. A complete set of these indices has not been measured for any of the vitamins or minerals for persons living or working in the cold or at high altitudes. Therefore, it is not possible to report here the actual requirements for any of these micronutrients as affected by prolonged exposure to cold or high-altitude environments. The approach used in this chapter will be to present a short statement on the major role of each vitamin and mineral in human metabolism, followed by a review of reported dietary intakes or status in various populations living or working in cold climates or at moderate to high altitudes. Finally, for each vitamin and mineral, a micronutrient intake goal will be suggested and compared to the current Recommended Dietary Allowance (RDA) (NRC, 1989), the Military Recommended Dietary Allowance (MRDA) (AR 40-25, 1985), and the anticipated intake of each nutrient provided by the Ration, Cold Weather (RCW) as formulated by the U.S. military. Each micronutrient intake goal has been derived by the author from a systematic reading of the relevant published literature and from personal consideration of widely-accepted, general nutrition principles. Construction of the micronutrient intake goals assumes that the individuals using it are in generally good health and nutrient status prior to beginning the expedition or maneuver. Thus, the aim of the micronutrient intake goals is to keep the individuals healthy rather than to make them healthy. Construction of the micronutrient intake goals should not be constrained by the limitations of the micronutrient availability from foods in the amount normally consumed. Therefore, use of the terms dietary or dietary intake as they relate to the micronutrient intake goals are not necessarily appropriate.

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--> ESTIMATED ENERGY INTAKES Many of the nutrients, especially the B vitamins, are involved with the production of energy via numerous steps in the Krebs citric acid cycle. Thus, consideration will be given in the construction of the micronutrient intake goal to the estimated intakes of total food energy by those persons engaged in strenuous work in the cold or at high altitudes. It has been estimated that in the cold, the average energy consumption by U.S. military soldiers who self-selected foods from the RCW was 2,800 kcal/d, and from the Meal, Ready-to-Eat (four meals) was 3,000 kcal/d (Personal communication, R. W. Hoyt, U.S. Army Research Institute of Environmental Medicine, Natick, Mass., 1994). The current MRDA for energy in cold weather is 4,500 kcal/d (AR 40-25, 1985). Jones and Lee (see Chapter 11 in this volume) estimate that energy requirements (expenditure) for soldiers range between 4,200 and 5,000 kcal/d during periods of physical exertion in the cold. The negative difference between energy intakes and expenditures thereby results in loss of body weight. During the conduct of Operation Everest II with simulated altitudes up to 8,848 m (29,000 ft), subjects consumed between 2,500 and 3,000 kcal/d, with 45 percent of the energy being provided by carbohydrates (Rose et al., 1987). Estimates of energy expenditure between 4,000 and 5,000 kcal/d have been reported for those engaged in strenuous exertion while at high altitudes (Reynolds et al., 1992). Therefore, for the construction of the micronutrient intake goal, an estimated average energy intake of 4,500 kcal/d will be assumed in order to keep the personnel in energy balance. Any deficit between energy intake and expenditure must result in loss of body tissue. FAT-SOLUBLE VITAMINS Vitamin A Functions Vitamin A is essential for the visual process, differentiation of epithelial cells, maintenance of the immune system, and integrity of the skin. The normal needs for vitamin A can be met by consumption of either preformed vitamin A or β-carotene, with the latter having a considerably lower toxicity (NRC, 1989).

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--> Intake and Status Of the fat-soluble vitamins, vitamin A has caused the most problems for polar explorers. Shearman (1978) presented a graphic description of vitamin A toxicity experienced by members of the three-man, 1912–1913 Mawson Australian Antarctic expedition, in which two men died, with only Sir Douglas Mawson surviving. Early in the expedition, Lt. Ninnis and one of the sleds loaded with most of the food fell into a crevasse and disappeared. Over the next 23 days, Xavier Mertz and Mawson were forced to reduce their daily food intake from the normal 34 oz (971 g) to 14 oz (400 g), much of which was dog meat that became available as each dog died (Mawson, 1915). As Mawson reported in his journal, ''It was a happy relief when the liver appeared; even if little else could be said for its flavor, it was easily chewed and demolished" (Shearman, 1978 quoting Mawson, 1915, p. 284). Over a 9-d period, Mertz's health rapidly deteriorated, culminating in his death, with intervening severe bouts of dysentery, fecal incontinence, depression, delirium, peeling skin, and loss of hair—all symptoms characteristic of acute vitamin A toxicity. Shearman (1978) estimated that as little as 100 g of husky dog liver could contain upwards of 1,000,000 international units (IU) (300,000 µg retinol equivalents [RE]) of vitamin A, which was sufficient to cause the toxic symptoms experienced by Mertz. This was not the only polar expedition that was thwarted by hypervitaminosis A. Today, with knowledge of the potential vitamin A toxicity from consumption of dog, seal, polar bear, or reindeer liver (Shearman, 1978), polar explorers or workers are unlikely to repeat such experiences, but warnings must still be given to those planning to spend long periods of time under such environmental conditions. Sundaresan and Therriault (1969) studied rats chronically exposed to air temperatures of 5°C (41°F) and observed that the total liver levels of retinol did not differ from rats maintained at 25°C (77°F). Rats kept at 5° or 25°C (41° or 77°F) were injected daily with one of six different levels of retinoic acid. Thirty-day survival was used as the marker for adequacy of retinoic acid. At 5°C (41°F), at least 100 µg retinoic acid daily was necessary for survival and growth, whereas only 5 µg retinoic acid was required daily for survival and growth at 25°C (77°F). From this, they concluded that cold-adapted rats required a 20-fold greater intake of vitamin A than did their room temperature-acclimatized counterparts. Lui and Roels (1980) reported that vitamin A deficiency did not affect energy production by the Krebs cycle, but glycogen synthesis from lactate and glycerol appeared to be slowed down. Thus, restoration of depleted energy stores may be impaired by a severe deficiency of this vitamin. Draper (1976) reported that Norwegian and Finnish Lapps (a race of formerly nomadic people residing in Northern Scandinavia) consume upwards of 50,000 to 62,000 IU of vitamin A (15,000 to 18,600 µg RE) per day,

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--> usually in the form of reindeer liver. Hasunen and Pekkarinen (1976) also reported that Finnish Skolt (a subpopulation of the Lapps) children consumed amounts of vitamin A well in excess (two- to fourfold) of the RDA. Rodahl and Issekutz (1965) reported that Alaskan Eskimos consumed approximately twice as much vitamin A as did U.S. Army or Air Force personnel living under similar conditions (3,750 versus 1,900 µg RE, respectively, or 3.6- and 1.9-fold higher than the RDA, respectively). Thus, indigenous persons living in the higher latitudes of the world continue to demonstrate comparatively high dietary intake of vitamin A. Compared to the 1989 RDA (NRC, 1989), it was reported that subjects in the 38-d Operation Everest II consumed increased amounts of vitamin A (Rose et al., 1987) as did subjects during an exercise at 3,500 to 4,050 m (11,475 to 13,279 ft) altitude in Bolivia (Edwards et al., 1991). However, Hannon et al. (1976) reported a transient reduction in the consumption of vitamin A by eight women during a sojourn to 4,300 m (14,098 ft) altitude. In addition to a decreased intake, it has been suggested that a malabsorption of fat may occur at high altitudes (Boyer and Blume, 1984; Ward et al., 1989b). This interpretation of the data has been criticized because two of the three subjects were reported to have been fat malabsorbers at sea level altitude prior to the experiment. Thus, it is not known whether impaired intestinal fat absorption occurs at high altitude, and if it does, whether it would be sufficient to affect absorption of the fat-soluble vitamins. Author's Recommendation Caution is recommended regarding the consumption of even modest supplements of vitamin A by persons working in cold or high-altitude environments due to the mobilization and utilization of body fat that often occurs under these circumstances and the resultant release of vitamin A stores into the circulation. It has been reported that 6 months to 2 years is required for a person consuming a vitamin A-deficient diet to become deficient. Thus, there is little justification to recommend intakes of vitamin A above RDA or MRDA levels because most expeditions or maneuvers are of a much shorter duration than the time required to become deficient. Deficiency of the vitamin is initially characterized by a reduction in night vision and dark adaptation, symptoms that are readily noticed by the person involved. Only under such instances of suspected vitamin A deficiency should a supplemental regimen of no more than the current RDA of 1,000 µg RE (NRC, 1989) be considered. Administration of supplemental vitamin A to a person with an adequate vitamin A status does not improve night vision further. Therefore, the suggested micronutrient intake goal for vitamin A is set at the RDA and MRDA level of 1,000 µg RE (Table 13-1). This recommendation is in contrast to the Thomas et al. (1993b)

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--> TABLE 13-1 Suggested Micronutrient Intake Goal for Use in the Cold and at High Altitudes Nutrient Micronutrient Intake Goal* RDA† MRDA‡ RCW as provided§ RCW as consumed || Fat-soluble vitamins           Vitamin A (µg RE) 1,000 1,000 1,000 8,022 6,016 Vitamin D (µg) 10 10 — — — Vitamin E (mg α-RE) 400 10 10 21 15.8 Vitamin K (µg) 70–80 70–80 — — — Water-soluble vitamins           Thiamin (mg) 3 1.5 1.6 5.7 4.3 Niacin (mg) 20 19 21 31 23.2 Riboflavin (mg) 3 1.7 1.9 2.6 1.9 Vitamin B6 (mg) 2 2 2.2 3.9 2.9 Vitamin B12 (mg) 3 2 3 0.8 0.6 Pantothenic acid# (mg) 10 4–7 — — — Biotin# (µg) 30–100 30–100 — — — Folic acid (µg) 400 200 400 141 106 Vitamin C (mg) 250 60 60 329 247 Minerals           Calcium (mg) 800–1,200 800–1,200 800–1,200 1,379 1,034 Phosphorus (mg) 800–1,200 800–1,200 800–1,200 2,168 1,626 Magnesium (mg) 400 350 350–400 529 444 Iron (mg) 15–20** 10 10–18 19 14 Zinc (mg) 20 15 15 10.8 8.1 Copper# (mg) 1.5–3 1.5–3 — — —

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--> NOTE: RE, retinol equivalents; α-TE, α-tocopherol equivalents. * As recommended by the author. † Recommended Dietary Allowance for males aged 19–24 and 25–50 (NRC, 1989). ‡ Military Recommended Dietary Allowance (AR 40-25, 1985). § Micronutrient content of the Ration, Cold Weather (RCW). || Average intake of nutrients contained in the RCW, assuming an average of 75 percent consumption of the contents (see Hoyt and Honig, Chapter 20 in this volume). # Estimated Safe and Adequate Daily Dietary Intake (NRC, 1989). ** 15 mg suggested for men, and 20 mg suggested for women.

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--> suggestion that "…the body's need for vitamin A…may increase at altitude" (p. 36). Whereas β-carotene does not share the toxicity that preformed vitamin A does, there is no RDA or MRDA for this provitamin. Should supplemental amounts of vitamin A be found to be beneficial as a result of future research in this area, thought should be given to supplementing with β-carotene rather than with vitamin A. Vitamin D Functions Vitamin D is important for the absorption and metabolism of calcium and phosphorus. Other roles for this vitamin have recently been elucidated (Suda et al., 1990), but they do not appear to be pertinent to the present discussion. Intake and Status Due to the synthesis of previtamin D3 from 7-dehydrocholesterol during exposure of the skin to ultraviolet irradiation, it is unlikely that a person operating in the cold or at high altitudes would develop a vitamin D deficiency, unless a prolonged expedition occurred during the long polar nights. Even though the sunlight is very oblique at the polar latitudes, the direct solar radiation on the face and any other exposed skin, along with the reflected radiation from the ice and snow, should be adequate to synthesize sufficient vitamin D3 to meet the nutritional needs of the explorer. High altitudes generally lie at more moderate latitudes with the result of more direct solar radiation. Thus more direct exposure, in addition to increased intensity of the radiation due to the decrease in absorption of ultraviolet rays by the thinner atmosphere, should result in sufficient exposure to ultraviolet irradiation for synthesis of adequate amounts of vitamin D to meet nutritional and functional needs. Author's Recommendation The exposure of the skin to ultraviolet radiation, coupled with a possible release of vitamin D stores from mobilization and utilization of fat stores during prolonged and strenuous physical exertion coupled with inadequate energy intake, should provide sufficient vitamin D to meet nutritional needs in both cold and high-altitude environments. In addition to sunlight, the inclusion of vitamin D-supplemented milk powder in the diet should provide

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--> sufficient vitamin D to meet nutritional needs. Thus, the suggested micronutrient intake goal for vitamin D is set at the RDA level of 10 µg (Table 13-1). Vitamin E Simon-Schnass (see Chapter 21 in this volume) reviews and discusses the literature on vitamin E; therefore, a short comment on another consideration of vitamin E supplementation is included here. Author's Recommendation Due to the reduction or prevention of oxidation of fatty acids by vitamin E, it is theoretically possible that supplementation with the vitamin could increase the oxygen supply for such purposes as energy production by the Krebs cycle (Williams, 1989). In addition to the beneficial effects of 400 µg/d vitamin E discussed by Simon-Schnass (see Chapter 21 in this volume), consideration should be given to the powerful epidemiological data recently reported by Stampfer et al. (1993) and Rimm et al. (1993), which demonstrated that consumption of 200 mg or more of vitamin E per day for periods of at least 2 years resulted in a significant reduction in the incidence of heart attacks. Due to the unknown etiology of high-altitude cerebral or pulmonary edema (HACE and HAPE, respectively), consideration should be given to instituting a regimen of elevated vitamin E intakes for substantial periods of time prior to exposure to high altitudes, as well as maintaining this level of intake during exposure to altitude. In view of the extremely low toxicity of vitamin E, this regimen may result in a reduction in either the incidence or severity of HACE and HAPE, both deadly consequences of working at high altitudes. Maintenance of membrane fluidity in a cold environment by supplemental vitamin E also supports such a supplemental regimen for this vitamin (see Simon-Schnass, Chapter 21 in this volume). For these reasons, the suggested micronutrient intake goal for vitamin E is set at 400 mg α-tocopherol equivalents (α-TE) (Table 13-1). Vitamin K Functions Vitamin K functions as a cofactor for several enzymes that modify and activate a number of blood clotting factors (NRC, 1989). Deficiency of the vitamin results in hemorrhages, whereas toxicity from intake of large doses is rare (NRC, 1989). Intestinal microbial synthesis also serves as a source of the vitamin (NRC, 1989).

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--> Status and Intake There are no known data on either changes in dietary intake or changes in status of this vitamin as a result of exposure to cold or high altitudes. Author's Recommendation A long period of time is required to produce deficiency in vitamin K. In addition, the vitamin is synthesized intestinally. Hence, it is suggested that the micronutrient intake goal for vitamin K be set at the RDA level of 70 to 80 µg, which is commensurate with what is found normally in foods. WATER-SOLUBLE VITAMINS All of the water-soluble vitamins, with the exception of folic acid and vitamin C, are intimately involved in the oxidation and conversion of food to energy at multiple steps leading up to and in the functioning of the Krebs cycle (see Hunt and Groff, 1990). Thus, an adequate nutritional status with respect to the water-soluble vitamins is essential for the production of sufficient energy for thermogenesis and for physical exertion while in the cold and at high altitudes. Thiamin Functions Thiamin is essential for energy metabolism; it serves as a cofactor in the oxidative decarboxylation of pyruvate to acetylcoenzyme A (acetyl-CoA) immediately prior to its entrance into the Krebs cycle, in the conversion of α-ketoglutarate to succinylcoezyme A (succinyl-CoA) in the Krebs cycle with subsequent oxidation to adenosine triphosphate (ATP), and in the hexose monophosphate shunt. If the thiamin supply is insufficient, the increased demand for acetyl-CoA during physical activity or for thermogenesis may not be met. As a result, more pyruvate will be converted to lactate with subsequent early onset of fatigue (Williams, 1989). Intake and Status Draper (1976) reported that inadequate intake of thiamin was one of the primary dietary deficiencies among Norwegian Lapps. Hasunen and Pekkarinen

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--> (1976) reported a similar observation among Finnish Skolt children. Hannon et al. (1976) reported a transient reduction of thiamin intake by women during their sojourn to 4,300 m (14,098 ft). However, there was an increased intake during Operation Everest II (Rose et al., 1987) and during the military exercise in Bolivia (Edwards et al., 1991). It has been suggested that 10 to 14 days are required for consumption of a thiamin-deficient diet to result in the appearance of deficiency symptoms. Author's Recommendation Due to thiamin's role in the metabolism of carbohydrate, the RDA for thiamin is based on total anticipated energy intake from carbohydrate and some amino acids (NRC, 1989). There have been many reports of increased intake of carbohydrate, at the expense of fat intake, at high altitudes (Frisancho, 1981; Ward et al., 1989b). Thus, the suggested micronutrient intake goal of this vitamin for those working either in the cold or at high altitudes is set at 3 mg, which is twice the existing RDA of 1.5 mg/d and higher than the MRDA of 1.6 mg/d. The suggested micronutrient intake goal of 3 mg/d is a prudent dose considering the low toxicity (NRC, 1989) of this vitamin. Niacin Functions Niacin as nicotinamide adenine dinucleotide (NAD) functions mainly to produce ATP from glycolysis and from the Krebs cycle. It is also necessary for the conversion of pyruvate to acetyl-CoA, in the hexose monophosphate shunt, and in the synthesis of fatty acids from acetyl-CoA. A deficiency of niacin could impair glycolysis and the Krebs cycle, whereas excessive niacin supplementation may suppress the release of free fatty acids from adipose tissue through decreased lipolysis, resulting in a decreased availability of a major fuel source for utilization during strenuous exercise (Bulow, 1981; Carlson et al., 1963). Thus, if muscle glycogen levels are low, as may occur during prolonged physical exertion in the cold or at altitude, excessive niacin supplements could actually impair physical performance (Williams, 1989). Intake and Status An increase in niacin intake was reported among participants in Operation Everest II (Rose et al., 1987) during their simulated exposure to high altitudes and by U.S. Army soldiers during the exercise in Bolivia (Edwards et al.,

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--> (Hannon et al., 1976). Pugh (1965) reported that two climbers who kept a cursory food journal while climbing from 6,400 to 7,600 m (20,984 to 24,918 ft) altitude on Mount Everest consumed 500 and 750 mg of calcium. Rupp et al. (1982) measured a variety of electrolyte changes in blood and urine of members of a Kanchenjunga expedition. They reported a slight, transient (8 percent) decrease in plasma calcium at higher altitudes, which returned to normal within a few days at altitude. During the 9-wk On Top Everest '89 nutrition research expedition, the 15 members (10 males, 5 females) maintained full dietary records every day, regardless of their camp location. While some of the records were inadvertently lost, there were a total of 842 dietary records collected (out of a possible 945, for an 89 percent completion rate) and analyzed for intake of energy and various nutrients. The calculated intake of calcium was 910 ± 579 mg/d, with no changes or trends occurring as a result of increasing altitudes. During this expedition, 24-h urine samples were collected at 28 separate times and analyzed for various compounds. Urinary excretion of calcium and of hydroxyproline, both markers for bone demineralization, were within normal ranges, which suggests that there was no loss of calcium. Also, single and dual photon absorptiometry performed immediately prior to departure from the United States, and at approximately 4 weeks following return to the United States, showed no indication of bone demineralization during exposures to these high altitudes. Author's Recommendation Because intakes of calcium have been reported to be near that suggested by the RDA and there have been no indications of an inadequate intake or adverse change in status of calcium during the 9-wk period at high and extreme altitude, there is no basis for recommending the micronutrient intake goal of calcium above the current RDA and MRDA levels of 800 to 1,200 mg/d (Table 13-1). Phosphorus Functions Phosphorus is required for bone formation and integrity, with approximately 85 percent of the total phosphorus in the adult body localized in the bones (NRC, 1989). Phosphorus is involved also in energy metabolism in its role as an enzyme modulator and its essentiality for high-energy bonds in ATP and creatine phosphate.

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--> Intake and Status Consumption of phosphorus generally parallels that of protein (NRC, 1989). Since elevated intakes of phosphorus increase calcium reabsorption (NRC, 1989) by the kidneys, this largely offsets the adverse effect of high protein intake on calcium reabsorption. Rodahl and Issekutz (1965) reported that Alaskan Eskimos consumed about 1.5- to 2-fold greater amounts of phosphorus than that suggested in the RDA (NRC, 1989). This high intake probably resulted from the relatively high protein intake of these people. Increased intakes were also reported during Operation Everest II (Rose et al., 1987) and during the military exercise in Bolivia (Edwards et al., 1991), but Hannon et al. (1976) reported a transient reduction in phosphorus consumption by women during a sojourn to 4,300 m (14,098 ft). During the 9-wk On Top Everest '89 expedition, there was a reported intake of 1,249 ± 720 mg of phosphorus per day, with no changes in intake occurring as a function of increasing altitudes. Urinary excretion during this expedition was within normal values. Jain et al. (1987) reported that phosphate supplementation of persons living at low altitudes resulted, by an unknown mechanism, in a more rapid acclimatization to moderate altitudes as evidenced by increased blood levels of 2,3-dephosphoglycerate, a rightward shift in the oxygen-hemoglobin dissociation curve, and improved performance on cognitive function tests. Author's Recommendation Because there has been no indication of abnormally low intake or excessive excretion of phosphorus in the cold or at high altitudes, there is no justification for recommending the micronutrient intake goal for phosphorus above the RDA and MRDA level of 800 to 1,200 mg/d (Table 13-1). Magnesium Functions Magnesium is essential for the production of ATP and for numerous other enzymatic reactions. As stated in the RDA (NRC, 1989), "As the complex Mg-ATP2-, magnesium is essential for all biosynthetic processes, glycolysis, formation of cyclic-AMP, energy-dependent membrane transport, and transmission of the genetic code" (p. 188).

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--> Intake and Status A decreased intake of magnesium was reported by soldiers working in Bolivia (Edwards et al., 1991). Rupp et al. (1982) reported no changes in plasma concentrations of magnesium when climbers were on Kanchenjunga, but daily urinary output of magnesium decreased approximately 50 percent with no comparable decrease in urinary volume. This output returned to normal following a return to lower altitudes and rest. In general, plasma magnesium concentrations are remarkably constant and are not indicative of magnesium status except under conditions of extreme magnesium deficiency (NRC, 1989). During the 9-wk On Top Everest '89 expedition, magnesium consumption averaged 236 ± 145 mg/d in the 842 daily diet records, with no significant changes or trends in intake related to increasing altitudes possible. Possible magnesium content of snow-melt used for drinking water was not taken into account in determining total intake. Author's Recommendation Due to the essentiality of magnesium in energy production, its low toxicity, and the observed low intake during the On Top Everest '89 expedition, the suggested micronutrient intake goal for magnesium is set at 400 mg/d, which is at the upper range set by the MRDA (Table 13-1). Iron Functions Iron is an essential constituent of hemoglobin and myoglobin as well as several enzymes. As much as 30 percent of the body iron is located in storage forms such as ferritin and hemosiderin (NRC, 1989). Intake and Status Numerous studies have documented an increase in erythrocytes (polycythemia) during exposure to altitude, wherein hematocrit values reach upwards of 0.65 to 0.70 (Ward et al., 1989a). At these higher hematocrits, the viscosity of blood becomes an issue of concern, especially in the peripheral tissues, which may have become vasoconstricted due to cold exposure. The causes of this polycythemia are beyond the scope of this chapter (for discussion, see Frisancho, 1981). However, due to the essentiality of iron in hemoglobin for

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--> the binding and transport of oxygen by erythrocytes, attention must be given to iron status, especially for those working at high altitudes. Increased consumption of iron occurred during Operation Everest II (Rose et al., 1987) and by soldiers working in Bolivia (Edwards et al., 1991). Worme et al. (1992) reported consumption of an average 30.8 ± 1.06 mg of iron by a group of men and women working at moderate altitudes (2,400 to 4,300 m or 7,875 to 14,110 ft), with a corresponding increase in serum transferrin during the 31-d expedition. There were no changes in transferrin saturation or serum ferritin, with none of the subjects having a serum ferritin concentration of less than 12 g/liter (although plasma volume was not measured). The lack of decline in serum ferritin indicates that there was an adequate intake of iron and of iron stores to prevent onset of iron deficiency brought on by the altitude-induced polycythemia. Consumption of iron averaged 15.8 ± 12.4 mg/d during the On Top Everest '89 expedition, with a gradual reduction in intake as altitudes increased. Intakes averaged 16.0, 17.1, 15.8, 7.5, and 11.3 mg/d at base camp and at camps 1, 2, 3, and 4, respectively. All members experienced polycythemia for the duration of the expedition. Reynafarje and Ramos (1961) reported an increase in intestinal iron absorption during the first several days following exposure to 4,500 m (14,754 ft), reaching a maximum after 1 week and gradually declining over the following month. Conversely, iron absorption decreased in natives from high altitudes when they were brought down to sea level, reaching a minimum at 3 weeks and increasing back to their normal high-altitude absorption by 16 months. Following this adaptation to different altitudes, intestinal absorption of iron was similar for natives living in their own localities at either low or high altitudes. Beard et al. (1990) reported that nonmalnourished women with diagnosed iron-deficiency anemia were more susceptible to exposure to cold water, resulting in a lower rectal temperature, lower plasma thyroxine and triiodothyronine, and a lower rate of oxygen consumption when compared to women who had an equivalent percentage body fat and were at the same stage of their menstrual cycle, but who were not anemic. Beard et al. (1988) also reported that iron-deficient individuals living in La Paz, Bolivia, at altitudes of 3,600 to 4,100 m (11,803 to 13,443 ft) had a lower maximal workload and a lower voluntary maximal oxygen uptake when compared to controls who had an adequate iron status. Author's Recommendation With the increasing number of women of child-bearing age being included in expeditions and military maneuvers, special consideration needs to be given to maintaining the adequacy of iron status. Due to the consistently observed

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--> increase in hematocrit as a function of exposure to altitude and to the importance of adequate iron in maintaining core body temperature, it is suggested that the micronutrient intake goal iron be set at 15 mg/d for men and at 20 mg/d for women (Table 13-1) (in comparison, the RCW contains 19 mg of iron). These amounts are low enough to prevent the onset of constipation, which is reported by some persons taking higher amounts of supplemental iron. Due to the high suggested dose of vitamin C, which enhances absorption of nonheme iron, the suggested micronutrient intake goal level of iron should be sufficient to prevent onset of an iron deficiency. Zinc and Copper Functions Both zinc and copper are required as enzyme cofactors, with substantial amounts of zinc stored in bone and muscle tissue (NRC, 1989). Trauma and stresses of various types have been shown to increase the rate of urinary zinc excretion (Moser et al., 1985). Intake and Status There was an increased consumption of zinc during Operation Everest II (Rose et al., 1987) but a decreased intake during the military exercise in Bolivia (Edwards et al., 1991). Rupp et al. (1982) reported a 2.3-fold increase in plasma zinc and a significant (12-fold) increase in urinary excretion of zinc during exposure to 8,453 m (27,715 ft) on Kanchenjunga. Both levels remained elevated for the duration of the expedition. It has been shown that muscle breakdown will produce elevated plasma and urine zinc concentrations (Moser et al., 1985). Rupp et al. (1982) were apparently not aware of this phenomenon, however, as they were unable to provide a reasonable explanation for the observed changes in plasma and urine zinc. In an expedition conducted at moderate altitudes (2,400 to 4,300 m [7,869 to 14,098 ft]), Deuster et al. (1992) reported dietary intakes of zinc and copper of 10.6 mg and 1.0 mg/d, respectively, prior to beginning the expedition. These intakes rose to 16.9 mg and 3.5 mg/d, respectively, during the 31-d expedition, and intakes were 15.5 mg and 1.9 mg/d, respectively, following return to sea level. There were no significant changes in concentrations of plasma or urine zinc, or plasma copper at any of these times. Consumption of zinc during the On Top Everest '89 expedition averaged 8.4 ± 8.7 mg/d, with no variation in intake related to increasing altitudes. Urinary excretion of zinc averaged between 0.5 and 0.9 mg/d, with the maximum excretion being 3.2 mg (Rose et al., 1987). The highest excretion

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--> generally occurred during periods of maximum physical exertion, which suggests that the abnormally elevated excretion was the result of muscle tissue breakdown. Neither intake nor status was determined for copper during this expedition. Author's Recommendation Because various stresses increase urinary excretion of zinc, the exertion of living and working in the cold or at high altitudes may be sufficient to warrant intake of zinc above the current RDA and MRDA of 15 mg/d. Thus, the suggested micronutrient intake goal for zinc is set at 20 mg/d (Table 13-1). Due to consistently low intakes and periodic high excretions of zinc during the On Top Everest '89 expedition, it is suggested that the micronutrient intake goal of zinc be provided in the foods actually consumed by soldiers or others working in the cold or at high altitudes. This increased amount of zinc is not high enough to adversely affect copper status (NRC, 1989). There is no indication that recommended intake of copper needs to be set at levels different than the current ESADDI of 1.5 to 3 mg/d. The high micronutrient intake goal for vitamin C (Table 13-1) is probably not high enough to adversely affect copper status. Other Trace Minerals Author's Recommendation There are no known data on the intake of trace minerals by those working in the cold or at high altitude. Until such data exist, there is no basis to recommend intakes of any of the trace minerals at other than RDA or ESADDI levels. AUTHOR'S CONCLUSIONS AND RECOMMENDATIONS Although it is generally agreed that a vitamin deficiency will impair physical performance, Williams (1989) stated that "in general, vitamin supplementation to an athlete on a well-balanced diet has not been shown to improve performance" (p. 163). While this may be true for athletes performing at moderate temperatures and at low altitudes, increased intake of several of the micronutrients may be useful to enhance various aspects of health related to survival for those working for prolonged periods in the cold or at high altitudes.

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--> Persons who live or who engage in prolonged physical exertion for extended periods of time in the cold or at high altitudes appear to have special nutritional needs. Thus, they are justified in consuming levels of specific vitamins and minerals in amounts greater than that set in either the RDA or the MRDA. The micronutrient intake goals (Table 13-1) are recommended to meet these special needs and should be safe for all the nutrients. Performance may not be enhanced by consumption of vitamins and minerals at the micronutrient intake goal level, but various critical bodily functions may be spared stress and chances of survival may be enhanced under these harsh environmental conditions. The predominantly Western foods now being consumed by expeditions in the cold and at high altitudes as well as the RCW that is consumed during military operations provide the majority of the essential nutrients in adequate amounts (according to the RDA and MRDA). Supplementation with additional vitamins E, C, and pantothenic acid, however, would ensure that these critical nutrients would be provided in amounts that cannot be obtained from the consumption of food alone. Including other vitamins and minerals at or near the RDA and MRDA levels should ensure adequate nutrient status for all metabolic functions. Amounts of each of the nutrients suggested in the micronutrient intake goal in Table 13-1 are recommended as a prudent and safe guide. The recommended amounts are subject to revision as more data are generated. As with most areas of scientific investigation, substantially more research is needed to establish the effects of cold and altitude on vitamin and mineral requirements. Due to the critical role of vitamins and minerals in energy production, a well-designed human study should be conducted in a location such as the South Pole or Fort Greely, Alaska. During such a study, intake, excretion, and numerous status indices for each of the vitamins and minerals can be measured accurately and correlated with energy intake, expenditure, changes in body composition, and environmental conditions. Duration of this study must be sufficient to allow for changes in nutrient status if any, to occur; that is, a range of 3 to 4 months. Whereas simulated high altitudes, such as those produced by the excellent studies conducted during Operation Everest II, can be used to investigate the effects of low oxygen pressure on nutrient intakes and changes in status, such hypobaric chamber experiments cannot duplicate the massive expenditures of energy, the prolonged exposure to cold, and the psychological concerns regarding danger and isolation that occur during actual expeditions to high altitudes. For this reason, hypobaric chamber experiments should be viewed as providing supplemental rather than primary information on the effects of high altitudes on vitamin and mineral requirements. There is still a need for well-designed experiments to be conducted at high altitudes to provide the data necessary to determine vitamin and mineral requirements. Even though such research expeditions are logistic nightmares compared to strict mountain

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--> climbing expeditions, research expeditions such as the American Medical Research Expedition on Everest (West, 1984) and On Top Everest '89 demonstrate unequivocally that they can be done. Good nutritional status is mandatory for those facing prolonged periods in the cold or at high altitudes. Survival in such environments depends, in large part, on adequate mental, physical, and nutritional preparation prior to and during exposures to extreme cold or high altitude. REFERENCES AR (Army Regulation) 40–25 1985 See U.S. Departments of the Army, the Navy, and the Air Force. Askew, E.W. 1989 Nutrition for a cold environment. Physician Sportsmed. 17:77–89. Beard, J.L., J.D. Haas, D. Tufts, H. Spielvogel, E. Vargas, and C. Rodriguez 1988 Iron deficiency anemia and steady-state work performance at high altitude. J. Appl. Physiol. 64:1878–1884. Beard, J.L., M.J. Borel, and J. Derr 1990 Impaired thermoregulation and thyroid function in iron-deficiency anemia. Am. J. Clin. Nutr. 52:813–819. Bonjour, J-P. 1991 Biotin, Pp. 393–427 in Handbook of Vitamins. L.J. Machlin, ed. New York: Marcel Dekker. Boutwell, J.H., J.H. Cilley, L.R. Kransno, A.C. Ivy, and C.J. Farmer 1950 Effect of repeated exposure of human subjects to hypoxia on glucose tolerance, excretion of ascorbic acid, and phenylalanine tolerance. J. Appl. Physiol. 2:388–392. Boyer, S.J., and F.D. Blume 1984 Weight loss and changes in body composition at high altitude. J. Appl. Physiol. (Respir. Environ. Exerc. Physiol.) 57:1580–1585. Brody, T. 1991 Folic Acid. Pp. 453–489 in Handbook of Vitamins. L.J. Machlin, ed. New York: Marcel Dekker. Bulow, J. 1981 Human adipose tissue blood flow during prolonged exercise. Effect of beta-adrenergic blockade, nicotinic acid and glucose infusion. Scand. J. Clin. Lab. Invest. 41:415–424. Carlson, L., R. Havel, and L. Ekelud 1963 Effect of nicotinic acid on the turnover rate and oxidation of the free fatty acids of plasma in man during exercise. Metabolism 12:837–845. Cooperman, J.M., and R. Lopez 1991 Riboflavin. Pp. 283–310 in Handbook of Vitamins. L.J. Machlin, ed. New York: Marcel Dekker. Czeizel, A.E. 1993 Prevention of congenital abnormalities by periconceptional multivitamin supplementation. Brit. Med. J. 306:1645–1648. Deuster, P.A., K.L. Gallagher, A. Singh, and R.D. Reynolds 1992 Consumption of a dehydrated ration for 31 days at moderate altitudes: Status of zinc, copper, and vitamin B-6. J. Am. Diet. Assoc. 92:1372–1375. Draper, H.H. 1976 A review of recent nutritional research in the arctic. Pp. 120–129 in Circumpolar Health, R.J. Shephard and S. Itoh, eds. Toronto: University of Toronto Press.

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