Nutrient requirements at high altitude.

Many people seek recreation and adventure at high altitudes. Skiing and snowboarding are sports that require at least short-term exposure to altitudes above 2500 m; trekking and mountain climbing, although still for the particularly stalwart, are drawing more and more adventurers every year to base camps as high as 5000 m or 6000 m. Ascent to high altitude results in acute and long-term physiologic changes that have the potential to profoundly affect nutrient requirements for the traveler and adventurer. Immediately upon arrival at high altitude, because of the decrease in inspired oxygen (O2) pressure, alveolar and arterial O2 pressures decrease, and maximal O2 consumption (VO2MAX) decreases. 10 , 12 , 33 , 70 , 86 Submaximal work performance is also dramatically reduced. 2 , 70 , 86 Some individuals experience symptoms, including headache, nausea, vomiting, insomnia, lassitude, and malaise 40 known as acute mountain sickness (AMS), a condition that also often results in severe anorexia. Over the first few days of high-altitude exposure, AMS subsides, and a cluster of physiologic adaptations occur that restore, at least partially, O2 homeostasis. The low arterial O2 pressure stimulates chemoreceptors in the heart and respiratory centers of the brain to increase heart and breathing rates. This adaptation increases the acquisition and delivery of O2 to tissues 24 , 59 and is, in part, responsible for an increase in basal energy expenditure at high altitude. The increase in respiratory rate decreases arterial carbon dioxide content, creating an alkalosis and limiting respiratory stimulation. 24 The kidney responds to the alkalosis by increasing the excretion of bicarbonate, with an associated diuresis and decrease in plasma volume. The resultant hemoconcentration helps to normalize arterial O2 content. 36 , 37 , 51 , 86 With additional time at high altitude, the O2-carrying capacity of the blood is further expanded by the secretion of erythropoeitin (EPO), 37 , 38 , 65 which stimulates red blood cell production in, and release of reticulocytes from, bone marrow and thereby potentially increases red cell mass and O2-carrying capacity. The ability to perform maximal exercise (VO2MAX) remains depressed in comparison to at sea level, 10 , 12 , 24 , 36 , 37 , 59 , 86 but performance of submaximal work improves 52 , 86 relative to immediate high-altitude exposure. With prolonged exposure to high altitude, anorexia may or may not subside, and basal energy requirements may or may not remain elevated. The most consistent finding in studies of prolonged exposure of visitors to high altitude is weight loss. * The primary cause of this weight loss is thought to be the energy imbalance created by the opposing phenomena of inadequate energy intake and elevated energy expenditure. 14 , 15 , 47 This series of physiologic responses to high altitude suggests an increase in the need for some nutrients at high altitudes compared with that at sea level. This article discusses the evidence for these increased requirements and makes recommendations regarding appropriate intakes at high altitude. Nutrients to be discussed include energy and the food components that supply it (i.e., protein, carbohydrate, and fat), water, vitamins (especially vitamin E), and minerals (especially iron).