Nutrition at High Altitude
by Phil 'Darby' Allen
High altitude (HA) is defined as above 3050m or 10,000 feet.
Abrupt exposure to elevations greater than this is frequently associated with symptoms of altitude illness (AI). AI is a combination of symptoms which include headaches, anorexia, nausea, vomiting, and lethargy. The combined effect of some or all of these symptoms, of which hypoxia is a major cause, is usually a profound loss of appetite and a subsequent reduction of food intake.
The human body requires 10% more energy just to exist at altitude and food intakes are typically reduced 10 to 50% during acute altitude exposure, depending upon the individual and rapidity of ascent. The average-sized male climber can expect to burn upwards of 500-800 calories per hour at higher altitudes. Add to this the increased exertion of operating and climbing at HA and a loss of appetite just at the time when the climber needs energy the most, makes a dangerous combination.
The effects on the body of inappropriate thirst due to increased water loss in urination and breath, is due to air at altitude being cold and dry. Each inhaled breath needs the body to warm the air before reaching the lungs and each expired air contains water and heat which is lost to the environment. The rate of respiratory water loss at altitude is about twice the rate for an equivalent activity at sea level. Combined with excessive sweat and energy expenditure, this can lead to rapid dehydration and energy depletion, resulting in extreme weight loss if adequate food and fluid are neglected.
So how can we combat AI? Gradual acclimatization to progressively higher altitude exposure is the best preventive medicine. Climbers that anticipate the consequences of appetite loss may at least minimize the secondary consequences of the effects of altitude: reduced energy intake, depleted muscle energy stores, negative nitrogen balances and loss of critical lean body mass.
One of the most effective and practical performance-sustaining measures that can be adopted upon arrival at altitude, is to consume a minimum of 3 to 4 litres of fluid per day, containing 200 to 300g of carbohydrate in addition to that contained in the diet. This should prevent dehydration, improve energy balance and the oxygen delivery capability of the circulatory system, replenish muscle energy and conserve body protein levels.
Caffeine has been reported to enhance short-term, high-intensity work at simulated HA and high carbohydrate diets have been recommended as a method to reduce the symptoms associated with acute AI. In addition to reducing fatigue and preventing low blood sugar levels it has been shown that a diet high in carbohydrate resulted in fewer symptoms. Studies have shown that a high carbohydrate intake can reduce the effects of altitude by 300m - 600m at a height of 4000m and 5200m respectively. This is most likely due to the fact that carbohydrate requires 8-10% less oxygen for use, compared to fat and protein.
A high carbohydrate diet should be fed prior to and during the initial 3 to 4-day critical period of acute altitude exposure.
Fat, while tolerated relatively well in the cold at sea level, may not be so in diets at HA. The symptoms of acute altitude exposure may be increased if fat displaces carbohydrate from the diet. Although high-fat foods are high in energy and reduce the weight/calorie aspect of food carried on climbs, fat requires more oxygen for metabolism than carbohydrate and will place a small, but added, burden upon the already overtaxed oxygen economy of the climber. Fat absorption may also be reduced at extremely high elevations.
Adequate food intake can be achieved at altitude, but it requires a concerted, conscious effort of dietary management and forced eating, which can be unpleasant.
The key to success at altitude is to hydrate regularly above all else and to eat plenty of food, as much as your body will tolerate, with the bulk of calories in the form of carbohydrates.
