Summer is beginning, bringing the desire to escape the heat and head to the mountains for hiking, trail running or cycling. Many athletes also complete altitude camps during this period, both to avoid hot cities and to benefit from physiological adaptations such as increased red-blood-cell production. One essential factor must not be overlooked during altitude stays: nutrition.
What happens to the body at altitude?
Atmospheric pressure falls at altitude; contrary to a common misconception, oxygen concentration remains constant. The amount of oxygen we can inhale therefore decreases, reducing maximal oxygen uptake (VO2max) and overall performance. One study in endurance athletes found that VO2max fell by an average of 6.3% for every 1000 meters above sea level (1). Athletes visiting the Sierra Nevada High Performance Center at 2300 meters, for example, would experience a decline in VO2max of around 12%. The athletes suffered an even greater performance loss—14.5% per 1000 meters—in a VO2max test to exhaustion.
The importance of carbohydrates during exercise at altitude
As discussed previously, carbohydrates are the primary energy source during exercise, especially at high intensities (>60-80% VO2max). Carbohydrate use increases during exercise at altitude, traditionally because the body requires less oxygen to generate energy from carbohydrates than from substrates such as fat. A study in PNAS (2) confirmed that carbohydrate metabolism increases during exercise at altitude. Its most interesting finding, however, was that the same amount of carbohydrate is metabolized at the same percentage of VO2max, whether at sea level or altitude. Exercising at 60% of VO2max therefore uses the same amount of carbohydrate at altitude and sea level, but that physiological intensity corresponds to a lower external workload—a lower speed or power—at altitude. We consequently tend to require more carbohydrates at altitude because we usually exercise at a higher relative intensity than at sea level.
If carbohydrate supplementation benefits endurance exercise generally, its value may be even more apparent at altitude. One study conducted a series of tests at 5192 meters, including a form of time trial, in 41 mountaineers assigned to carbohydrate or placebo supplements. Participants who consumed carbohydrates perceived effort as 18% lower and completed the test in 17% less time than those receiving placebo (3). Similarly, another study in trained participants (4) conducted cycling time trials at sea level and after 3 and 10 days at 4300 meters. Half consumed carbohydrates during the test and half received placebo. At altitude, oxygen saturation was similar in both groups and every participant performed worse than at sea level. However, the decline was smaller in the carbohydrate group, with a difference of almost 20% from placebo. Importantly, these participants were in an energy deficit, which is common at altitude because both resting energy expenditure and the energy cost of a given exercise increase. A later study in habitual altitude residents without an energy deficit found no benefit from carbohydrate supplementation (5).
Conclusions
Carbohydrates are the primary energy source during moderate- or high-intensity exercise, and altitude increases the relative intensity of a given workload. For the same effort, such as running at 5 min/km, carbohydrate use will be much higher. Supplementation may therefore help prevent glycogen depletion and improve performance, or attenuate its decline. These findings are particularly relevant for people with little altitude acclimatization—for example, those who have spent only 1-2 weeks at altitude—and especially for anyone in an energy deficit, a common occurrence because altitude increases the energy cost of every activity, including basal metabolism.
AUTHOR
Pedro Valenzuela
Researcher in the Physiology Unit at the University of Alcalá and the Performance Monitoring Unit at the Sports Medicine Center (AEPSAD, Madrid High Performance Center).
Website: www.fissac.com
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References
- Wehrlin JP, Hallén J. Linear decrease in VO2max and performance with increasing altitude in endurance athletes. Eur J Appl Physiol. 2006;96(4):404–12.
- Mcclelland GB, Hochachka PW, Weber JM. Carbohydrate utilization during exercise after high-altitude acclimation: A new perspective. Proc Natl Acad Sci U S A. 1998;95(17):10288–93.
- Oliver SJ, Golja P, MacDonald JH. Carbohydrate supplementation and exercise performance at high altitude: A randomized controlled trial. High Alt Med Biol. 2012;13(1):22–31.
- Fulco C, Kambis KW, Friedlander AL, Rock PB, Muza SR, Cymerman A. Carbohydrate supplementation improves time-trial cycle performance during energy deficit at 4,300-m altitude. J Appl Physiol. 2005;99(3):867–76.
- Fulco CS, Zupan M, Muza SR, Rock PB, Kambis K, Payn T, et al. Carbohydrate supplementation and endurance performance of moderate altitude residents at 4300 m. Int J Sports Med. 2007;28(5):437–43.
