Quite simply, the body freaks out when first exposed to high places: Your brain tells you to breathe more and deeper. Your heart rate increases. But within 24 to 48 hours the level of natural EPO—the hormone that regulates the volume and number of red blood cells—starts to rise, and in time this leads to more red blood cells and acclimatization. Andrew Luks, a pulmonary specialist at the University of Washington with expertise in high-altitude medicine, elaborates.
Expect Some Discomfort You might experience a combination of headache, poor sleep, fatigue, and dizziness. That’s acute mountain sickness, the most common malady to strike people who go from sea level to altitude quickly, Luks says. (This usually subsides over time, but if doesn’t, you need to descend.) The good news? It probably won’t get worse. Some who are rapidly exposed to high altitudes suffer from high-altitude pulmonary edema (HAPE), a buildup of fluid in the lungs. But during 30 years of research, the U.S. Army Pikes Peak Research Laboratory has had to evacuate only about 1 percent of its volunteers with suspected HAPE.
The Longer You Can Acclimatize, The Better In a 2007 study, elite cyclists were brought from sea level to 7,700 feet and tested weekly on the bike. The first day, their time to exhaustion decreased by 27 percent compared with sea level. On Day 7, it had dropped by 21 percent. By Day 21, it was 16. But, says Luks, adaptation can only help so much: “Above a certain altitude”—sometimes as low as 5,000 feet—”you won’t be able to gain back your sea-level performance, no matter how long you’ve acclimated.”
You Can’t Fool Your Body Some endurance athletes believe that if you can’t arrive a week in advance, the next best thing is to land the day before. Not true, says Charles Fulco, of the Army Research Institute of Environmental Medicine. The reason lies in basic human physiology. The body compensates for the drop in barometic pressure by working harder to increase blood volume (by having the heart pump more blood, faster). But the body can compensate only so much before reaching VO2 max. So the same task (say, riding at 10 mph) will be performed at a higher percentage of VO2 max at altitude—meaning it will feel hardest the first day and gradually less hard the more you adjust.
New Sensation Luks believes part of the value of arriving well in advance of your event is that you can gain familiarity with the discomfort of riding at altitude. “It will be safer,” Luks says, “and it will help you adjust to the sensation.” Again: If you can spend a week of vacation time at altitude right before your ride, you’re almost certain to see a better result.
By Evelyn Spence from Bicycling.com