There's a science to your weakness. At higher elevations, air pressure decreases and thus the amount of oxygen consumed in each breath declines. As a result, oxygen levels in your blood drop, and your aerobic capacity dips, says Barry Braun, Ph.D., an exercise biochemist at the University of Massachusetts. And that's bad news, because aerobic capacity is nothing more than your ability to use oxygen.
In the short term, your body compensates for these changes by boosting your respiratory rate so you can push more air through your lungs and by raising your heart rate to pump more oxygen to your muscles. Over the long term, your body acclimatizes by producing additional oxygen-carrying red blood cells and increasing the levels of 2,3-diphosphoglycerate, a molecule that facilitates oxygen transfer in your cells, says Neal Henderson.
Your metabolic needs shift slightly, too. "Especially initially, you burn more carbs at altitude," says Henderson, though this may be less true for women. Altitude tends to suppress appetite, which can lead to undereating and decreased energy levels.