Yes—you're more likely to get dehydrated when you exercise at altitude than you are at sea level. There are three factors that combine to up your risk of dehydration:
- Increased breathing rate
- Increased production of urine
- Loss of appetite (that means thirst, too)
To summarize, even though your body loses fluids faster at altitude, you're less likely to crave the water you need to perform well. Your risk is even higher in cold temperatures, which can further reduce your impulse to drink. Your body will attempt to compensate by pumping out more aldosterone—a hormone that helps you retain water—but it can't reverse the effect completely. That's why it's so important to sip water frequently when you're exercising at high altitude—especially since you can't always rely on thirst to be your guide.