Preparing for high altitude hiking starts weeks before you hit the trail, not at the trailhead. Above 8,000 feet, the air holds significantly less oxygen, and your body needs time, fuel, and the right strategy to perform well and stay safe. The difference between a miserable slog and a strong summit day often comes down to what you did in the weeks and days leading up to your hike.
What Altitude Does to Your Body
The moment you arrive at elevation, your body starts compensating for the thinner air. The single most important response is breathing faster and deeper. This hyperventilation kicks in almost immediately and is your body’s primary tool for getting enough oxygen. Over days, your blood gradually produces more red blood cells to carry oxygen more efficiently, but this process is slow. If you’re spending two or three weeks at altitude, the red blood cell increase barely matters. What matters in the short term is how well your lungs and cardiovascular system adapt.
Your body also loses water faster than normal at altitude. Low humidity and all that heavy breathing increase water loss through your lungs. On top of that, your kidneys produce more urine during the first three to four days at elevation, a natural part of acclimatization. Meanwhile, your thirst signals actually decrease, so you feel less like drinking right when you need fluids most. This combination can create a fluid deficit of two to three liters in just the first few days.
Your metabolism speeds up too. Resting metabolic rate can jump 16 to 27 percent in the first couple of days above 14,000 feet. You’re burning more calories just sitting in your tent, and even more once you start hiking.
Build Fitness With Purpose
Altitude punishes poor cardiovascular fitness. Start training at least six to eight weeks before your trip, focusing on sustained aerobic effort. Long hikes with a loaded pack, stair climbing, and uphill treadmill sessions all build the kind of endurance you’ll need. Your heart and lungs will have to work harder at elevation no matter how fit you are, but a strong aerobic base gives you a wider margin before you hit your limit.
Strength training matters too, particularly for your legs, core, and the muscles that stabilize your ankles and knees on uneven terrain. If your hike involves significant elevation gain, practice hiking uphill with the pack weight you’ll actually carry. Many people train on flat ground and are blindsided by how much harder sustained climbing feels with a 30-pound pack at 12,000 feet.
Check Your Iron Stores
Your body needs iron to build the extra red blood cells altitude demands. If your iron stores are low before you go, your body simply can’t ramp up oxygen-carrying capacity the way it needs to. Research on athletes training at altitude recommends a ferritin level above 50 ng/mL before heading to elevation. Iron deficiency without anemia, where ferritin drops below 30 ng/mL but your hemoglobin looks normal on a standard blood test, is common and often missed, particularly in women and endurance athletes.
If your trip involves multiple days above 10,000 feet, consider asking your doctor for a ferritin test a couple of months before departure. That gives you time to correct a deficiency through diet or supplementation if needed. Iron-rich foods like red meat, lentils, and spinach help, but if your levels are genuinely low, food alone may not raise them fast enough.
Ascend Slowly and Sleep Low
The CDC recommends that once you’re above 9,000 feet, you increase your sleeping elevation by no more than 1,600 feet per day. This is the single most effective way to prevent altitude sickness. “Climb high, sleep low” is the classic mountaineering principle: you can hike to a higher point during the day, but come back down to a lower camp to sleep. Every extra night your body gets at a given elevation makes the next day’s gain easier to handle.
If you’re flying directly from sea level to a high-altitude city like Cusco (11,000 feet) or La Paz (12,000 feet), plan at least one or two rest days before doing anything strenuous. Your body needs time to ramp up that breathing response and begin adjusting. Jumping straight into a demanding hike on day one dramatically increases your risk of getting sick.
Recognize Altitude Sickness Early
Acute mountain sickness (AMS) is diagnosed based on four symptoms: headache, nausea, fatigue, and dizziness. Headache is the hallmark. The standard clinical scoring system requires at least a headache plus enough total symptom severity to cross a diagnostic threshold. A severe headache alone, even without other symptoms, qualifies as AMS.
Mild AMS feels like a hangover: dull headache, low appetite, tiredness, maybe some lightheadedness. Most people experience at least some of this above 10,000 feet. It typically resolves in one to two days if you stop ascending and let your body catch up. Moderate to severe AMS brings vomiting, crushing headaches, and fatigue so intense that you can’t continue. At that point, the treatment is descent. Going down even 1,000 to 2,000 feet often brings rapid relief.
One useful signal: people who develop AMS tend to gain weight from fluid retention, while those acclimatizing well tend to lose a small amount of weight. If your rings feel tight and your face looks puffy, pay attention.
Medication for Prevention
Acetazolamide (commonly known as Diamox) is the most widely used preventive medication for altitude sickness. It works by making your blood slightly more acidic, which tricks your body into breathing faster and deeper, essentially accelerating the natural acclimatization process. The standard preventive dose is 125 mg twice a day, starting the day before you ascend and continuing through the first two days at your target altitude, or longer if you’re still gaining elevation.
Common side effects include tingling in your fingers and toes and a flat taste to carbonated drinks. These are annoying but harmless. Some people also notice increased urination, which means you’ll need to be even more diligent about hydration. Talk to your doctor well before your trip if you want a prescription, and do a trial run at home to make sure you tolerate it. Acetazolamide is a sulfonamide, so it’s not an option if you have a sulfa allergy.
Hydration and Nutrition Strategy
Plan to drink more water than feels natural. Because your thirst response decreases at altitude even as your fluid losses increase, relying on thirst alone will leave you dehydrated. A practical approach is to set a schedule: drink a fixed amount every hour rather than waiting until you feel thirsty. Carrying a water bottle with volume markings helps you track intake throughout the day.
For food, prioritize carbohydrates. A carb-rich diet is easier to digest at altitude and provides the most efficient fuel when oxygen is limited. Your body extracts more energy per unit of oxygen from carbohydrates than from fat or protein. During the first few days at elevation, keep meals light and salty-food intake low. Appetite suppression is common, so bring foods you genuinely enjoy eating. Forcing down bland energy bars when you’re nauseous is harder than snacking on things you actually crave. Given the metabolic increase, you’ll likely need several hundred extra calories per day just to break even.
Sun and Weather Protection
UV radiation increases roughly 18 percent per 1,000 meters of elevation gain for the wavelengths that cause sunburn. At 4,000 meters (about 13,000 feet), you’re getting roughly 70 percent more burn-causing UV than at sea level. Snow and exposed rock reflect even more radiation back at you. Sunburn at altitude happens fast and can be severe.
Wear SPF 50 or higher sunscreen and reapply it frequently, especially on your nose, ears, lips, and the underside of your chin (where reflected UV hits). Sunglasses with UV protection are essential, not optional. Snow blindness, a painful sunburn of the cornea, can happen in just a few hours of exposure on snow or glaciers. A wide-brimmed hat or cap with a neck flap rounds out your protection. Lip balm with SPF is easy to forget and one of the first things you’ll wish you had.
Managing Sleep at Elevation
Sleep quality drops at altitude, sometimes dramatically. Periodic breathing is a common and unsettling experience: your breathing pauses for several seconds while you sleep, then resumes with a gasp. This is a normal neurological response to low oxygen, not a sign of something dangerous, but it fragments your sleep and leaves you feeling unrested.
Sleep at the lowest elevation you can. If your campsite options vary by a few hundred feet, choose the lower one. Elevating your head slightly can help. Avoid alcohol in the evening, as it worsens both periodic breathing and dehydration. Acetazolamide, if you’re already taking it for AMS prevention, also reduces periodic breathing and can improve sleep quality. Expect your first two nights at a new altitude to be the worst, with gradual improvement after that.
Gear Considerations for Altitude
Beyond the standard hiking gear list, altitude-specific preparation includes a few extras. Layering is critical because temperatures drop roughly 3.5°F per 1,000 feet of elevation gain, and conditions can shift from warm sun to freezing wind in minutes. A windproof outer layer, insulating mid-layer, and moisture-wicking base layer give you flexibility.
Trekking poles reduce stress on your knees during long descents and help with balance when fatigue sets in. A pulse oximeter, a small clip-on device that reads your blood oxygen level, costs under $30 and gives you objective data on how well you’re acclimatizing. Normal readings at sea level are 95 to 100 percent. At 12,000 feet, readings in the low to mid-80s are typical and not cause for alarm, but a sudden drop or readings below 75 percent suggest you need to descend. Pack a basic first aid kit that includes pain relievers for headaches, anti-nausea medication, and blister care.