Preparing for higher altitudes starts weeks before your trip and continues during your ascent. The single most important factor is how quickly you climb: the Wilderness Medical Society recommends gaining no more than 500 meters (about 1,650 feet) in sleeping elevation per day once you’re above 3,000 meters (9,800 feet). Beyond pacing your ascent, preparation involves hydration, nutrition, fitness, and understanding what your body will go through as oxygen becomes scarcer.
What Happens to Your Body at Altitude
At higher elevations, the air contains less oxygen per breath. Your body detects this drop through specialized sensors on the carotid arteries in your neck, which send signals to your brain to increase breathing rate and depth. This response, called the hypoxic ventilatory response, kicks in almost immediately and continues to strengthen over days to weeks as those sensors become more sensitive to low oxygen levels.
Within the first hours and days, your blood plasma volume decreases, which concentrates your existing red blood cells and temporarily improves oxygen delivery. Over the following days and weeks, a hormone called erythropoietin ramps up red blood cell production, further increasing your blood’s oxygen-carrying capacity. There’s a trade-off, though: more red blood cells make your blood thicker, which forces your heart to work harder to pump it. This is one reason you feel exhausted during the first few days at elevation.
Interestingly, populations that have lived at high altitude for generations, like Tibetans living above 4,000 meters, don’t rely on extra red blood cells at all. Their hemoglobin levels fall within normal sea-level ranges. Their bodies have found other ways to cope, a reminder that acclimatization is a complex, whole-body process rather than a single switch that flips.
Control Your Ascent Rate
Your ascent schedule matters more than any pill, supplement, or training plan. The key metric is sleeping altitude, not the highest point you reach during the day. You can hike high and sleep low, which is a classic acclimatization strategy. The specific guidelines from the Wilderness Medical Society:
- Below 3,000 meters (9,800 feet): Avoid jumping straight to a sleeping altitude above 2,750 meters (9,000 feet) in a single day from low elevation.
- Above 3,000 meters: Increase your sleeping altitude by no more than 500 meters (1,650 feet) per night.
- Rest days: Plan an extra acclimatization day for every 1,000 meters (3,300 feet) of sleeping altitude gained.
If you’re flying into a high-altitude city like Cusco (3,400 meters) or La Paz (3,640 meters), you’re skipping the gradual ascent entirely. In that case, plan at least one to two full rest days before any strenuous activity. Some travelers spend a night or two at an intermediate altitude first.
Recognize Altitude Sickness Early
Acute mountain sickness (AMS) is diagnosed using the Lake Louise scoring system, which rates four symptoms on a scale of 0 to 3: headache, gastrointestinal problems (nausea, vomiting, poor appetite), fatigue or weakness, and dizziness. A total score of 3 or more, with at least some headache present, qualifies as AMS. Scores of 3 to 5 indicate mild illness, 6 to 9 moderate, and 10 to 12 severe.
Most people with mild AMS can manage symptoms and continue acclimatizing at the same altitude. Moderate to severe symptoms call for descent. Two dangerous complications can develop from untreated AMS. High-altitude cerebral edema (HACE), which involves brain swelling, typically appears 24 to 72 hours after gaining altitude and shows up as confusion, loss of coordination, or altered mental status. High-altitude pulmonary edema (HAPE) involves fluid in the lungs and causes breathlessness even at rest. Both are medical emergencies that require immediate descent.
Stay Hydrated, but Know the Real Numbers
You lose more water at altitude than you realize. Low humidity and the faster, deeper breathing your body adopts both increase the amount of moisture you exhale. Combined with reduced thirst cues that many people experience at elevation, this can produce a fluid deficit of 2 to 3 liters during the first few days at high altitude. That’s on top of whatever you’d normally need.
There’s no single magic number for daily intake because it depends on your exertion level, the temperature, and how dry the air is. A practical approach: drink enough to keep your urine pale yellow, and start increasing your water intake a day or two before you arrive at altitude. Carry water with you and sip consistently rather than chugging large amounts at once.
Prioritize Carbohydrates
Carbohydrates produce more energy per unit of oxygen consumed than fats or protein. At altitude, where every breath delivers less oxygen, this efficiency advantage becomes meaningful. The harder you exercise, the more your body relies on carbohydrates for fuel. At maximum exertion, carbohydrates provide essentially 100% of your energy.
You don’t need to follow a rigid macronutrient plan. The practical takeaway is to favor carbohydrate-rich foods like rice, pasta, bread, potatoes, oats, and fruit, especially during the first several days of acclimatization and on days with heavy physical activity. Many people lose their appetite at altitude, so eating smaller, more frequent meals that are carb-heavy tends to work better than forcing large protein-rich meals.
Avoid Alcohol in the First Days
Alcohol and altitude are a bad combination, particularly during sleep. Research simulating cabin pressure at cruising altitude (roughly equivalent to 1,800 to 2,400 meters) found that alcohol dropped blood oxygen saturation to an average of just over 85% during sleep, well below the 90% threshold considered clinically normal. Without alcohol at the same altitude, oxygen levels stayed around 88%. Heart rate also spiked to nearly 88 beats per minute with alcohol, compared to about 73 without it.
Alcohol also disrupted sleep architecture. The deepest stage of sleep was cut nearly in half, dropping from about 67 minutes to 46 minutes. REM sleep was shortened too. Since sleep quality at altitude is already compromised by periodic breathing patterns and frequent waking, adding alcohol to the mix leaves you poorly rested and less able to acclimatize. Avoid alcohol for at least the first 48 hours at a new altitude, and keep it minimal after that.
Build Fitness Before You Go
Cardiovascular fitness won’t prevent altitude sickness, but it gives you a larger reserve to draw from when your effective oxygen intake drops. If you’re planning a trek or climb, start aerobic training at least six to eight weeks beforehand. Focus on sustained efforts like hiking, running, cycling, or stair climbing. If you have access to hilly terrain, use it. Training at moderate intensity for 30 to 60 minutes most days builds the aerobic base you’ll rely on.
Strength training for your legs and core also pays off, since most high-altitude activities involve carrying a pack uphill for hours. The fitter you are at sea level, the more comfortable you’ll feel when your aerobic capacity drops by 20% to 30% at altitude.
Pre-Trip Iron Stores
Your body needs iron to manufacture new red blood cells, and it will be making a lot of them during acclimatization. If your iron stores are low before you arrive, your body may struggle to ramp up red blood cell production effectively. Getting a blood test to check your ferritin level (a marker of stored iron) a few weeks before your trip gives you time to address any deficiency. This is especially relevant for women, vegetarians, and frequent blood donors, who are more likely to have low iron stores. Eating iron-rich foods like red meat, lentils, spinach, and fortified cereals in the weeks before your trip helps build those reserves.
Medication for Prevention
A prescription medication called acetazolamide is the most commonly used drug for preventing altitude sickness. It works by slightly acidifying your blood, which tricks your brain into increasing your breathing rate, effectively speeding up the acclimatization process. It’s typically started one to two days before reaching high altitude and continued for the first couple of days at your target elevation. Common side effects include tingling in the fingers and toes, increased urination, and an altered taste of carbonated drinks.
Acetazolamide is worth discussing with a healthcare provider if you’re ascending rapidly with no opportunity for gradual acclimatization, if you’ve had altitude sickness before, or if you’re heading above 3,000 meters. It’s a tool to use alongside a sensible ascent plan, not a substitute for one.
A Pre-Trip Checklist
- 6 to 8 weeks out: Begin or increase aerobic training. Check iron levels if you’re in a higher-risk group for deficiency.
- 2 to 4 weeks out: Plan your itinerary around recommended ascent rates. Build in rest days. Identify intermediate-altitude stops if flying directly to a high destination.
- 1 to 2 days before arrival: Start acetazolamide if prescribed. Increase water intake. Pack carbohydrate-dense snacks.
- First 48 hours at altitude: Keep activity light. Drink consistently. Eat carb-rich meals. Skip alcohol. Monitor yourself for headache, nausea, dizziness, or unusual fatigue.