Altitude does affect metabolism, and the changes are more dramatic than most people expect. When you ascend above roughly 2,500 meters (about 8,200 feet), your body enters a state of low oxygen availability that triggers a cascade of metabolic shifts. Your resting metabolic rate can spike by 10% to 27% in the first few days, your appetite drops, your body changes which fuel sources it prefers, and your blood sugar regulation shifts in ways that persist as long as you stay at elevation.
How Much Your Metabolic Rate Increases
The initial jump in resting metabolic rate at high altitude is surprisingly large. In one well-documented study, men brought from 1,600 meters to 4,300 meters (about 14,100 feet) saw their resting metabolic rate climb 27% by day two. That spike gradually tapered to about 17% above baseline by day ten. Other research at similar elevations has measured increases ranging from 5% to 16% on the first day, depending on how high and how fast people ascended.
This metabolic surge doesn’t last forever. Your body begins to adjust within about a week. One study found that metabolic rate climbed significantly after seven days at 3,353 meters (about 11,000 feet) but returned to sea-level values by day 14. The general pattern is consistent: a sharp spike in the first 48 hours, followed by a gradual decline toward normal over one to two weeks. Full physiological acclimatization typically takes 7 to 10 days, though early adaptations can begin within the first 72 hours.
What Drives the Metabolic Surge
Two major systems power the initial metabolic increase. First, your sympathetic nervous system ramps up, releasing stress hormones like norepinephrine. This has been confirmed across dozens of studies and is likely responsible for the elevated heart rate, increased blood pressure, and higher energy expenditure you experience in the first days at altitude.
Second, your thyroid hormones rise. Levels of the active thyroid hormones T3 and T4 increase at altitude independently of the normal pituitary signaling pathway. These hormones boost heart rate and cardiac output, helping your body push more oxygen to tissues that are getting less of it from each breath. Because thyroid hormones are central regulators of your metabolic rate, their increase directly contributes to the calorie-burning spike. As your body acclimatizes and oxygen delivery becomes more efficient, thyroid activity and metabolic rate settle back down.
At the cellular level, low oxygen triggers a protein called HIF-1 alpha, which acts as a master switch for how your cells produce energy. It increases the number of glucose transporters on cell surfaces, ramps up the enzymes needed for sugar-based energy production, and dials down the oxygen-intensive energy pathways your mitochondria normally use. The net effect is that your cells shift toward burning glucose in a less efficient but more oxygen-sparing way.
Your Body Prefers Carbohydrates at Altitude
One of the most consistent metabolic changes at altitude is a shift toward burning more carbohydrates and less fat. This makes biochemical sense: extracting energy from carbohydrates requires less oxygen per calorie than extracting energy from fat. When oxygen is scarce, your body pivots to the more oxygen-efficient fuel.
Research on high-altitude-adapted animals has shown this clearly. Highland deer mice increased carbohydrate oxidation by 34% after acclimating to low oxygen, while simultaneously reducing fat oxidation. Their muscles also showed increased activity of enzymes that pull glucose from the bloodstream. Interestingly, lowland mice exposed to the same conditions did not make this shift as effectively, suggesting it’s a specific adaptation to altitude rather than a generic stress response. In humans, the same general trend holds: carbohydrate becomes the preferred fuel, which is one reason that high-carbohydrate diets are often recommended for mountaineers and altitude travelers.
Why Appetite Drops at Altitude
Most people eat significantly less at high altitude, a phenomenon sometimes called “altitude anorexia.” This isn’t just about feeling nauseous from mountain sickness. Your appetite hormones shift in ways that actively suppress hunger.
During acute altitude exposure, levels of leptin (a hormone that signals fullness) rise, while levels of ghrelin (the hormone that drives hunger) fall. One study found ghrelin concentrations dropped 12% to 22% at altitude compared to sea level. That may sound modest, but related research found that an 18% reduction in ghrelin corresponded with a 31% drop in voluntary food intake. During a 12-day trek to 5,140 meters, both fasting ghrelin and voluntary calorie intake fell by roughly 25%.
The appetite suppression tends to be strongest in the first days at altitude and may partially resolve with acclimatization. However, ghrelin remained suppressed even after 21 days in one study, suggesting that some degree of reduced appetite can persist throughout an altitude stay. People at altitude also tend to prefer sweet, carbohydrate-rich foods over protein-heavy meals, which aligns with the metabolic shift toward carbohydrate burning.
Weight Loss at Altitude
The combination of higher energy expenditure and lower calorie intake creates a caloric deficit that leads to real weight loss. Research on high-altitude trekkers found average losses of about 3.3 kilograms (roughly 7 pounds), with approximately two-thirds of that loss coming from fat and one-third from lean body mass.
Dehydration also plays a role. At altitude, you breathe faster and the air is drier, which means you lose more water through your lungs with every breath. Insensible water loss (through breathing and skin) can reach 700 to 1,000 milliliters per day under normal conditions, and altitude pushes that figure higher. Respiratory heat loss at 4,572 meters can be 1.6 times greater than at sea level at the same breathing rate. Climbers working hard at extreme altitudes may need 3 to 4 liters of fluid daily just to maintain adequate hydration and urine output.
Some of the early weight loss at altitude is simply water, but sustained stays produce genuine changes in body composition. This has generated interest in whether altitude exposure could be useful for weight management, though the practical challenges of living at altitude make it an unlikely obesity intervention for most people.
Blood Sugar Improvements at Altitude
One of the more intriguing metabolic effects of altitude is improved blood sugar regulation. People who live at higher elevations tend to have lower fasting blood sugar and better glucose tolerance compared to those living near sea level. Lowlanders exposed to very high and extreme altitude for three to eight weeks showed significant reductions in fasting blood sugar, and similar improvements have been documented in people who relocated to high altitude for 24 months, regardless of physical activity levels.
The mechanisms behind this are not fully understood, but the increased glucose uptake driven by HIF-1 alpha, the shift toward carbohydrate metabolism, and the heightened sympathetic nervous system activity all likely contribute. The benefit appears to be an acquired trait rather than a genetic one, since it develops in lowlanders who move to altitude and fades when high-altitude natives migrate to sea level. Early observations by the Peruvian physiologist Carlos Monge-Cassinelli found that Andean natives who had lived at sea level for at least a year had the same fasting blood sugar as coastal natives.
What This Means for Altitude Travelers
If you’re heading to high altitude for hiking, skiing, or work, a few practical realities follow from these metabolic changes. Your body will burn more calories than usual for the first one to two weeks, potentially 10% to 27% more at rest alone, before settling back toward normal. At the same time, your appetite will likely decrease, making it easy to under-eat without realizing it. Prioritizing carbohydrate-rich foods makes sense because your body is already shifting toward carbohydrates as its preferred fuel source, and these foods deliver more energy per unit of oxygen consumed.
Staying well hydrated takes deliberate effort. The dry air and faster breathing at altitude pull water from your body faster than you might expect, and dehydration compounds fatigue and impairs performance. Planning for higher fluid intake, ideally 3 to 4 liters per day during physical activity at altitude, helps offset these losses. Most metabolic adjustments stabilize within 7 to 14 days, so the first two weeks are when the mismatch between energy output and energy intake is greatest, and when paying attention to nutrition matters most.