When considering how altitude influences the body, it’s important to distinguish between weight and mass. Mass, the amount of matter in an object, remains constant regardless of location. Weight, the force exerted by gravity, changes with gravitational pull. Understanding both physical principles and biological responses is key to knowing how altitude affects weight and body mass.
The Physics of Weight at Altitude
A person’s measured weight is influenced by two primary physical factors at different altitudes: Earth’s gravitational pull and air buoyancy. Gravity slightly decreases with distance from the Earth’s center. At higher altitudes, being further from the core results in a slightly weaker gravitational force, making a person weigh infinitesimally less. For instance, at 9,000 meters (30,000 feet), gravitational pull causes about a 0.29% weight decrease.
The second factor is air buoyancy, the upward force exerted by displaced air. Like water, air provides a small upward lift. At higher altitudes, lower air density reduces this buoyant force, which in isolation, would cause a person to register a slightly higher weight. For example, at 9,000 meters, this effect could increase apparent weight by about 0.08%. However, these physical changes in measured weight are typically negligible for practical purposes.
Physiological Changes Affecting Body Mass
Beyond direct physical effects on measured weight, altitude leads to actual changes in body mass through physiological adaptations. One notable change is an increased basal metabolic rate (BMR), as the body works harder with less available oxygen. This increased energy expenditure can range from 6% to 27% in the first few days at 3,650 to 4,300 meters, though it may decline with acclimatization. Hypoxia can also alter how the body processes sugars and fats, potentially leading to lower body weight over time.
Appetite is significantly affected at high altitudes, with many experiencing decreased hunger and food intake. Caloric intake reductions can be substantial, sometimes decreasing by 30% to 40% initially. This suppressed appetite, combined with an elevated metabolic rate, creates a negative energy balance where the body expends more calories than it consumes. Fluid shifts and increased fluid loss through respiration in drier, thinner air also contribute to initial weight changes and dehydration risk.
Prolonged stays at higher altitudes impact body composition, specifically muscle and fat mass. Exposure to hypobaric hypoxia can lead to muscle atrophy, where muscle protein breakdown increases and synthesis decreases. Studies indicate that significant weight loss at high altitudes, particularly above 4,500 meters, can be attributed to muscle mass reduction. While fat mass can also decrease due to increased energy expenditure, muscle loss can be more pronounced, especially if caloric intake remains insufficient.
Practical Considerations for Travelers
For travelers, direct physical effects on measured weight at higher altitudes are minimal and imperceptible. The slight reduction in gravitational pull and minor increase in apparent weight due to reduced air buoyancy largely cancel out or are too small to notice on a standard scale. These theoretical changes do not typically account for perceived weight shifts.
More significant changes result from the body’s physiological responses to the lower oxygen environment. Travelers might experience decreased appetite, an increased resting metabolic rate, and fluid shifts, all contributing to a reduction in actual body mass. These physiological adaptations are the primary drivers behind observed weight loss during altitude exposure, particularly during longer stays or at very high elevations. Awareness of these biological responses helps travelers understand and prepare for potential changes in their body mass.