The human body constantly works to maintain a stable internal temperature of approximately 98.6°F (37°C). This thermal regulation requires energy, supplied through metabolism, which converts food into usable fuel. When the external environment deviates significantly from this optimal range, the body must expend additional energy to prevent overheating or excessive cooling. These automatic, energy-demanding adjustments activate distinct physiological mechanisms that change the body’s total energy expenditure.
How the Body Burns Calories to Stay Warm
Exposure to cold environments triggers thermogenesis, a process that significantly increases the body’s energy expenditure above its basal metabolic rate (BMR). The most visible response is shivering thermogenesis, involving the rapid, involuntary contraction of skeletal muscles. This muscular activity is demanding, and intense shivering can increase a person’s metabolic rate by as much as five to six times the resting rate.
Before shivering begins, or in response to milder cold, the body employs non-shivering thermogenesis (NST). This process is primarily mediated by Brown Adipose Tissue (BAT), or brown fat, which burns energy stores directly for heat production. Unlike typical white fat, BAT is packed with mitochondria and uses fat and glucose as fuel to generate heat without muscle movement.
The activation of brown fat represents a metabolic boost, capable of increasing overall energy expenditure by 10% to 30% above BMR, depending on the cold exposure’s severity. This mechanism allows the body to maintain its core temperature without shivering. Total energy expenditure is higher in cold climates due to the cumulative effect of these thermogenic responses.
Energy Costs of Responding to Heat Stress
While cold exposure forces the body to create heat, hot environments require the body to actively dissipate heat, which also costs energy. When the ambient temperature rises above the body’s thermoneutral zone, the body must work harder to transfer heat from its core to the skin’s surface. This is accomplished through vasodilation, where blood vessels near the skin widen, increasing blood flow to the periphery.
This increased blood flow places an extra load on the heart, forcing it to pump harder and faster to maintain circulation to both the working muscles and the skin. This cardiovascular effort requires more energy, contributing to the total calorie burn. Furthermore, the body initiates sweating, relying on the evaporation of moisture from the skin to cool the surface.
While the phase change of water from liquid to gas during sweating is a passive cooling event, the underlying physiological adjustments supporting this function require energy. The metabolic increase associated with heat stress is modest compared to the boost seen in the cold, typically resulting in only a 2% to 8% higher calorie burn than in temperate conditions. In extreme heat, passive heating can increase the resting metabolic rate by 11% to 23% as the body struggles to maintain thermal balance.
Practical Differences in Calorie Burn During Exercise
When exercise is added, the effects of temperature on calorie burn become more complex and often relate to performance capacity. In a cold environment, the body’s need to generate heat combines with the calories burned from physical activity, resulting in a higher net energy expenditure per minute. However, cold temperatures may limit the duration or intensity of a workout due to discomfort, restrictive clothing, or a need to reduce exposure. This reduction in total work volume can lead to a lower overall calorie burn for the entire activity, despite a higher rate of expenditure per minute.
Exercising in the heat also increases the metabolic rate because the cardiovascular system must simultaneously support muscle activity and thermoregulation. The heart rate is elevated to shunt blood to the skin for cooling, which increases the energy required for a given pace. However, heat stress accelerates fatigue and fluid loss, often forcing the exerciser to slow down or stop sooner than in a moderate climate.
Consequently, the higher metabolic rate per minute in the heat often does not translate into a greater total calorie burn because the overall volume of work performed is reduced. Furthermore, exercising in high heat tends to cause a metabolic shift, favoring the use of carbohydrates as fuel and decreasing the body’s reliance on fat burning. While mild cold exposure may provide a marginal metabolic advantage, extreme temperatures primarily compromise performance and safety, overwhelming any small gains in passive calorie burning.