The human body maintains a constant internal temperature of approximately 98.6°F (37°C) through homeostasis. This stability requires continuous energy expenditure, known as the basal metabolic rate (BMR), which is the minimum energy needed to keep the body functioning at rest. When environmental temperatures deviate from the body’s comfort zone, specialized energy-consuming mechanisms, collectively called thermoregulation, activate to maintain this core temperature. The energy cost of these involuntary processes determines whether winter or summer weather demands more calories from the body.
How the Body Generates Heat in Cold Weather
When the body is exposed to low temperatures, it activates internal heating systems to prevent a drop in core temperature. The initial response is non-shivering thermogenesis, a metabolic process driven primarily by specialized cells called brown adipose tissue (BAT). BAT is densely packed with mitochondria and contains a unique protein that uncouples energy storage from heat production. Instead of creating energy (ATP), BAT burns fat and glucose substrates to dissipate energy directly as heat.
Exposure to mild cold, such as temperatures around 60°F (16°C), can activate BAT and increase whole-body energy expenditure by up to 1.8-fold in some individuals. If the cold exposure is more severe or prolonged, the body initiates shivering, involving rapid, involuntary muscle contractions. Shivering is the body’s most intense heat-generating mechanism, converting chemical energy into mechanical energy and heat. This muscular activity can dramatically increase the body’s metabolic rate, sometimes raising energy expenditure to five or six times the BMR.
How the Body Manages Heat in Warm Weather
In warm environments, the body’s primary goal is to shed excess heat to prevent overheating. The first line of defense is vasodilation, where blood vessels near the skin surface widen to increase blood flow, allowing heat to radiate away from the body. The most significant energy-consuming cooling mechanism is the production and evaporation of sweat. Sweat glands draw fluid from the plasma and pump it to the skin surface, a process that requires a small metabolic cost.
The cooling effect occurs when the liquid sweat evaporates, carrying heat away from the skin. While the increase in core body temperature caused by heat stress can raise the overall metabolic rate by about 10–13% per degree Celsius, the metabolic cost of sweating is modest. This heat-dissipation strategy generally requires less internal energy expenditure than the body’s active heat-generation processes.
The Comparative Calorie Cost: Cold Versus Heat
Comparing the internal regulatory mechanisms, cold-induced thermogenesis typically demands a much higher increase in calorie expenditure than heat management. The intense, involuntary muscle activity of shivering represents a massive, temporary energy drain, often burning between 100 and 400 calories per hour depending on its intensity. Even without shivering, the sustained activation of brown adipose tissue in response to mild cold adds a significant, continuous metabolic load.
In contrast, the energy cost of producing sweat and increasing blood flow during heat exposure is a lesser burden on the body’s fuel reserves. While both extremes of temperature pull the BMR above the minimum, the difference is substantial when the environment forces a strong response. Exposure to sufficiently cold conditions to induce shivering or sustained BAT activation will result in a greater caloric expenditure increase than exposure to hot conditions. This significant burn requires being inadequately insulated, forcing the body to actively fight to maintain its core temperature.
The Role of Seasonal Activity Levels
While the metabolic cost of thermoregulation favors the cold, the overall number of calories burned—the total daily energy expenditure (TDEE)—is often dictated by behavior. In many populations, the physical activity level (PAL) tends to be higher during the summer months. People are generally more inclined to engage in outdoor activities, such as hiking, swimming, and walking, when the weather is favorable and daylight hours are extended.
This voluntary, activity-related energy expenditure is highly variable but can easily outweigh the metabolic increases from temperature regulation. Studies show that while BMR may change slightly, the total energy expenditure often remains statistically similar between seasons because lower activity levels in winter offset any gains from thermogenesis. A significant seasonal difference in energy expenditure is more likely to reflect a change in exercise habits than a forced metabolic response to the weather.