Metabolism is the process that converts food into energy, powering functions from breathing to movement. The majority of daily calories burned are used simply to keep the body functioning at rest—a measure called the Basal Metabolic Rate (BMR). While a seasonal change in temperature does influence this rate, the ultimate answer involves a closer look at the body’s highly efficient temperature regulation systems.
Calorie Burn and Temperature: The Basic Mechanism
The human body constantly strives to maintain an internal temperature of about 37 degrees Celsius (98.6 degrees Fahrenheit) through a process known as homeostasis. This regulation requires the body to expend energy, which directly impacts the number of calories burned. The environment’s temperature dictates how much energy the body must divert toward generating or dissipating heat to maintain this internal balance.
Scientists define the Thermoneutral Zone as the range of ambient temperatures where the body expends the least amount of energy for thermal regulation. For a lightly clothed, resting person, this zone typically falls between 20 and 30 degrees Celsius (68 to 86 degrees Fahrenheit). When the surrounding temperature moves outside this comfortable window, the body must actively engage mechanisms that consume extra calories to bring the core temperature back to its set point.
Temperatures dipping below this zone require the body to increase heat production, while temperatures rising above it require increased heat loss. Any temperature that forces the body to work to maintain its core temperature adds to the overall calorie expenditure.
The Summer Effect: Why Heat Reduces Basal Calorie Burn
The body’s response to cold is fundamentally different and more calorically expensive than its response to heat. When exposed to cold, the body actively produces heat through mechanisms like shivering, which involves rapid muscle contractions and significantly increases the metabolic rate. Non-shivering thermogenesis, primarily involving brown adipose tissue, also burns calories to generate warmth.
When the body is faced with high summer heat, its primary goal is to dissipate excess heat. This is accomplished mainly through vasodilation, where blood vessels near the skin widen to allow heat to escape, and through the evaporation of sweat. These heat-loss mechanisms require comparatively less metabolic energy than the active heat-generation processes triggered by cold.
Since the body expends fewer calories to cool down than to warm up, the basal metabolic rate tends to slightly decrease in high heat. In a warm environment, the body is not working as hard to maintain its warmth, resulting in a lower overall calorie burn from thermoregulation. This adjustment means that, at a resting state, the body is more metabolically efficient in the heat of summer compared to the cold of winter.
Total Daily Expenditure: The Role of Summer Activity
While the body’s basal calorie burn from temperature regulation may be lower in the summer, this does not necessarily mean the total daily calories burned decreases. Total Daily Energy Expenditure (TDEE) includes BMR but is heavily influenced by physical activity and exercise. Summer weather typically encourages a shift in lifestyle and behavior, leading to increased time spent outdoors.
Many people increase their activity level during the warmer months, engaging in more walking, gardening, swimming, or other outdoor leisure activities. This increased activity-related energy expenditure often outweighs the slight reduction in BMR from the environmental temperature. The behavioral choice to be more active is the dominant factor in total calorie burn, not the seasonal temperature shift itself.
Exercising in high heat can also increase the active calorie burn, though not always efficiently. The heart works harder to pump blood to both the working muscles and the skin’s surface for cooling, which elevates the heart rate faster than in cooler conditions. However, the heat may also reduce exercise performance and duration, which limits the total calories burned from activity. Therefore, while summer heat lowers the resting metabolic cost of temperature regulation, the potential for increased physical activity often makes the greatest difference in total energy expenditure.