When the ambient temperature rises, the human body must work harder to maintain a stable internal temperature of approximately 98.6°F (37°C). This process of thermoregulation requires energy, meaning the body does expend a few more calories in a hot environment. However, the resulting increase in metabolic expenditure is marginal, often representing only a 2 to 8 percent rise above a person’s baseline resting burn. This minimal extra energy expenditure is a side effect of the body attempting to prevent overheating, which is a significantly less costly process than the energy required to shiver and generate heat in cold conditions.
The Energy Cost of Cooling Down
Maintaining a stable core temperature in the heat triggers several energy-consuming physiological responses. One mechanism is vasodilation, where blood vessels near the skin surface widen, diverting warm blood away from the core to the periphery. The heart must pump harder to circulate this increased volume of blood to the skin, which raises the heart rate even at rest. This increased cardiovascular effort accounts for a small portion of the extra calories burned.
Another key cooling mechanism is the production of sweat, which draws heat away from the body as it evaporates from the skin. The act of generating and transporting the sweat itself requires a small amount of metabolic energy. While a person may observe weight loss after intense sweating, this drop in mass is almost entirely due to water loss, not the expenditure of stored fat or calories. Once fluids are replaced through drinking, this temporary water weight is immediately regained.
How Heat Affects Exercise Performance
While the body expends a small amount of extra energy for passive cooling, this effect is often negated during exercise. High temperatures negatively impact the ability to maintain a high-intensity workout, which is the primary driver of overall calorie burn. As the core temperature rises, the heart rate increases more rapidly than in cooler conditions, a phenomenon known as cardiovascular drift. This earlier spike leads to premature fatigue and a higher perceived effort for the same level of work.
Most people instinctively reduce their pace or shorten the duration of their activity to compensate for the discomfort and exhaustion caused by the heat. Since calorie burn is directly proportional to both intensity and length, a lower output often results in an equal or even lower net calorie expenditure compared to exercising in optimal temperatures. Furthermore, exercising in the heat can shift the body’s fuel preference toward burning carbohydrates instead of fat.
Safety and Environmental Factors
The ability to cool down is severely compromised when the surrounding air contains high moisture. High relative humidity prevents the efficient evaporation of sweat, which is necessary for heat to leave the body. When sweat cannot evaporate, it merely drips off the skin, and the core temperature continues to rise. This environmental limitation increases the risk of heat-related illnesses because the body’s most effective cooling system is rendered ineffective.
Pushing physical limits in hot and humid conditions can lead to dangerous health consequences. Continuous sweating without adequate fluid replacement quickly causes dehydration, straining the cardiovascular system. This can progress rapidly to heat exhaustion and, eventually, heat stroke, where the body’s temperature regulation mechanisms completely fail. Focusing on the minimal increase in calorie burn is not worth the health risks associated with overheating.