When the weather turns hot, many people wonder if increased heat means they are burning more calories just by existing. Burning calories refers to energy expenditure, the total energy your body uses to perform all its functions. The short answer is that heat slightly increases your baseline metabolic effort as your body works to cool itself down. However, this effect is generally minimal compared to the energy burned during exercise. The main difference in calorie burn occurs when physical activity is added, though extreme heat often reduces overall activity levels.
How Thermoregulation Affects Energy Use
The human body maintains a stable internal temperature through thermoregulation. When the ambient temperature rises above the thermoneutral zone—roughly between 68 and 77 degrees Fahrenheit for a clothed, resting person—the body must actively work to dissipate heat. This increased effort requires energy, and thus, a minor increase in calorie expenditure.
One of the first mechanisms the body uses is vasodilation, where blood vessels near the skin surface widen to move internal heat outward. Pumping this blood to the skin requires the heart to work slightly harder, contributing to the elevated energy use. This process acts like a radiator, transferring heat from the core to the periphery where it can be released.
The body also initiates sweating, a process that requires energy to transport fluid to the skin’s surface. Evaporation is the most effective cooling mechanism, but the creation and secretion of sweat itself consumes a small amount of energy. While these cooling processes do increase the body’s basal metabolic effort, the increase is relatively small.
The energy cost of these passive, resting cooling mechanisms is not substantial enough to be a meaningful contributor to daily calorie burn. The temporary drop in weight seen after heavy sweating is almost entirely due to water loss, which is regained immediately upon rehydration.
The Calorie Cost of Physical Activity in Warm Climates
The most significant change in calorie expenditure occurs when physical activity is introduced. When exercising in a hot environment, the body must perform two demanding tasks at once: powering muscle movement and regulating an increasing core temperature. This dual demand substantially increases the overall cardiovascular workload.
During exercise, the heart rate increases to deliver oxygen and nutrients to working muscles. In the heat, the heart must also pump a greater volume of blood toward the skin for cooling, diverting it away from central circulation. This results in a much higher heart rate for the same level of exercise intensity compared to a cooler environment, directly increasing the energy cost.
The combined effort of muscle contraction and the circulatory strain of heat dissipation means that performing a standardized task, such as running a mile at a set pace, will generally burn more calories in hot weather. Studies indicate that the body may also shift its fuel preference, potentially increasing the oxidation of carbohydrates during exercise in the heat compared to cooler temperatures.
To support this increased internal workload, adequate hydration is paramount. The body’s ability to sweat and perform the necessary cooling functions relies heavily on sufficient fluid reserves. Without this support, performance drops rapidly, and the risk of heat-related illness rises, ultimately limiting the duration and intensity of the activity.
Why Heat May Not Lead to Overall Weight Loss
While cooling and exercising in the heat burn slightly more energy, this increase often does not translate into greater overall daily weight loss. Behavioral changes in response to the heat frequently negate the small increase in physiological calorie expenditure.
Many people naturally reduce their exercise intensity or duration when temperatures climb because the activity feels much harder. Moving at a slower pace or finishing a workout sooner means the total calories burned from the exercise are lower than they would be in a moderate climate. The body’s protective response to heat often limits physical output.
Throughout the rest of the day, individuals in hot climates tend to experience increased lethargy and a reduction in non-exercise activity thermogenesis (NEAT). NEAT includes calories burned from daily spontaneous movements like fidgeting, walking around, and standing. Seeking refuge in air conditioning or moving less substantially decreases this component of total daily energy expenditure.
The dangers of heat exhaustion and dehydration limit how much a person can safely push themselves. Prioritizing safety by moderating activity or resting entirely means the potential for a higher total daily calorie burn is not realized. Ultimately, the slight increase in metabolic effort from resting thermoregulation is typically outweighed by a decrease in voluntary physical movement.