The human body maintains a stable internal temperature, a process requiring continuous energy expenditure. When running, muscles generate heat, but a cold environment forces the body to work harder to balance heat production against heat loss. This challenge to thermal regulation, combined with other physical factors, can increase the total calories burned during a run.
Internal Mechanisms of Heat Generation
Maintaining a core temperature requires continuous metabolic activity regulated by the hypothalamus. When exposed to cold, the body first attempts to generate heat through non-shivering thermogenesis (NST). NST activates specialized cells, such as those in brown adipose tissue (BAT), which burn fat to produce heat without muscle contraction, increasing the resting metabolic rate.
If NST is insufficient, the body initiates shivering, an involuntary response involving rapid, small muscle contractions. Shivering is a highly energy-intensive emergency mechanism that can increase the basal metabolic rate by five to six times, primarily fueled by glycogen stores.
Quantifying the Increased Calorie Expenditure
The metabolic increase from thermoregulation is often minimal for a well-dressed runner due to the heat generated by the exercise itself. Running is inefficient, with approximately 80% of the energy consumed released as heat, often negating cold stress. Significant thermogenesis or shivering is unlikely unless the temperature is extremely low or clothing is inadequate.
In moderately cool temperatures, many runners perform more efficiently; the optimal running temperature is suggested to be around 45 degrees Fahrenheit (7 degrees Celsius). However, when the cold is severe enough to require thermogenesis, the body shifts its preferred fuel source. Cold exposure stimulates hormones that increase carbohydrate utilization, causing muscles to rely more heavily on glycogen stores than fat.
This fuel shift can negatively impact performance, leading to earlier fatigue from glycogen depletion. While the metabolic rate is higher, the runner may be forced to slow down or shorten the distance, ultimately negating the total caloric advantage.
Physical Resistance and Gear Weight
Beyond internal metabolic adjustments, several external physical factors increase the mechanical work required for running. Cold air is denser than warm air, leading to a subtle but measurable increase in air resistance, or drag. A temperature drop from 68°F (20°C) to 14°F (-10°C) can increase air density and drag by about 10%.
Running into a cold headwind significantly compounds this issue, demanding a much higher caloric expenditure to overcome the resistance. Running at 10 miles per hour into a nine miles per hour wind can produce the same wind chill factor as standing still in a 19 miles per hour gale. This external force requires greater effort from the muscles, directly increasing the mechanical work and the calories burned.
The necessary layered clothing also adds weight and bulk, further increasing the energy cost of running. Moving a heavier total mass requires more force with every stride. Running on snow or ice demands continuous small muscle adjustments for stabilization and balance, which contributes to the total energy demand. These external factors, rather than internal thermogenesis alone, often account for the most noticeable increase in total calories expended during a cold-weather run.