How the Human Body Regulates Heat During Exercise

Environmental Influence on Heat Balance During Exercise

The ability to maintain heat balance during exercise is critically dependent on the surrounding environmental conditions. The psychrometric chart below illustrates zones of compensable and uncompensable heat stress. Compensable heat stress refers to environments where the body can maintain thermal equilibrium through physiological cooling mechanisms. In contrast, uncompensable heat stress represents conditions where heat gain exceeds heat loss, resulting in continuous core temperature rise. For instance, this curve represents unacclimated women exercising at a metabolic rate of 150 W/m². Upon acclimation to heat, this threshold shifts significantly, expanding the compensable zone.

As shown, heat acclimation is a critical adaptive response that enhances thermal tolerance.

However, acclimation is not the only variable impacting heat balance. Convective cooling, largely influenced by wind speed, also plays a significant role by increasing the transfer of heat from the skin surface to the environment. The effects of wind speed on heat tolerance are represented by the dashed line below.

Metabolic Heat Production and Heat Load

The primary source of internal heat during exercise is metabolic heat production, which scales with activity intensity, load carriage, environmental resistance, and individual fitness. For example, military personnel marching at 3.7 mph with a 20 kg pack through loose sand may generate approximately 1221 W of heat—comparable to the output of a household hair dryer. Similarly, elite distance runner Kenenisa Bekele, weighing 59 kg and running at 14.2 mph with a VO₂ max of 4.7 L/min, produces around 1516 W of internal heat.

This heat must either be dissipated through physiological mechanisms or stored, increasing body temperature. The inefficiency of human energy metabolism exacerbates the problem:

For instance, shoveling involves 92–94% of energy being converted into heat, with only 6–8% used for actual mechanical work. If no thermoregulatory exchange occurred, the time required to reach a lethal core temperature (approx. 40.5°C) would be alarmingly short.

Fortunately, humans possess robust thermoregulatory systems. Tour de France cyclists climbing a hill at 450 W can sustain performance for much longer than the 8 minutes theoretically required to reach a critical temperature—thanks to heat dissipation.

Heat Transfer Mechanisms: Conduction and Convection

Heat is transferred from working muscle to skin via two main mechanisms: conduction and convection. Conduction through tissue is limited due to low thermal conductivity.

While copper has a thermal conductivity of 385 W/m/°C, human tissues are significantly less conductive—muscle (~0.47), fat (~0.17), and skin (~0.21). Thus, convection via blood flow becomes the dominant pathway for internal heat transfer during exercise.

Blood has a high specific heat and acts as a thermal conduit between the core and skin. The rate of convective heat transfer is driven by three variables:

1. Specific heat of blood

2. Core temperature

3. Skin temperature

Among these, only skin blood flow is under active physiological regulation. At rest, skin blood flow averages 0.3 L/min, but under thermal stress it can rise dramatically to 7–8 L/min.

This increase corresponds with a significant jump in heat transfer capacity. For example, increasing skin blood flow from 0.3 to 6 L/min results in a nearly 20-fold increase in heat transfer capacity.

Thermal signals regulate this process: as core temperature rises, both sweating and cutaneous vasodilation increase.

Thermoregulatory Impairment with Age and the Role of Nitric Oxide

Aging is associated with a decline in skin blood flow due to reduced vasodilatory capacity. This impairment contributes to greater vulnerability to heat-related illness in older populations.

Nitric oxide (NO) is a key mediator of vasodilation. NO production is essential for full expression of skin blood flow in response to heat. When NO synthesis is inhibited with L-NAME (a non-specific nitric oxide synthase inhibitor), cutaneous vascular conductance decreases despite rising core temperatures, as shown in the graph below.

NO is synthesized from L-arginine, a semi-essential amino acid.

However, L-arginine metabolism is subject to competing pathways: one pathway leads to NO production and vasodilation, while the other results in urea formation, eliminating its vasodilatory potential. Moreover, oxidative stress impairs NO availability by promoting the formation of peroxynitrite, which deactivates nitric oxide synthase (NOS). This uncoupling diminishes NO production.

One strategy to combat this degradation is through the use of ascorbic acid (vitamin C). Vitamin C scavenges oxidative species, preserving NO bioavailability and thus supporting vasodilation.

When both NO synthesis (via L-arginine or its precursor L-citrulline) and oxidant scavenging (via vitamin C) are addressed together, significant improvements in cutaneous blood flow are observed in both younger and older populations.

Application: Nutritional Strategies to Support Thermoregulation

Heat Hydration, a targeted hydration and electrolyte supplement, utilizes this physiological understanding to enhance thermoregulation. It contains L-citrulline, a more bioavailable precursor to L-arginine that bypasses initial metabolism and increases plasma L-arginine levels more effectively. This leads to enhanced NO production and vasodilation. Additionally, it provides 100% of the daily value of vitamin C to minimize oxidative inhibition of nitric oxide.

The result is increased skin blood flow, improved internal heat transfer, and more efficient heat elimination during exercise. Below shows a table of a few additional methods to increase skin blood flow and a few factors that decrease skin blood flow.

Sources

All photos and the majority of this content was published by Kenny, W. in his paper titled "INTERNAL HEAT TRANSFER IN EXERCISING HUMANS"

Huberman Lab. (2023, May 1). How to improve cardiovascular & metabolic health with sauna use | Huberman Lab Podcast #93 [Video]. YouTube. https://www.youtube.com/watch?v=jmIEwBeM2o4