Overview

Thermoregulation refers to the body’s ability to maintain its core temperature within a narrow range. During physical activity in hot conditions, the primary mechanism for heat dissipation is sweat evaporation, which helps the body achieve thermal balance by managing both heat production and loss. This process is especially crucial for endurance athletes who often compete in environments characterized by high temperatures and humidity. Under such conditions, the body’s thermoregulatory efficiency can be impaired, increasing the risk of heat-related illnesses and negatively affecting athletic performance.

To address these challenges, various cooling methods have been explored in research, given the increasing frequency of extreme conditions in major sporting events. For instance, the 2021 Tokyo Summer Olympics saw average temperatures of 32.2°C with 70% relative humidity, and the 2022 FIFA World Cup in Qatar posed the likelihood of even higher temperatures exceeding 40°C. Cooling interventions such as ice vests, neck coolers, cold beverages, and ice slurries have been studied as potential strategies for improving heat management. However, these approaches present practical challenges during exercise, such as additional weight, discomfort, and difficulties with consumption. Furthermore, certain sports, including marathons, triathlons, and long-distance cycling, impose rules that may restrict the use of such cooling devices during competition.

The physiological stress experienced during intense physical activity in hot environments depends on the individual’s metabolic rate and the body’s ability to exchange heat with the surrounding environment. Physical exertion increases the metabolic rate by 5 to 15 times the resting rate to generate energy for muscle activity. Depending on the type of activity, 70–100% of this energy is released as heat, which must be dissipated to maintain thermal balance. The body’s ability to regulate temperature effectively depends on factors such as acclimatization (Wenger, 1988), aerobic fitness (Armstrong & Pandolf, 1988), and hydration (Sawka & Pandolf, 1990). Those who are aerobically fit, acclimatized to heat, and well-hydrated tend to store less body heat and perform better under heat-stress conditions.

The autonomic nervous system manages heat regulation through mechanisms such as redirecting blood flow from the core to the skin and sweating. Thermoreceptors in the skin and core send signals to the hypothalamic centre, which processes this information and activates responses to release heat, including sweat production and changes in skin blood flow (Sawka & Wenger, 1988).

Core temperature responses during exercise

During physical activity, core body temperature rises rapidly at first and then stabilizes as heat loss equals heat production. While metabolic rates increase immediately with exercise, the body’s cooling mechanisms, such as sweating and increased blood flow to the skin, take time to respond. These thermoregulatory processes scale with the rise in core temperature until equilibrium is achieved.

Research has shown that the extent of core temperature elevation is influenced primarily by metabolic rate rather than environmental factors (Gonzalez et al., 1978; Nielsen, 1938, 1970). For example, Nielsen (1938) observed that even under varying temperatures (5°C to 36°C with low humidity), total heat loss remained constant during exercise at specific intensities. However, the balance between radiative and evaporative heat loss depended on the surrounding temperature. Cooler environments allowed more heat loss through radiation and convection, while hotter conditions relied heavily on evaporation. In extremely hot environments, where ambient temperatures exceed skin temperature, the body may even gain heat from the surroundings.

Although research suggests core temperature elevation is mostly independent of environmental conditions, the practical experiences of athletes often differ. For instance, runners often experience more pronounced hyperthermia in 35°C conditions compared to cooler settings (Robinson, 1963). This discrepancy arises because temperature elevation is environment-independent only within specific conditions, known as the “prescriptive zone” (Lind, 1963). Within this zone, the core temperature increase is closely related to metabolic rate, but outside the zone, further temperature increases occur due to environmental extremes.

Lifestyle factors that can exacerbate sweating and feelings of heat during exercise

Clothing choices

Clothing selection plays a crucial role in supporting thermoregulation and reducing heat stress during physical activity in hot conditions. Traditional clothing can hinder heat loss by increasing thermal insulation and limiting sweat evaporation, while modern sportswear, made from lightweight, breathable synthetic fabrics, enhances moisture-wicking and comfort, supporting athletic performance. Sports Medicine Australia (SMA) recommends minimal, lightweight clothing with reduced skin coverage to improve sweat evaporation and heat loss. Different fabrics offer varying thermoregulatory benefits: polyester is durable but has low moisture absorption, nylon absorbs moisture better but dries slowly, cotton provides good absorption but can feel sticky, and wool offers wicking and heat dissipation but dries slowly and may be heavy. Chemically treated fabrics, like phase change materials (PCM), show promise in enhancing thermoregulation, though research is limited. Factors such as fit, knit structure, and air permeability also influence thermoregulation, with loose-fitting clothes generally improving airflow and heat dissipation. More research is needed to determine optimal fabric types, fits, and designs for varying conditions and activity levels.

Dehydration

Sweat evaporation and increased blood flow to the skin are key mechanisms for cooling the body, but dehydration reduces the body's ability to sweat and release heat. Additionally, hot and humid conditions or unsuitable clothing can hinder effective heat loss.

When exercising in the heat, the body loses more water through sweat than it takes in by drinking. This causes dehydration (hypohydration), along with a loss of important salts (electrolytes). Over a day, this water loss can add up, so it’s important to drink water not just during exercise but also at mealtimes.

Here’s how heat and dehydration affect the body:

1. Performance Drops: Aerobic (endurance) exercise, like running or cycling, becomes harder in hot weather, especially when dehydrated. The hotter it is, the worse the performance can get
2. More Heat Stored in the Body: Dehydration causes the body to store extra heat, making it harder to stay cool
   • This happens because:
     • Sweating slows down, reducing the body’s ability to cool off through evaporation
     • Blood flow to the skin decreases, which means less heat can be released into the air

Some believe that drinking extra water before exercise (hyperhydration) might help the body handle heat better, but there isn’t strong evidence to prove this works.

In short: Staying hydrated is key to performing well and keeping the body cool, especially in hot weather.

Poor fitness level

Regular exercise helps the body get better at handling heat during physical activity. It does this in three main ways:

1. Sweating Sooner: The body starts sweating at a lower core temperature, which helps cool it down faster
2. Sweating More Efficiently: The sweat response becomes more sensitive, so the body produces the right amount of sweat to cool off without wasting fluids
3. More Blood Volume: Exercise increases the total amount of blood in the body, which improves blood flow to the skin. This helps carry heat away from the core and release it through the skin

Together, these adaptations make it easier for the body to stay cool and perform better in hot conditions.

Caffeine 

When caffeine is consumed whether alone, with water, or with a sports drink it does not raise body temperature (thermogenesis) or interfere with the body’s ability to cool down.

However, when caffeine is combined with a carbohydrate-electrolyte solution (like a sports drink), it slightly increases core body temperature compared to the sports drink alone.

Caffeine also causes:

1. Increased urine output (you may pee more)
2. More electrolytes (like sodium and potassium) are lost through sweat

Despite these effects, they are not strong enough to cause dehydration or changes in blood electrolyte levels during 2 hours of exercise in a hot environment.

In simple terms: Caffeine has some small effects on body temperature, urine, and sweat, but it doesn’t cause dehydration or harm performance in hot conditions over 2 hours of exercise.

High environmental temperature and humidity

When the air is very humid, sweating becomes less effective for cooling the body because the air is already full of moisture (high water vapour). This makes it harder for sweat to evaporate and release heat.

If there’s no wind or air movement, the body also loses less heat through the air (this is called convective cooling).

When exercising in the sun, the body absorbs more heat because of infrared radiation from the sun’s rays, which adds to the heat the body is already generating.

High humidity, still air, and intense sunlight make it difficult to cool the body, increasing heat stress. Heat acclimatization, which occurs gradually through exercise in the heat, allows the body to adapt and cool itself more effectively by improving sweating, blood circulation, and sodium retention while reducing heart strain.

Impact of diet on thermoregulation during exercise

Diet plays a crucial role in regulating body temperature during exercise, particularly in hot conditions. Proper energy, carbohydrate, and protein intake are essential to maintain exercise performance and heat tolerance. Inadequate energy availability can impair thermoregulation by reducing the body's ability to adapt to heat stress and sustain performance. 

Carbohydrates are a key fuel source during prolonged or high-intensity exercise in the heat, as they support energy production while minimizing the strain on the cardiovascular system. Protein intake also supports recovery and muscle adaptation, helping maintain overall physical performance.

Heat stress can alter macronutrient metabolism and gastrointestinal function, leading to reduced appetite and nutrient absorption, which further impacts energy balance. Training programs designed for heat acclimation (HA) often involve low-to-moderate-intensity exercise, but newer protocols incorporate high-intensity exercises. 

While these methods enhance heat adaptations, they can strain energy reserves and impair post-HA performance if nutritional needs are not met. Therefore, appropriate dietary strategies, such as increasing carbohydrate intake, ensuring adequate energy availability, and optimizing protein consumption, are vital for supporting thermoregulation, energy metabolism, and overall exercise performance in hot environments.

Effect of body fat on thermoregulation during exercise

Research to examine the effect of body fat on thermoregulation during aerobic exercise in hot environments suggests that body fat did not significantly influence thermoregulation outcomes, such as core temperature, skin temperature, or sweat loss, during exercise in the heat. However, many studies did not adequately control for confounding factors like metabolic heat production and body mass, which can affect thermoregulation. Given the limited number of studies meeting the criteria, further research is needed to make accurate comparisons and better understand the independent role of body fat in regulating body temperature during exercise in hot conditions.

What is exercise-related heat exhaustion?

Exercise-related heat exhaustion occurs when your body overheats during physical activity. Normally, the brain regulates body temperature around 98.6°F (37°C) to maintain optimal function. However, during heat exhaustion, your temperature can rise from 101°F (38.3°C) to 104°F (40°C). When your body overheats, it activates mechanisms like sweating to cool down. The evaporation of sweat helps lower body temperature. Additionally, blood flow increases to the skin and extremities, allowing heat to dissipate. If these cooling methods fail, body temperature can escalate to between 101°F (38.3°C) and 104°F (40°C). This can lead to symptoms like dizziness, weakness, and even fainting due to insufficient blood circulation.

If untreated, heat exhaustion can progress to heat stroke, a more severe condition that can cause organ failure and potentially be life-threatening.

Heat exhaustion is common in athletes, particularly in hot and humid environments, as well as in military recruits undergoing intensive training.

Preventing heat exhaustion

To minimize the risk:

• Exercise during cooler parts of the day, like early morning or evening
• Stay hydrated and take frequent breaks
• Wear lightweight, loose-fitting clothing
• Avoid outdoor activities during extreme heat and humidity
• Recognize early warning signs of overheating and stop exercising immediately if they occur