Exercising in hot and humid weather imposes additional stress to the body during exercise. This additional layer of stress paired with the physiological stress from exercise requires the body to allocate more energy to maintain homeostasis, specifically, where the body attempts to maintain a core temperature of ~37 °C. While there are behavioral and technological solutions to mitigate the impacts of these environmental stressors, they are often not sufficient to maintain homeostasis. As a result of the heat, there is an alteration of the relative proportion of carbohydrate and fat utilized during exercise. In part 1 of this blog, we will discuss how exercising in a hot environment impacts the body. In part 2, we provide considerations and strategies on how to adjust fueling and hydration during endurance exercise in the heat.
Physiological response to exercise in the heat
The body produces heat as a result of increased metabolic activity during exercise. This heat is typically well regulated by the body to maintain core temperature. However, exercising in the heat presents a significant challenge to the body’s ability to manage heat. Exercising in the heat typically leads to an elevated core temperature from the additional thermal stress related to the body’s decreased efficiency to release heat produced during exercise and from the external thermal source in a hot environment.
Figure 1: Describing heat exchange between the body and environment. From Physiology of Sport and Exercise. Fifth edition. Kenney, Wilmore, Costill
Much of the physiological responses to heat are related to altered blood flow distribution, altered sweat rates, and altered metabolic activity. Exercising in the heat is challenging to the cardiovascular system. In the heat, more blood flow is directed to the surface of the skin in an attempt to move more heat from the core to the surface of the skin where it is more easily dissipated. Because of this, less blood flow is delivered to the working muscle. Thus, heart rate must increase in an attempt to maintain blood flow to the working muscle.
Sweat rates will increase to cool the body in response to the higher core temperatures typically associated with the heat. This is the primary way for the body to dissipate heat during exercise. Sweating is intended to remove body heat through evaporative cooling; the energy from heat is transferred to the sweat where it causes the sweat to evaporate. The challenge that comes with sweating is the loss of water from the body. If the water loss through sweat is not replaced, performance and physiological function can be compromised. Depending on the individual, sweat rates can be as high as 2-3 liters of sweat per hour (Burke et al., 2001). It is well known that endurance performance can decrease with water loss as little as 1.8% of body mass (Walsh et al., 1994, Burke et al., 2001). While it is cited that some individuals can sustain similar performances as in a normally hydrated state with up to a >5% fluid loss, this is highly dependent on the individual. The reduction of body water during exercise leads to further increases in core temperature due to reduced skin blood flow, reduced capacity of blood to manage heat, and impaired cooling from sweat as sweat rates may decline through an exercise bout.
Finally, additional heat stress can alter the function of metabolic and contractile proteins in the working muscle. Exercise in hot conditions leads to an increased use of carbohydrates (i.e. muscle glycogen), and a decrease use of fats (Febbraio et al., 2001, Jeukendrup et al., 2003). The nuance being elevated muscle glycogen utilization occurs in the heat if the intensity of exercise is submaximal and there is approximately a 0.5°C increase in core temperature. However, if core temperature remains stable despite the heat stress and exercise intensity, then carbohydrate utilization is unlikely to differ from exercising in temperate conditions (Febbraio et al., 2001). Interestingly dehydration in hot conditions, but not temperate conditions appears to increase carbohydrate utilization (Mougin et al., 2025). So what is driving this altered metabolic activity when exercising in the heat? A few reasons have been proposed such as reduced oxygen and fuel sources delivered to the working muscle due to redistribution of blood to the skin for cooling, altered muscle recruitment such that more fast twitch muscle fibers (more reliant of carbohydrates for energy) are recruited, altered enzyme activity due to an increase in muscle temperature, and increased sympathetic nervous system activity (Febbraio et al., 2001).
Physiological adaptations to the heat
Fortunately, humans are highly adaptable to the heat. The table below highlights the primary adaptations to the heat. The full timeline of heat adaptations can be broken into three periods of short-term (<7 days: most of the improvements in heartrate, skin and core temperature, and sweat rate are achieved during this period), medium-term (8-14 days: thermoregulatory benefits of heat acclimation are generally thought to be complete), and long-term acclimation (15-21 days: thermal tolerance continues to improve until maximal acclimation is achieved) (Garrett et al., 2011). It has been reported that 75-80% of heat acclimation occurs in the first 4-7 days (Pandolf, 1998, Shapiro et al., 1998).
Figure 2: The estimated timeline of various physiological factors in response to heat exposure over a 14-day period. Figure from Périard et al., 2016.
Table 1: Physiological adaptations to the heat. From Périard et al., 2015.
Total blood volume typically increases after 3-4 days of consistent heat exposure, and increases 5-7% in totality after full heat acclimation (Bass et al., 1955, Wyndham et al., 1968, Sawka & Coyle,1999, Patterson et al., 2004a). However, there is a range in response among individuals. For example, during acute heat acclimation, plasma volume can expand 3-27% although in some cases, an individual may not experience an expansion in plasma volume (Bass et al., 1955, Senay et al., 1978, Nielsen et al.,1993, Patterson et al., 2004a). This range is dictated by the intensity of heat stress, hydration status, skin temperature, and whether exposure was during rest or exercise (Sawka et al., 1983b, Harrison, 1985, Kenefick et al., 2014). This expansion of plasma volume is driven by the hormones (i.e. aldosterone and arginine vasopressin) that drive fluid retention, which are released due to dehydration that typically occur during exercise in the heat. As a result of elevated blood volume, the body is better able to “carry” and transport heat leading to improved core temperature management and heat tolerance, while also able to deliver adequate amounts of blood to the working muscle. Thus, cardiovascular strain is reduced as more blood is pumped per beat, equating to a lower heart rate compared to pre-adapted conditions.
It is still important to note that despite additional body water, fluid replacement is still critical and a primary concern for most individuals exercising in the heat. As a result of increased sweat rate, fluid and sodium loss becomes significant and can contribute to the decrease in exercise performance. As outlined in the table above, one of the early and key adaptations to the heat is the earlier onset of sweating to heat exposure, but also the dilution of sweat meaning less sodium is lost. It has been reported that sodium loss through sweat can decrease between 30-50% depending on the individual (Périard et al., 2015). This is due to the sweat glands' improved ability to reabsorb sodium before it leaves the body and hormone activity (Périard et al., 2015).
Adaptation to the heat leads to an alteration in the metabolic activity of the working muscle. It has been noted that muscle glycogen use can be reduced after heat acclimation (King et al., 1985, Kirwan et al., 1987, Febbraio et al., 2001, Xu et al., 2025). However, this sparing of muscle glycogen has been found to be minor and observed during exercise in temperate and cool conditions after heat acclimation (Young et al.,1985). Lactate accumulation in the muscle and blood has been observed to be lower resulting in increased power output at an individual's lactate threshold. This is thought to be due to the increase in plasma volume which may be a result of a greater volume of water able to circulate the blood through the liver where lactate can be removed (Rowell et al., 1968).
Finally, there are modifications to gene expression and cellular adaptations that occur with heat acclimation that increase the body's ability to tolerate heat stress (Sonna et al., 2002). This increase in thermal tolerance allows for humans to improve their ability to handle heat strain that they would not be able to otherwise handle in a non-acclimated state. The basis of this thermal tolerance is related to heat shock proteins which support, protect, and accelerate repair of cells from heat stress (Kregel, 2002). Heat shock protein production is stimulated by exercise in addition to heat exposure. Together, exercise and heat exposure lead to a greater expression of heat shock proteins and is also dictated by the intensity and duration of the heat stress (Skidmore et al., 1995, Febbraio & Koukoulas, 2000).
Summary
Exercise in the heat is physiologically challenging. This results in impaired performance in the non-acclimated state resulting in elevated carbohydrate usage and fluid loss. Fortunately, the human body is highly adaptable to the heat and can make physiological adaptations to better tolerate the heat. Regardless, the elevated carbohydrate usage and fluid loss by the body still remain. With this in mind, an individual's fueling and hydration plan should be modified in order to account for the additional carbs used and replenish the lost fluids through sweat. In part 2, we will discuss the practical applications of the information from this blog and provide recommendations for fueling and hydrating in the heat.
