Heat Acclimation

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Top Contributors - Kapil Narale, Uchechukwu Chukwuemeka and Vidya Acharya  

Introduction

Heat Acclimation is the body's process of adapting to the heat. Heat acclimation is a process that can occur to individuals at any age. Individuals who desire to achieve maximal heat acclimation must exercise in the heat, although they can achieve the heat acclimation in a cool environment. Exercising in the heat poses a demand in maintaining normal body temperature and fluid balance.

Heat acclimation occurs when the body physiologically adapts to the hot environment, and can maintain homeostasis. The adaptations improve exercise tolerance in hot environments and reduce the risk of heat injury.

It is interesting to note that heat acclimation can occur within 1-2 weeks.

Body temperature needs to be regulated within a homeostasis value of roughly 37 degrees Celsius, since cellular structures and metabolic pathways are affected by body temperature. An increase in body temperature to 45 degrees celcius can be quite dangerous to the body, as it can lead to an inability to produce cellular energy, and lead to cellular death, and death of the organism. Therefore heat loss of the body needs to equal heat gain in the body.

The homeostasis of the body can be referred to as the human 'furnace' or 'thermostat'. The body's temperature is relatively high, and is regulated with gradual heat loss.

Exercise in a Hot Environment

Hot environmental conditions decrease performance in both submaximal (marathon or triathlon) and high-intensity exercise (rugby or soccer).

There are three major contributors to impaired performance in the heat:

1. Quicker muscle fatigue
2. Impaired cardiovascular function
3. Impaired central nervous system (CNS) function

These factors can separately contribute to impaired exercise performance, though they are more likely to work together to result in an impaired exercise performance. These contributions depend on the exercise duration and intensity.

Quicker muscle fatigue

There are three major changes that occur in muscle metabolism that interact to cause muscle fatigue:

1. Muscle glycogen breakdown is accelerated - this contributes to muscle fatigue during prolonged exercise
2. Muscle lactate production is increased with exercise in a hot environment, thus decreases pH and contributes to muscle fatigue
3. Free radical production in skeletal muscles is increased with exercise in a hot environment, which can also contribute ot muscle fatigue

The effect of the CNS also can cause a decrease in the activation of motor units, and thus a decrease in muscle force production.

Physiology

If heat loss is less than heat gained, then there is an increase in body temperature, thus body temperature rises. This is case where homeostasis needs to be maintained.

Contracting skeletal muscle produce a large amount of heat, thus prolonged exercise in hot environments poses a challenge to maintaining homeostasis.

An important function of the circulatory system is to transport heat. Blood is a facilitator in transporting heat, since it has a high capacity to store heat. Whe the body wants to lose heat, blood flow is increased to the skin to facilitate heat loss to the environment.

Individuals who are heat acclimated have a lower body temperature and a lower heart rate with submaximal exercise. It is recommended that individuals should partake in strenuous interval training or continuous exercise at an intensity higher than 50% VO2max to facilitate the higher core temperature, since a higher core temperature is what drives heat acclimation.

Initial physiological changes that take place with heat acclimation are:

• Increased plasma volume
• Earlier onset of sweating
• Higher sweat rate
• Reduced salt loss in sweat
• Reduced skin blood flow
• Increased synthesis of heat shock proteins

As mentioned heat acclimation can occur quickly, in as fast as 1-2 weeks.

Some of these physiological changes will be explained:

There is a 10-12% increase in plasma volume. This maintains central blood volume, stroke volume, nd sweat capacity, and allows the body to store more heat with a smaller temperature gain.

There is an earlier onset of sweat production, and an increased sweat rate. This indicates that sweating begins right as soon as the exercise starts, with less heat storage at the beginning of exercise, and decreased core temperature. Infact, heat acclimation can increase the sweat volume to almost 3x the amount without being acclimatized to the heat.

Higher amounts of evaporative cooling in the body is possible, which is a benefit in minimizing the storage of heat during prolonged work.

Environments with high heat and humidity make the body less able to lose heat by radiation/convection and evaporation. The inability to lose heat produces a greater core temperature and a higher rate of sweat loss., compared to a more comfortable environment. With fluid loss, and a high core temperature, together, there is a risk of hyperthermia and heat injury. More information on heat injuries and illnesses is discussed on this page, Heat Illness in Sports.

As mentioned on the Physiology of Sweat page, the only way to lose body heat, when exercising in hot environments, is through through the evaporation of sweat.

See the page Physical Activity and Perspiration for a section on sweat rates and exercise.

Effects

Heat gain of the body, and thus effects of exercising in the heat, would occur when environmental temperatures are greater than skin temperatures.

Exercise in the heat, for 10-14 days, at

• low-intensity, <50% VO2 max, and a long duration, 60-100 mins, or
• moderate to high intensity, 75% VO2 max, and a short duration, 30-35 mins,

results in,

• increased plasma volume, blood flow, perspiration
• increased VO2 max, maximal cardiac output, power output at the lactate threshold
• reduces body temperature and heart rate responses with submaximal exercise
• reduces salt loss on sweat and the occurrence of sodium depletion
• increases aerobic fitness capacity and performance

This is the process of acclimation and helps with safeguarding against heat illnesses.

As noted the effects of heat acclimation take about 7-14 days of consecutive exercise to occur. Individually, the timeline ot acclimation of these effects are:

• Decreased heart rate, within 3-7 days
• Increased plasma volume, within 3-6 days
• Decreased perceived exertion, with 5-9 days
• Increased sweat rate, within 8-14 days

Benefits

As mentioned above, with the higher sweat production and rate from heat acclimation, the storage of heat is minimized with prolonged activity, and there is a higher rate of evaporative cooling in the body.

With heat acclimation, there is a lower loss of sodium and chloride, from an increased secretion of aldosterone. Despite this adaptation resulting in a lower loss of electrolytes during exercise, there is still definitely a need to replace the water loss during exercise, especially prolonged exercise in the heat.

With becoming heat acclimated, not only does one have a lower body core temperature and heart rate throughout their exercise duration, there is a decreased rating of perceived exertion, and an improved exercise performance in the heat.

Heat Shock Proteins

With heat acclimation, there is cellular production of heat shock proteins in skeletal muscle fibers, the heart, and all other cells in the body. These 'stress' proteins are produced as an adaptation to the heat stress, and they protect from cellular damage from stresses such as heat. They protect cells from thermal injury by stabilizing and remodeling damaged proteins.

Risks of prolonged exposure to the Heat

As mentioned on the Heat Illness in Sports page, prolonged sporting activities in the heat can cause adverse reactions, which could even result in death. It is important to recognize signs and symptoms, and prevent the onset of these illnesses rather than treat or manage the conditions.

Loss of Acclimation

Similar to the use it or lose it principle, there is a rapid loss of heat acclimation with the cessation of exercise.

There is a decrease in heat tolerance within a few days of no activity in the heat. Heat tolerance is significantly reduced within 7 days of no heat exposure. Heat tolerance completely removed after 28 days without heat exposure.

To maintain heat acclimation, one must have continuous or repeated exposure to the heat.

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