The following is an extract from the Extreme Physiology chapter of our Jungle Marathon title.  The book is currently available to buy online at Amazon and Waterstone's.

  

Extreme Physiology:

Exercise in Hot and Humid Conditions

 
 
The Jungle Marathon presents a unique challenge to its competitors. Of all of the extreme environments in which we can perform physical activity, the hot and humid jungle is one of the most challenging, perhaps second only to high altitude climbing (i.e. to summit Mount Everest without oxygen).
   Many Jungle Marathon competitors may have participated in other multi-stage, ultra-endurance adventure races around the world. It cannot be stressed strongly enough, that the jungle environment is unique. Mile for mile, it is likely to be the toughest adventure race in the world. Exhaustion, due to poor heat management, is the single biggest factor to prevent competitors completing the JM, followed by physical injury (i.e. muscle damage) and external physical factors (serious bites, stings, infection, etc.). The humidity encountered in the jungle severely compromises our ability to cool ourselves, the consequences of which can be disastrous.
 
 
The Physiology of Temperature Regulation
During Exercise
 
 
Heat gained during exercise must be matched by heat lost, in order to preserve the body's core temperature of 37 degrees Celsius. If the core temperature rises above 38 degrees, then the athlete will begin to feel tired and performance will decline. If this rise is not corrected, then the athlete will become too exhausted to continue. Ultimately, the hypothalamus in the brain will cause the individual to collapse, so as to prevent them from causing further damage.
   Heat gain is affected by external factors such as the ambient temperature, wind speed, humidity, solar radiation, ground thermal radiation, and clothing. Internal factors that affect heat gain include the metabolism and muscle activity. The greatest concern for the endurance athlete in the jungle is the combination of prolonged exercise in a hot and humid environment, with little or no wind. Sweating is compromised by the saturated air, which severely inhibits our ability to lose heat.
   Depending upon the intensity, physical activity increases metabolism 5-15 times above normal resting levels. Approximately 70-90% of the energy produced is heat, which needs to be dissipated to maintain normal functioning. The proteins and enzymes required for life processes begin to slow down when the cells approach 40 degrees Celsius.
   Heat is usually lost in our breath and via the skin. The heat in the core of the body is taken to the blood vessels near the surface of the skin, from where that heat is then transferred into the cooler outside air. The problem in the jungle is that the outside air is hotter than our skin temperature, which means that our body's normal mechanisms to cool down do not work!
   The only means by which the body can cool itself during exercise in a hot environment is via sweating, and this mechanism is far less effective if the air is very humid. Sweating works by moving water from the blood and glands out onto the skin. From there the water evaporates into the surrounding air, and the energy involved has a cooling effect on the skin. The problem is that if the air is saturated, then there is nowhere for the sweat to evaporate to. If the body cannot cool itself, then the core temperature will rise and exhaustion will very quickly ensue.
   The body attempts to maximise cooling by increasing the rate of sweating. The body produces sweat in an attempt to cool itself, but there is a very limited effect. In this case, not only should the athlete be concerned about the amount of heat that they are gaining through exercise, but they should also be concerned about the amount of water that they are losing through sweat.
   The sweat comes from liquid in the blood. The blood carries plasma, which is more than 90% water, along with various nutrients and waste products, in addition to cells (red cells, white cells and platelets). When we sweat, we begin taking some of that water away, and we have to compensate for it. The blood is naturally 'topped up' by water that exists in and around the cells of the body. When this happens the cells will be less 'healthy' and will no longer be able to function as well as if they were perfectly hydrated. Water regulation is managed by the electrolytes, such as sodium, which are lost in varying concentrations in the sweat. Sodium, potassium, chloride and other electrolytes, are all kept within very particular concentrations within and without the cells of the body. If water is lost from the blood, the cells, or the spaces in between cells, then this will affect the concentration of electrolytes in those areas. To help prevent imbalances, electrolytes are lost in the sweat to maintain internal balance (homeostasis).
   Unless the lost water is replaced, the volume of blood in the body will decrease. This has serious repercussions for the whole cardiovascular system. Our heart rate is determined by how much blood our heart pumps around the body each minute. Because there is less blood, the heart has to pump faster to compensate. If one is used to exercising at a particular workload (i.e. speed), then the heart will now have to beat faster to be able to maintain that workload, or else one will have to slow down in order to maintain the usual exercising heart rate. If exhaustion is usually achieved at a particular heart rate, then it will be reached much sooner if blood volume is reduced.
   So, we are at risk of exhaustion as our temperature rises above 37 degrees, and we are at risk of exhaustion due to an increasing heart rate. Not only this, but more and more blood is being directed to the skin, so as to aid cooling. In doing so, blood flow to the working muscles, as well as the organs, is declining, which creates an additional limiting factor to performance. This increased blood flow to the skin has also been found to decrease blood pressure, and to further increase heart rate in an effort to compensate.
   The increased heart rate is an attempt to maximise cooling whilst still preserving other requirements, which in our case includes exercise. However, the attempt by the heart to preserve the total amount of blood pumping around the body each minute eventually fails, and this value, our cardiac output, begins to decrease. Our heart is beating faster, but it is still not sufficient to meet requirements for exercise. As a general rule, heart rate will increase by four beats per minute, for every one percent increase in dehydration.
 
 

Managing Hydration

 
Maintenance of normal levels of hydration (euhydration) is essential for optimal exercise performance. This allows for effective heat dissipation, via increased skin blood flow and sweating rate, and maintains blood volume (and therefore blood pressure, heart rate and cardiac output).
 
   Our capacity to perform endurance exercise becomes significantly reduced once our body has become dehydrated to 2-3% of total body weight. Dehydration must be avoided by matching fluid consumption with fluid losses (primarily urinary and sweat losses). Some athletes have attempted to 'hyperhydrate' in an attempt to increase their water stores and prolong exercise. Early studies that supported this were generally flawed, as they tended to compare hyperhydrated subjects with dehydrated ones. Hyperhydration is only a transient state, as excess water is quickly removed from the body. Maintaining normal hydration is far more effective during prolonged exercise than attempting to hyperhydrate beforehand.
   Athletes can lose anything from 1-2.5 litres of water per hour during exercise in the heat. With such a huge amount of variation between individuals, it is strongly advised not to consume fluids based solely upon general recommendations.
   Fluid replacement needs to be based upon individual requirements. The goal of fluid replacement during exercise is to maintain a level of hydration within 2% of normal body weight. Thirst is generally not perceived until an individual has already reached 2% dehydration. Studies have shown that athletes that remain effectively hydrated will have lower core and muscle temperatures than those that are dehydrated. Efficient fluid replacement during prolonged exercise reduces carbohydrate usage, improves cardiovascular function and temperature regulation, and improves athletic performance. Following exercise, any water deficit should be compensated for within two hours of finishing. This will allow the cells to work optimally again, and facilitate effective recovery.
   On an individual level, each athlete must learn to match his or her own fluid intakes with fluid losses. This can normally be managed by measuring changes in body weight. Typically, the amount of fluid ingested should be 25% or so greater than that which is lost, so as to account for subsequent sweat and urinary losses. Even if such calculations were made in advance, the novel environment of the jungle would create different requirements to those associated with other climates. For this reason, each athlete should be attentive to his or her own needs, and seek advice if they are not confident that their strategy is effective.
 
 

Conclusions

 
Endurance exercise in the jungle presents some unique challenges to the athlete, far greater than those experienced in most other environments. The key is to prevent core temperature from rising too far. Once it has risen more than a degree, everything becomes less efficient, including how one cools oneself, how the cardiovascular system functions, and how the muscles work and produce energy. Only a small level of dehydration is necessary to begin this process, and the effective maintenance of body water levels is essential for managing body heat, optimal health and exercise performance.
 
 
The full version of this article can be found within our Jungle Marathon title, currently available to buy online at Amazon and Waterstone's.
 
 

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