By Dominique Di Vincenzo , head doctor for UTAT
As a homeothermic living being, human must maintain a constant central temperature. In order to do so, it is essential that the addition of the heat produced and the heat captured in the environment is equal to the losses of heat. The heat produced by the body is called endogenous heat.
It is produced:
On the one hand, by the basic metabolism, that is to say the energy used for the functioning of the human body at rest; this is the energy necessary for breathing, for the functioning of the heart, for digestion…
On the other hand, by the muscular contraction during the effort
Exogenous heat corresponds to heat exchanges with the environment, including both heat gains and losses. These heat exchanges are carried out using four mechanisms: Convection, Conduction, Radiation, Evaporation
These heat exchanges can only take place if there is a temperature gradient, i.e. a temperature difference between the body and the environment.
Convection allows both gain and loss of heat. Heat exchanges are made in contact with air.
If the temperature of the air is lower than that of the skin, there is therefore a temperature gradient between the two allowing an elimination of heat from the body. The effects of convection can be felt during warm weather when you ride your bike. You drive fast, the air circulates quickly around you, which tends to refresh this air: you then feel a sensation of fresh. As soon as you slow down, the air becomes stifling again, the loss of heat is then diminished.
Conduction also allows gains and heat losses. The exchanges take place in direct contact with a liquid or a solid. The typical example is that of swimming, where the whole body is in contact with water which is at a temperature lower than the body. There is therefore a loss of heat which can become very significant in cold water given the exchange surface. Hence the importance of the isothermal suit. The reverse example of that of the hand placed on an electric plate: the gain of heat is immediate.
Radiation allows also heat gains and losses. Any body emits radiation: the human body, the sun, the earth, plants, minerals … The exchanges of heat are thus made by reception and emission of these radiations.
Evaporation only allows the loss of heat by elimination of the water contained in the organism. Three mechanisms allow the evaporation of:
The exhalation of air saturated with water vapour during breathing
The perspiration which is the elimination of water on the surface of the skin without intervention of the sweat glands
And finally, sweating which is the elimination of sweat consisting of water and minerals and that is produced in the sweat glands located under the skin.
At rest, endogenous heat –the heat produced by the body– comes only from the basic metabolism and is evacuated by:
Convection plus radiation (75%)
Evaporation (25%) of which 2/5 by breathing and 1/5 by perspiration and sweating
On exertion, endogenous heat can be considerably increased. It is no longer exclusively produced by basic metabolism. Muscular contraction then takes a large part in the production of heat. Indeed, the energy produced from carbohydrates and lipids is transformed:
In mechanical energy used for physical exertion for 25%
While 75% of this energy is converted into heat. This excess heat is then evacuated very predominantly by sweating (80%).
The above four heat control mechanisms allow heat exchange between the environment and the surface of the skin. However, endogenous heat is produced inside the body and must therefore be routed to the skin surface. It is the role of blood circulation to transport heat. The blood is always warm and allows to transport heat from the inside of the body (the core) to the periphery (the envelope) and vice versa.
At the so-called noble organs (the brain, the heart, the lungs and the abdominal viscera), there is always a minimum amount of blood circulating to bring them the amount required for them to operate. On the other hand, for all the other organs, the blood volume passing through them depends on their level of activity:
It increases when the organ functions in order to bring it the oxygen it needs.
It sometimes diminishes very importantly when the organ is at rest.
Thus, the blood flow of the digestive system increases during digestion. Similarly, on exercise, muscle blood flow increases significantly.
These few notions concerning the distribution of the total blood volume have a direct effect on the thermal regulation which will be carried out largely by variations in the blood volume of the subcutaneous vessels.
At rest, the blood volume present in the subcutaneous vessels represents only 5% of the total blood volume.
In a cold environment, the temperature of the skin is higher than the temperature of the environment. There is therefore a temperature gradient between the skin and the environment which favours the loss of heat by convection and by radiation.
There is then a risk of a drop in core temperature. Since human being is a homeotherm, it must maintain its central temperature constant.
What is happening to prevent core cooling?
The size of the subcutaneous vessels will shrink, it is the vasoconstriction. Since blood transfers heat, vasoconstriction results in a reduction in the cutaneous blood volume and thus a limitation of heat loss.
Moreover, this quantity of blood which does not leave in the subcutaneous tissues is redistributed to the deep zones allowing the maintenance of the heat of the core.
Thanks to its two adaptations we have:
A cold envelope
A constant temperature core
In warm conditions, the temperature of the skin is lower than the temperature of the environment. There is therefore a temperature gradient between the skin and the environment which favours the gain of heat by convection and radiation. There is then a risk of increasing the core temperature.
What happens to avoid a warming of the core?
The size of the subcutaneous vessels will increase: it is vasodilation. The cutaneous blood volume increases thus allowing excess heat to be transported from the core to the periphery to be removed by convection and radiation.
Often, convection and radiation are sufficient to control the excess of heat: evaporation is then started.
Thanks to these two adaptations, we have:
A hot envelope
Maintaining the core at constant temperature
At work in a warm environment, the excess heat captured in the environment adds up the heat produced by the muscular contraction, which constitutes a considerable excess of heat to be evacuated (75% of the energy produced for the muscle contraction turns into heat). So sweating is systematically put in predominant action (up to 80% of the elimination against less than 5% at rest).
In hot and wet conditions, sweating can only occur if there is a gradient of moisture between the skin and the environment. On exertion, the moisture level of the skin is very high due to sweating. If the air is dry, there is the gradient necessary to eliminate the water present on the surface of the skin. On the other hand, if the air is moist (as in Hawaii), the difference between the moisture at the surface of the skin and the humidity of the ambient air is too low. The water accumulated on the skin can no longer evaporate preventing the elimination of excess heat. There is a significant risk of hyperthermia.
The first recommendation that arises from these physiological reminders when performing a hot and humid test is:
Do not water because you add to the surface of the skin water that on the one hand will heat, but on the other hand will prevent the elimination of sweat that continues to be manufactured.
It is therefore necessary to soak so as to eliminate water on the surface of the skin thus allowing the sweat that continues to be produced to spread on the surface of the skin.
The second recommendation in the course of this type of race is to ensure rigorous hydration. Indeed, sweating results in large water losses of up to 12 litres per 24 hours. The losses must be absolutely compensated by a suitable water supply.
In case of insufficient hydration, the risks are multiple:
Hyperthermia: in fact, the body no longer has enough water to generate sweat and therefore can no longer eliminate excess heat. The temperature of the core will start to rise causing a malfunction of the internal thermostats.
Dehydration: major losses of water result in a decrease in total blood volume resulting in the risk of cardiovascular failure
It is therefore imperative to hydrate abundantly with the following reserve, the maximum absorption capacity of the stomach being 1 l / h, it is recommended to drink small amounts very regularly.
Also note that the appearance of the thirst sensation is offset from the state of hydration in other words when you begin to feel thirst, your body is already in a state of dehydration.
Another recommendation concerns the wearing of clothing
Choose them wide to favour convection heat losses.
Choose them clear to limit heat gain by radiation.
These physiological findings also have implications for how to manage diet during these long distance events. Remind that whatever the level of activity, the body redistributes permanently a minimum part of its total blood volume to noble organs.
It should also be remembered that, in a warm environment, the blood volume in the subcutaneous tissue is greatly increased and that, on exertion, a large part of the total blood volume is redistributed to the muscles. Now, the total blood volume contained in the body varies only in very small proportions between rest and effort. This results in a very large drop in the blood volume to be distributed to the other organs and in particular to the digestive system.
This hypoperfusion is responsible for a lack of oxygen at the digestive level at the origin of the digestive disorders that can be encountered on the long distance tests. Hence the importance of continuing to eat throughout the race, to continue to activate a minimum digestion and thus stimulate a minimum of blood supply to the digestive tract.
Finally, since the human body is full of resources, it is possible to gradually adapt the body to physical exercise in a warm environment: this is the phenomenon of acclimatization. The repetition of workouts in warm environment allows the implementation of cardiovascular adaptations and sweat adaptations.
HOW TO ACHIEVE THE ACCLIMATIZATION?
Cardiovascular adaptations are carried out by:
A reduction in the amount of subcutaneous blood volume to the benefit of the muscles. The muscles are then more efficient because of a greater O2 intake.
An increase in the total blood volume allowing a better removal of heat by sweating
A reduction in the heart rate at the effort, which leads to a saving in energy expenditure and a saving in heat production
While sweating adaptations are due to:
An increase in the ability to sweat
A shortening of the time between the beginning of exertion and the begining of sweating favouring better heat removal by sweating
A reduction in the loss of minerals limiting the risk of dehydration
Adaptations result in a decrease in body temperature and a delay in the beginning of fatigue.
Cardiovascular adaptations take place in three to five days, and at least 10 days are required for sweating adjustments.
For acclimatization to occur, it is imperative to reduce the intensity of the sessions to 60-70% of the training plan initially planned for about two weeks.
For example, an initially scheduled 1½ hour running session at 145 bpm should be done around 100 bpm (70% of 145 bpm). The sessions should be divided according to their intensity. The sessions performed at a high intensity (fractional, threshold …) must take place in the morning in the cool.
Only sessions of lower intensity (endurance) should be done at the hottest time of the day. It is also important to use heart rate (not speed) as the intensity of the sessions because, at the beginning of the acclimatization period, the speed will be lower than the rate the athlete is used to for a given cardiac frequency range. Once the adaptations are in place, the athlete will return to his usual performance level for a given cardiac frequency range.
The acclimatization period of 15 days of warm and humid training must be in the last month before the race, with the need to stay the last week in a warm and humid environment.
Indeed, 50% of gains during the acclimatization period may disappear if the athlete quits the warm environment.
The period of sharpening (usually about seven days) should be reduced to five days. Indeed, during the sharpening period, the progressive reduction of the training volume is responsible for a decrease in the total blood volume. It is then essential to minimize this decrease in blood volume and to promote and anticipate the beginning of dehydration on the day of the race.
This post is also available in: French