TRAINING

Running Hot

Steve Bird

THE warm weather provides a great time to be out enjoying the environment. And as evidenced by full Summer-fixture lists, many of us will be competing in bush and street orienteering, as well as continuing to train throughout the hottest part of the year. However, whilst participating in regular exercise all year round is strongly advocated for health, as well as fitness, we must be aware that when exercising in hot and sunny conditions we need to take extra precautions, in order to do so safely. Obviously, some of the key concerns with exercising outside during the Summer are the risks associated with sunburn and exposure to UV. These are well known, highly publicized, and relevant whether we are exercising or not. So I won’t dwell on these, as the necessary precautions are equally well publicized. Therefore, the focus of this article will be on the specific issues associated with exercising in the heat, and what to do to reduce the risks of dehydration and hyperthermia: which may not only adversely affect our performance, but also pose a health risk. This is a perennial topic and one that has been given excellent coverage in previous articles by Gillian Woodward1-4 and Adrian Uppill5. However given recent incidents, including those reported in this magazine6 it is one that warrants revisiting.

Temperature control

In order for our bodies to function effectively, our temperature needs to be maintained within a specific and narrow range. To be more precise, it is our internal organs and muscles that need to be maintained at around 37oC (normal range 36.1 - 37.8oC). This is our internal or core temperature, and whilst small increases in muscle temperature of around 2 - 3oC are beneficial and enable our muscles to work more effectively, large increases tend to have the converse effect on our performance, and we get into difficulties if it rises above 40oC. If our core temperature rises too high we become hyperthermic, which is a serious and potentially fatal condition. Fortunately our bodies have a number of physiological responses that are very good at preventing this from happening, which means that we can comfortably exercise in quite a range of environmental conditions. But in extreme conditions such as exercising when its very hot (> 30oC), these thermoregulatory responses may struggle to keep our core temperature at the desired level, and we therefore need to be aware of the risks and minimize them through appropriate behaviour.

Maintaining a balance

Our ‘normal’ body core temperature of around 37oC is maintained by balancing the heat produced by our body’s metabolism with how much we lose. We lose heat mainly through the skin, with some additional losses via the air we exhale from the lungs. Incidentally, dogs don’t perspire in the way that we do and therefore they have to lose their excess heat via their exhaled air, which is why they pant when hot. For us, we lose heat through the skin via four processes: Radiation, Conduction, Convection and the Evaporation of Sweat. The first three of these mechanisms are dependent upon a substantial temperature difference between our skin and the external environment in order to be effective. Which means that they work quite well on cold days, but on hot days, when the temperature of the environment is quite close to that of our skin, these processes make very little contribution to our heat loss. So in hot conditions we are almost entirely reliant upon the evaporation of sweat for losing heat. This means that on hot days, even at rest, our thermoregulatory mechanisms are challenged with the task of evaporating off the heat we produce from normal resting metabolism in order to keep us cool. However, when we run strenuously, our metabolism can increase by about 800%, and therefore so does our heat production and we may struggle to lose this extra heat and risk increasing our core temperature.

It’s good to sweat

When our core temperature begins to rise, we start to sweat and more blood is sent to the skin, carrying with it the excess heat from our muscles and the core of our body. The evaporation of sweat off the surface of the skin, in which the liquid water is converted to water vapour, requires heat energy that it gains from our skin. The evaporation of 1 litre of sweat removes 580kcal of heat from the body and as it does so it cools the blood, which then returns to our core. So we have blood being heated up as it passes through our exercising muscles and core, it then loses this heat due to the cooling effects of evaporation as it passes through the skin, and then returns back to the core and muscles to once again collect excess heat. It is important to emphasise that it is the process of evaporating our sweat, or any other fluid on our skin, that cools us, not the process of sweating on its own. Sweat that simply drips off you has no cooling effect. To have a maximum cooling effect the sweat should be free to evaporate and not restricted. To aid the process of evaporation on hot days, clothing should be light and whilst also needing to protect from UV, should not restrict evaporation. If we are not able to lose heat in this way, our core temperature will increase and hyperthermia will rapidly ensue.

Conflict in the blood

So on hot days there is competition between the muscles that require large volumes of oxygenated blood in order to fuel their aerobic metabolism, and the skin, which also demands large volumes of blood, in order to effect cooling. Consequently the muscles may receive a reduced oxygen supply on a hot day, which is why endurance events, such as orienteering, are affected and times are generally slower on hot days.

Dehydration

During strenuous exercise on hot days, sweating rates can be between 1.5 – 3.5 Litres an hour whereas, even with the most assiduous drinking strategies, we cannot absorb fluids into our body at such a high rate. Note that the fluid needs to be absorbed from the gut into the blood before it can be of benefit, so just because you’ve drunk a litre of fluid doesn’t mean that its all in your system. Indeed we can only absorb around 150 ml every 15 minutes whilst exercising, so it will take a while for it to all be absorbed. Which is one reason why we should take lots of small drinks very frequently on hot days: in order to keep our system replenished and constantly absorbing the fluids that we need into the blood stream. (For details of what to drink and when, see the aforementioned articles by Gillian and Adrian). Dehydration is associated with a reduction in blood volume. This has two major implications. Firstly a reduction in the capacity of the blood to deliver oxygen to the muscles, thereby further impeding performance. Studies have shown that losing just 1.5 litres of body fluid will impair your running performance by about 4 – 5%, and that’s not counting any additional navigational errors caused by being less able to concentrate. And secondly, when the body becomes dehydrated, further sweating is impaired, and as a result there is an increased risk of overheating. This will again detract from performance or in more extreme cases may result in hyperthermia, collapse or even permanent damage to organs such as the kidneys.

Hot and humid

The risk of dehydration and over heating (hyperthermia) is greatly increased when the weather is humid as well as hot. This is because in humid conditions the sweat cannot evaporate very easily and hence its cooling effect is greatly reduced. So your temperature is liable to remain high and you sweat even more in order to try and compensate.

Take on fluids

Since sweating is the major means of temperature control on a hot day and we can lose several litres of fluid, it is important to be fully hydrated before you start an event. You will also need to drink during an event to try and reduce the risk of dehydration and on this point remember that the feeling of thirst is a relatively poor indicator of dehydration so you should drink on a hot day even if you don’t feel thirsty. This particularly applies to the early stages an event where you may be inclined to refuse a drink. Then thirdly, after an event competitors should drink copious amounts to replace the lost body fluid. Indeed on a hot day competitors may need to drink several litres of liquid in the hours following the event (so bring plenty with you). This post event drinking is essential because although the body can absorb water at a rate of about 150 ml every 15 minutes, in hot conditions your rate of fluid loss (sweating) will have greatly exceeded this and you may have lost several litres of sweat during an event. As a result you may complete an event in a semidehydrated state. For best effect and to avoid a ‘bloated feeling’ drinks should be taken ‘little and often’ both during and after an event. Although liquids may need to be drunk after an event, as a general rule alcohol is not recommended since it has a diuretic effect upon the kidneys (causes the production of more urine), which will counteract the rehydrating effect of the fluid. Therefore I suggest that as an initial means of replacing lost body fluids non-alcoholic drinks are most effective.

What to drink?

There is no absolute answer to this question. Water is good, but water containing a combination of electrolytes (salts) and some glucose (to replace used energy stores) is often better. But the best concentration of these will depend upon the individual, the environment, their state of hydration, the length of the event, and whether they are drinking, Pre-, Post- or during the event, indeed even how long they are in to it will affect what some people function best with. It also depends on whether your main concern is the replacement of glucose energy or fluid to prevent dehydration. The inclusion electrolytes helps to replace the salts lost through sweating. And as has been reported previously, excessive sweating over an extended period and fluid replacement with just water, can result in a lower than normal concentration of blood sodium (hyponatremia). This is not a common occurrence in a single event but can occur 6. Most sports drinks contain a small amount of electrolyte, and the amount lost in the sweat is around 1.5 g of sodium per litre. In most people’s diets there is ample sodium to replenish this, but if additional amounts are required then about a third of a teaspoon of salt in a litre of water should be adequate. Likewise for energy replacement a general recommendation is 4 - 8 g of carbohydrate per 100 ml, drunk as 100 – 150 ml every 15 minutes. Although as expertly outlined by Gillian 1 - 4, the exact content will depend upon the individual and the circumstances.

Fast and loose

When heat exhaustion and hyperthermia do occur as a general rule the victim should be quickly removed to a cool shady environment, clothing loosened or removed to aid heat loss and they should be given plenty of fluids to drink. Slightly salted water is often recommended and should be given in preference to very sweet drinks. In serious conditions medical assistance should be sought. Finally as a note of warning, there are certain misconceptions about the wearing of excessive amounts of clothing whilst exercising in hot conditions (often in an attempt to lose weight). This includes the wearing of ‘sweat suits’. It must be emphasised that the wearing of such clothing in hot conditions can result in excessive water loss and dehydration, which is potentially harmful and therefore cannot be recommended. Indeed there have been a number of fatalities caused by individuals dehydrating and overheating whilst exercising in such clothing. On a similar note: and whilst I wouldn’t want to rely upon the physiological accuracy of this incident it may serve as a precautionary warning about needing to sweat in the heat; some may remember the James Bond film Goldfinger, where the woman was covered in gold and died. My diagnosis - likely cause of death being hyperthermia due to the fact that she couldn’t sweat to keep herself cool in the hot conditions.

Adaptation

Exercising regularly in warm environments will enable your body to adapt and become more able to cope with the conditions. The reported adaptations included a more sensitized sweating response, in which you will begin to sweat sooner and at a lower temperature. This provides a more effective response to an initial increase in core temperature. So it’s a fallacy to think that sweating is an indication that you are unfit. Indeed quite the reverse. It’s also a fallacy to think that if you deprive your body of water during training you will be more able to deal with hot conditions. There is no research evidence to support this, indeed the reverse appear to be true, and you should apply the same hydration principles of drinking before, during and after training, as you do in an event.

Summary

Our bodies are capable of adjusting to and coping with a whole variety of climatic conditions and our ability to run in all weathers is largely due to the very effective temperature control mechanisms that we possess. However it is important that we do all we can to assist these mechanisms in their task to keep us at the correct temperature with the most important consideration being the wearing of appropriate clothing, ensuring a proper state of hydration, and sensible behaviour, such as training during the cooler part of the day if the temperatures, solar radiation, and humidity are extreme.

References

1. Gillian Woodward; Fluid: your secret weapon. The Australian

Orienteer, March 2002 p26. 2. Gillian Woodward; Preventing Fatigue During Long Workouts. The

Australian Orienteer, March 2005 p29. 3. Gillian Woodward; Summer: time to think Fluids. The Australian

Orienteer, December 2005 p47. 4. Gillian Woodward; Hydration – There’s more to it than fluids alone.

The Australian Orienteer, March 2006 p10 5. Adrian Uppill; Hydration. The Australian Orienteer, September 2003 p40. 6. Julie Flynn; Too much of a good thing. The Australian Orienteer,

March 2006 p8-9.

Professor Steve Bird is Director, Centre for Population Health, Sunshine Hospital, Melbourne. Steve worked with the Great Britain National Orienteering Squad for over 10 years and is now assisting the Victorian Junior Squad.