During a long run in hot weather, you do not want to dehydrate, because this would reduce your performances and put you at a higher risk of heat illness. You do not want to drink too much either, because you could develop hyponatremia, and die from it. How can you get it just right? Unfortunately, scientists are still arguing about it.
How your body keeps cool.
Whatever your body does, it produces heat, and the harder you work the warmer you get. It has different systems to get rid of the excess: conduction (transfer of heat to another solid object, for example to your bike’s handle bars), convection (using a gas or a liquid, such as when you are soaked or in a breeze), radiation (giving it off as infrared rays) and sweating.
The latter is the most important cooling system during exercise.
Thermo sensors in the skin, muscles and spinal cord send information to the anterior hypothalamus, an organ in the brain which acts as a thermostat. To keep your body cool, it will increase the blood flow to the skin, dilate the superficial veins and activate the sweat glands.
The brain will send signals to stop exercising when a critical core temperature is reached –probably about 40 to 41C. But of course it is smarter than that and it will slow you down well in advance when it “feels” that your temperature is going to increase too much.
Heat illness occurs if your body’s cooling mechanisms are overwhelmed by heat production and your core temperature rises above 41 C. In other words: it can happen when your brain has not sent the signal to slow down and stop, or when your cooling mechanism is not good enough. Heat illness ranges from cramps through heat exhaustion (nausea, headache, weakness...) to heat stroke (syncope, shock...) and death.
As sweating is your main tool for remaining cool during exercise, dehydration is a risk factor but it is not the only one. Other factors include a preceding viral infection, fatigue, obesity, poor physical fitness, some drugs and lack of acclimatisation. You can only lose heat when sweat evaporates, as all the drops that drip off are inefficient. They can cool you down slightly by convection, but using your own precious fluids for this is not cost-effective from your body's point of view. The system is therefore much less efficient when it is humid.
The problem with dehydration
To allow you to sweat while exercising, your cardiovascular system has not only to transport blood to the working muscles, but also to the skin.
As you are further losing fluid through sweat, the active muscles and the skin start to compete for the remaining blood plasma (the water part of your blood). Therefore your heart rate rises and blood is shunted away from your kidneys, liver and bowels.
To stay alive it is not only crucial to have enough blood but also to have the right concentration of sodium in it. Even if you are a “salty” sweater, you lose more fluid than salt and your blood becomes gradually more concentrated.
You are therefore facing two problems:
- You have to keep a healthy blood sodium concentration.
- You do not want a decreased plasma volume.
The problem with drinking
The remedy seems simple: drink.
But if you drink a lot, the sodium concentration in your blood drops, because drinks do not contain any significant amounts of salt. Normally your kidneys immediately start to produce more urine to keep the concentration right. But if at the same time your plasma volume is reduced because of sweating, your body faces a dilemma: produce urine and lose more plasma and therefore risk severe dehydration and collapse, or keep the water and risk hyponatremia (too little sodium in your blood). The choice will depend on how much volume you have lost, which means that if you are dehydrated your body goes for the second option, but the problem is that several people have died from it.
You body instructs the kidneys to conserve water via a hormone, called anti-diuretic hormone (ADH) or vasopressin. In some circumstances (pain, intense exercise, medication, drugs, stress...) the hormone is secreted even if you have enough plasma volume left, which puts you at risk of hyponatremia. This mechanism explains why so many slower runners suffer from this condition.
How much should I drink to be safe?
While everybody agrees that you need to drink to maintain a correct core temperature and to perform well, and that over-drinking can lead to hyponatremia, scientists are still arguing about how much we should drink during the run.
The viewpoint of the American College of Sports Medicine
According to the American College of Sports Medicine dehydration is the main danger. They state that you should avoid losing more than 2% of your body mass in sweat because otherwise you are at risk of heat illness. As there is an enormous variation of the volume of fluid lost in sweat, the advice is to weigh yourself nude before and after training sessions to determine how much weight you have lost. Since 1g is 1ml, you can then calculate how much water you have lost by sweating, after correcting for what you have drunk during and just after the run. That is the volume that you should have drunk, and if you do this on a regular basis you will get a good idea of how much you need.
There are two problems with this method:
- You also lose weight because you burn glycogen and fat for energy. Moreover, to store glycogen in your body, you have to bind it to some water. When you are burning it, the water comes free, and can be used by your body. In other words: body weight changes overestimate dehydration.
- When you run outdoors, you also cool down by giving off heat to the moving air around you (convection), which is an efficient cooling mechanism.
Another viewpoint
Prof Tim Noakes (University of Cape Town) and other scientists think that avoiding hyponatremia is essential while dehydrating slightly does not matter. Their calculations show that hyponatremia is very unlikely to develop due to salt losses through sweating, even during an Ironman race. It is therefore best to drink to your thirst. This will allow you to avoid excessive dehydration without risking hyponatremia, but it is not as easy as it sounds, because exercise suppresses thirst. You have to learn to listen to your body and to trust it.
Remember that everybody is different, and you should discuss this with your GP if you have concerns.
Which drink?
Sports drinks do not contain enough sodium to prevent hyponatremia. To rehydrate fully you need to have food (which contains electrolytes) and drinks.
Choose a drink that you like, because if you don’t you will not drink it. Medical studies on rehydration of people suffering from diarrhoea have shown that if you add some salt and carbohydrates to water it is better absorbed. Taking carbohydrates during an endurance event also delays the onset of fatigue and improves performance.
The basic recipe for a home-made drink would be between 2% and 8% sugar (20g to 80 g for each litre) and a pinch of salt. You can for example mix orange juice and water in equal amounts. Or prepare 500 ml cold caffeine-free tea; add lemon or orange juice and two tablespoons of sugar. You can add a pinch of salt to both drinks if you like the taste.
References:
Jack H. Wilmore, David L. Costill, W. Larry Kenny: Physiology of Sports and Exercise; Human Kinetics 2008, ISBN-13 978-0-7360-5583-3
Tim Noakes, IMMDA: Fluid replacement during marathon running; Clin J Sport Med 2003 Sep; 13(5):309-18
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