As we’re in the midst of summer, it is more important than ever to stay properly hydrated for optimum health and performance. But why?

Considering the human body is made up of roughly two-thirds water it is simple to see why we need to replace fluid lost so that the body and mind can function optimally. You have all heard that you should be drinking the equivalent of two litres of water per day. However, when exercising in hot conditions, this amount is still not enough given that exercise in hot environments can cause sweat rates as high as 2.5 litres per hour!

Summer sports competitions usually take place in environments whereby skin temperatures are kept at around 31-32^◦C. During these conditions it is usually dehydration rather than the heat itself that makes you feel drained and exhausted. Even in much cooler temperatures of around 7-18^◦C, dehydration of 3-5% of total body mass losses can cause your core body temperature to rise into dangerous figures. So, given that people are less able to tolerate dehydration in the heat, running dehydrated in hot conditions would cause a rapid rise in body temperature, leading to an early dropout.

With the advent of pre-cooling methods such as ice-baths and cooling jackets, as well as the process of heat acclimatization, coaches see the need to protect their athletes’ from the heat in order to gain a competitive advantage. However, the importance of simply being hydrated during such conditions is often being overlooked.

The basics of temperature regulation

Whether at rest or during exercise, your body is constantly working to keep itself at a temperature of 37 ^◦C. This is typically achieved through sweating and increased blood flow to your skin. When we sweat and it is evaporated heat is lost, likewise, when blood flow to the skin is increased, heat is radiated from its surface. These processes are ultimately controlled by a little part in your brain called the hypothalamus.

Dehydration on performance in the heat

Research has shown that when exercising in the heat there is a critical core temperature of roughly 40^◦C at which people experience heat exhaustion and collapse. In less fit runners this is even lower, about 38.5-39^◦C, creating a bigger problem for such individuals. It is therefore important that when running in the heat you do as much as you can to keep your core temperature down and avoid fatigue. 

When we are dehydrated the amount of circulating blood can decrease by up to four litres per minute, reducing the amount of blood to the working muscles. While research has surprisingly shown that this decrease in blood volume does not reduce oxygen delivery to muscles, as oxygen concentration within the blood increases, running performance can be impaired by an increased perception of effort, and by the increased amount of muscle glycogen being used for energy. The latter problem also being a huge problem for the marathon runners among you who are trying to avoid hitting the wall. Also, given the importance of blood flow to the skin for heat loss, it makes sense that this function is reduced with the drop off in blood volume, which in turn, would cause body temperature to rise.

Dehydration can also decrease your sweating responses during exercise due to less fluid availability to the sweat gland. While sweating causes water loss, it is essential in keeping us cool, and even more so in conditions greater than 36^◦C, in which sweating becomes the main method of heat loss. As a result, drinking plenty of water can slow your rise in core temperature down, and maintain performance.

While the majority of research indicates a negative impact on middle and long-distance running through hyperthermia and dehydration, in which oxygen supply is crucial for optimum performance, sprinting does not seem to be affected as long as one is properly hydrated before exercise. However, research has shown that repeated sprinting, which occurs in sports such as soccer and rugby, are affected by rises in core temperature.

How much water should I drink?

What I’ve done there is that I’ve made a relatively long-winded argument for drinking enough fluid to optimise performance or training. I will now get onto how much you actually need. A couple of factors influencing the effectiveness of a drink as a fluid replacement are the rate at which the drink is emptied from the stomach, and then absorbed in the intestine.

When possible, fluid intakes should match the fluid losses by sweating, which would depend on a number of factors including running time, genetics, acclimatization, running speed, and the ambient temperature. It has been shown that the amount of fluid emptied from the stomach is proportional to the stomach volume. Meaning that the more water in the stomach, the quicker it will empty, up to at least 600 ml anyway. Because of this, it has been previously recommended that you maintain this amount of fluid in your stomach during a run by repeated drinking. With that being said, the additional 0.6 kg been carried around in your stomach might be uncomfortable and would tend not to favor those of you who might be competing at longer distances. At the same time being dehydrated by 1% or more may also negatively affect your performance. It is therefore necessary to find an optimal amount of water replacement that would favor both running performance and health. It is also important not to wait until you are thirsty before having a drink because by then you will already be dehydrated!

For these reasons it is recommended that you drink 200-300 ml of water just before exercise, then 400-800 ml every hour during exercise, with the higher amount for larger or faster individuals. This is a good starting point for which you can begin a trial-and-error process by which you can tailor your fluid needs based on your individual fluid losses. This can be easily gauged by weighing yourself nude before and after exercise to give you a good idea of how much water you lost. The closer the two weights are to each other the better. Whereas if your weight is more than two percent lower after the run, then you really need to be drinking more during exercise. Observing urine colour is a simpler test of hydration. The paler, clearer urine colour as opposed to a darker one is a good indicator of being hydrated.

When it is practically impossible to consume fluid during an event (i.e. middle-distance races), simply ensuring your are sufficiently hydrated prior to performance. For weight training, consuming 400-800ml per hour will suffice; adjusting this amount to your own needs.

You should be careful not to overdo your water intake as this can lead to a condition called hyponatremia, in which sodium chloride (the equivalent of table salt) levels in the blood are diluted due to the increased water intake. This most typically occurs in slower runners who stop at every water station to have a drink, and though this condition is rare, especially when exercising in the heat, symptoms can range from mental confusion and in extreme cases, death.

So, get into the habit of drinking more every day and around your training, and not only will your performance improve, your health will too!