Dehydration: How lack of water affects your body

With the warmer climate approaching, it is increasingly important to stay properly hydrated, both for maintaining good health and optimum performance levels.

The human body is made up of roughly two thirds water so it is easy to see why we need to replace lost fluid for the body and mind to function optimally. Generally, we need to drink two litres of water per day (or 35ml per kilogram of bodyweight), but when exercising in hot conditions, this amount is not enough. Exercise in hot environments can cause sweat rates as high as 2.5 litres per hour, so the amount of water that we take in will need to increase.

Competing or training in the summer can be tough as skin temperatures are kept at around 31-32 degrees. During these conditions, it is usually dehydration rather than the heat itself that makes you feel drained and exhausted. So, given that people are less able to tolerate dehydration in the heat, training dehydrated in hot conditions causes a rapid rise in body temperature, leading to poor performance.

Temperature Regulation

Whether at rest or during exercise, your body is constantly working to keep itself at a temperature of 37 degrees. This is achieved through sweating and increasing blood flow to the skin. When we sweat and it evaporates, heat is lost; similarly, when blood flow to the skin is increased, heat dissipates through its surface.

Effects of Dehydration on Performance

Research suggests that when exercising in the heat, the critical core temperature at which people experience heat exhaustion is about 40 degrees. In less fit runners, this would be more like 38.5-39 degrees and would prove a bigger problem for such individuals. It is important, therefore, that when competing in activities such as running, you do as much as you can to keep your core temperature down and avoid fatigue.

When we are dehydrated, the amount of blood circulating the body can decrease by up to four litres per minute, reducing the amount of blood to the working muscles. Research has shown that this does not decrease the amount of oxygen delivered to the muscles (as oxygen concentration within the blood increases to compensate), but performance can still be impaired by an increased perception of effort and by the increased amount of glycogen being used for energy. It does, however, mean that your body prioritises the delivery of blood to the muscles and less to the surface of the skin, which means that it is more difficult for heat to radiate from the surface for cooling.

Dehydration also decreases your sweating responses during exercise due to less fluid availability to the sweat gland. While sweating causes water loss, it is essential for keeping us cool, especially in high heat where it is the main way the body is able to cool itself. As a result, drinking water allows you to slow the rise in core temperature and maintain performance.

But How Much Water Should I Drink?

A couple of factors influence how effective fluid replacement can be: the rate at which the drink is emptied from the stomach and how it is then absorbed in the intestine.

When possible, fluid intake should match the fluid lost through sweating. This depends on a number of factors: training time and duration, genetics, acclimatisation and ambient temperature. It has been shown that the more water in the stomach, the quicker it will empty. Due to this, it was recommended that you maintain around 600ml in your stomach during competition or exercise. However, carrying that amount of water can be uncomfortable, especially if you are working on endurance exercise. But being dehydrated by as little as 1% can negatively affect your performance. The key, therefore, is to find the optimal balance between replacing water for health and for performance.

It is recommended, therefore, that you drink 200-300ml of water prior to exercise and then 400-800ml every hour during exercise. Obviously, this is not a one-size-fits-all but it is a good starting point which you can adjust for personal requirements.

So, get into the habit of generally drinking more every day and especially around training since this will not only improve your performance but also your general vitality too.


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