The effects of dehydration on athletic performance

It is well known that what you eat can affect how you perform, but what are the effects of dehydration on athletic performance?

In physiology, dehydration is a deficit of total body water (Mange et al., 1997). Dehydration can be divided into two categories: voluntary and involuntary dehydration. Voluntary dehydration is much less common in the realm of sport, generally only occurring when athletes believe it gives them some apparent athletic advantage, such as weighing in for a weight class. However, voluntary dehydration is NOT recommended as it can lead to serious health complications. On the other hand, Involuntary dehydration occurs when fluid losses (perspiration) are greater than intake (fluid intake) leading to an imbalance in body water. This can be brought about in many different ways such as intense exposure to heat and humidity but, the consequences are the same.

...The 2% rule:

The American college of sports (ACSM) medicine has proposed that a state of dehydration greater than a 2% loss of body weight can significantly affect performance, especially aerobic performance. Additionally, it has been shown that if this percentage loss occurs through the lack of ingestion of fluid, this impairment to performance can be drastically greater (ACSM, 2007). For example, if you began your workout at a weight of 70kg, and ended it at 68kg, your loss would be greater than 2% at 2.9%. Therefore, it can be suggested that your performance would have been sub-optimal due to a greater percentage loss of 2%. However, in recent literature a loss of <2% (1-2%) has been suggested to cause significant cognitive impairment. Whether this is the case in all athletes, and in all sports can be disputed but, the bottom line is that maintaining a level of dehydration less than 2% is critical for sustaining an optimal level of sport performance.

Physiological effects of dehydration:

Water has been shown to play a huge role in helping maintain blood flow, regulating core temperature and establishing effective muscle contractions (Shirreffs & Sawka, 2011). Thus, consuming liquids helps replace the fluids lost during exercise via sweat. Physiological effects of dehydration can manifest in numerous ways including increased cardiovascular tension, reduction in blood flow, decreased sweat rate, decreased ability to loose heat and increased rate of muscle glycogen use (Coyle, 2004; Jeukendrup & Gleeson, 2010). The most prominent mechanism this may manifest in would be an individuals VO2 MAX. As blood flow is reduced due to increased viscosity of the blood, venous return is reduced leading to cardiac output being impaired. In simpler terms, dehydration reduces blood flow back to the heart, so less blood can be pumped from the heart to the working muscles (what goes in must come out). Overall, reducing the ability to produce a maximal effort.

Also, another major physiological mechanism which is impaired due to dehydration is thermoregulation ability in the bodye. As blood flow decreases through the skin, the more heat builds and can not be dissipated. In warmer climates, this factor becomes increasingly more important. If untreated, this can lead to heat exhaustion and heat stroke which may terminally affect sporting performance. Additionally, increased core temperature has been shown to increase the rate of glycogen breakdown contributing to a faster onset of fatigue.

Psychological effects of dehydration:

It is a well known fact that our brains are roughly 70% water, making it logical that dehydration has the ability to effect our psychological state. A prominent consequence of dehydration is the effect it can have on your energy levels. Reduction in blood flow to the brain due to increased viscosity of blood reduces the oxygen levels to the brain which as a result can impair your ability to want to expend energy. Research has shown that a dehydration level of less than 5% has the ability to reduce your energy levels by up to a staggering 30%!

Perhaps the most detrimental psychological effect of dehydration is how it can impact your ability to focus. Research by The Journal of Medicine and Science in Sport Exercise found that even mild dehydration has the ability to negatively influence cognitive performance in sport. This becomes increasingly important for sports which involve vast amounts of decision making, anticipation or any other highly cognitive processing such as golf, basketball or tennis.

Sweat rate:

Knowing your sweat rate can be vital for ensuring you do not fall into a state of dehydration. This straightforward method revolves around changes in body weight that occur during training as a result of weight loss due to sweat, whilst taking into account the weight of any food and/or liquid you additionally intake. This technique assumes 1g of mass lost is equivalent to 1ml of water lost. Obtaining sweat rate is a simple process involving few measurements and use of the following equation:

Once your sweat rate is calculated, it is possible to calculate the liquid requirements for a given training session to ensure that for each minute of exercise, the correct replacement occurs adjusted to individual needs. An important factor to note here is how the environmental conditions may affect sweat rate. Obviously, exercise in humid hot heat will cause an individual to perspire at a greater rate than if the same individual was to exercise in very cold conditions. Therefore, to take this into account, scientific literature suggests a recalculation of this sweat rate every quarter of the year to adjust the sweat rate to the changing environmental demands. This not only prevents hyper- and hypo-hydration, but also means that the body is supplied with the correct intake of liquids to optimise training and performance.

How to prevent dehydration:

It is pleasing to know that all these negative consequences of dehydration have the ability to be reversed by consuming adequate amounts of liquids before, during and after exercise. The most important way to help mitigate the effects of dehydration is to be aware. Knowing the environmental conditions, the time you will be exercising and your required water intake is vital to ensure you are prepared for the demands of your training. Should you be training in a hot climate, a possible idea could be to plan your training to take place during the cooler times of the day to help prevent the effects of dehydration. Planning your hydration is key to ensuring you do not get hit with any nasty consequences dehydration. Additionally, after exercise a urine chart is a useful tool to help you determine whether you have consumed enough fluids to adequately replace the fluid you lost through exercise. Although this tool is subjective, it is one of the best non-laboratory tools to use to assess hydration status.

Other ways to help mitigate the effects of dehydration may relate to clothing, diet and intake of other fluids. Ensuring you are wearing light-cloured and fitting clothing to prevent the absorption of heat may be vitally important for exercise in hot climates. Additionally, being mindful of your diet before exercise is important, as ensuring you eat a diet rich in high water foods (such as fruit and vegetables) will not only hydrate you, but will also help to keep your body temperature cool and provide essential nutrients.

To conclude, hydration is an aspect of training which is often overlooked. It is important to remember that staying hydrated not only ensures you are able to perform optimally, but also helps maintain a good overall health status.

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American College of Sports Medicine, Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and science in sports and exercise, (39)2, 377–390.

Shirreffs, S. M., & Sawka, M. N. (2011). Fluid and electrolyte needs for training, competition, and recovery. Journal of sports sciences, (29), 39–46.

Mange, K., Matsuura, D., Cizman, B., Soto, H., Ziyadeh, F. N., Goldfarb, S., & Neilson, E. G. (1997). Language Guiding Therapy: The Case of Dehydration versus Volume Depletion. Annals Of Internal Medicine, (127)9, 848-853.

Jeukendrup, A. E., & Gleeson, M. (2010). Sport Nutrition: An Introduction to Energy Production and Performance. Champaign, IL: Human Kinetics, N. pag. Print,increased%20rate%20of%20glycogen%20use.

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