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Dehydration in Athletes: What actually happens to your body during hard exercise efforts?

In this article we’ll explore some of the impacts of dehydration on athletes during endurance competition. Hopefully, you'll gain a greater understanding of what is happening within your body which may help you prioritize hydration in the days running up and during competition.

Throughout the body are sensors providing feedback to the brain on what's going on within different systems. Amongst other things, these indicate when the internal temperature is getting too high or fluid levels are getting too low. When these things happen the body’s normal metabolic processes become disrupted and consequently performance declines.

The fluids in the body carry a variety of essential nutrients, like taking glucose to the working muscles. So ensuring adequate hydration status is fundamental before, during, and after competition.

Dehydration can also impact cognitive performance as well as physical performance. This cognitive decline can actually happen before the level of dehydration reaches a point where physical performance starts to suffer. There is some evidence of cognitive impairments with only mild levels of dehydration. Even short periods of insufficient hydration (a loss of body mass of 1-2%) can result in reductions in the subjective perception of alertness, ability to concentrate and an increase in self-reported tiredness and headache.

The regulator of the body’s fluid balance is the hormone, arginine vasopressin often referred to as antidiuretic hormone. This helps to control blood pressure during times of dehydration by acting on the kidneys and the blood vessels. Its helps to conserve the body’s fluid volume by decreasing the quantity of water lost in the urine. Any athlete knows that you can be at the start line, hydrated and needing to pee, then the race starts and that feeling goes, this is the hormone responsible! Water from the urine can be moved back into bloodstream so urine concentration rises but water loss is reduced. When this hormone is present in higher levels (as you become more dehydrated) it can cause blood vessels to constrict (vasoconstriction) to increase blood pressure. This is a helpful survival strategy but is not helpful to athletic performance. Vasoconstriction limits blood flow to your extremities and so reduces the nutrients like glucose and oxygen from reaching your muscles. The more dehydrated you get, the less oxygen and glucose reaches your working muscles. You will quickly see declines in your ability to generate power.

An interesting 2007 study examined whether dehydration could benefit cycling hill climbing by improving the power-to-weight ratio. While they did measure the two extremes, either 2.4 liters (about 81 oz) of a 7% carbohydrate drink or 0.4 liters (about 13 ozs) of water with sport gels to match the carbohydrate content, but found that the effects of dehydration were greater than any benefit that could have resulted from the reduction in rider’s weight. Those athletes competing in events longer than three hours can be exposed to substantial heat stress and dehydration so the importance of replenishing fluids cannot be understated.

Dehydration has many negative consequences during a race. Your hydration strategy should not start at the start line. It should start days before by ensuring you are well hydrated in the days running up to an event. Hydrate well in the morning with a drink containing electrolytes, carbohydrates and caffeine (there's a whole other post coming up on caffeine). Then throughout the race you consistently consume fluids to try and give your body what it needs to move those essential nutrients to your muscles. Of course, nothing new on race day, so make sure that whatever you're using is tried and tested in training.


Cheuvront, S., Carter, R., & Sawka, M. (2003). Fluid Balance and Endurance Exercise Performance. Current Sports Medicine Reports, 2(4), 202–208.

Ebert, T., Martin, D., Bullock, N., Mujika, I., Quod, M., Farthing, L., Burke, L., & Withers, R. (2007). Influence of hydration status on thermoregulation and cycling hill climbing. Medicine and Science in Sports and Exercise, 39(2), 323–329.

Maughan, R. (2003). Impact of mild dehydration on wellness and on exercise performance. European Journal of Clinical Nutrition, 57(S2), S19–S23.

Rothenberg, J. A., & Panagos, A. (2008). Musculoskeletal performance and hydration status. Current reviews in musculoskeletal medicine, 1(2), 131–136.


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