The Physiology of Dehydration

(Written by BPC Coach Brian Barnett)

We all know that exercising while dehydrated doesn’t usually work out in our favor.  You’ve likely been told that to maintain high performance, you need to try and prevent drops in weight coming from fluid loss.

But have you ever been told why you should keep the fluid stores topped off as it relates to performance?

Here it is, the down and dirty on what happens during dehydration:

1. Loss of neurotransmitters – Neurotransmitters are chemicals within the body that aid in the transmission of signals (across synapses) from our brain from one neuron to another.  They are released between small junctions in the pathway between the brain and your muscles.  In regards to exercise, they allow for muscle contraction and relaxation. With decreasing levels of these (calcium, sodium, and potassium are the big ones here) our muscles can’t release after contracting and we cramp. For anyone who has experienced cramping, this definitely decreases performance.
2. Blood volume decreases – Sweat is often pulled from the plasma that makes up a 55% of our blood and helps to keep it flowing smoothly through our body.  When plasma volume decreases blood becomes thicker and your body must work harder to pump what blood is available through the body. If the body is working harder to pump your blood, it has less energy available to actually move your body in the direction you are trying to go.
3. Prevents thermoregulation – As blood and fluid volume in the body decreases, your body does its best to prevent this loss.  To do this, the body inhibits the pathways that vasodilate (vasodilation: the widening of blood vessels) your blood vessels preventing easy flow of blood to the skin. This is a problem because blood traveling to the skin is one mechanism for keeping your body cool (the other being sweating). When blood is not able to be moved to the skin, heat is retained in the body. The hotter you get, the more your body has to work to keep your body cool rather than propel you forward.
4. Heart rate increases – During prolonged exercise at a constant effort heart rate increases.  This phenomenon is called cardiac drift and is influenced by many factors.  How hydration alters it is through the decrease in blood volume.  The heart is forced to work harder to pump a smaller volume of blood through the body. To compensate, heart rate increases and the results in fatigue to set in earlier.  You get a double whammy here because cardiac output (the amount of blood pumped over a given time period), decreases for another reason as well. As your heart beats faster, it has less time for each chamber to fill with blood. This effectively limits your performance as your body cannot receive as many nutrients it needs in order to train or race harder.

There are a variety of other effects of dehydration, but from a performance standpoint, these 4 are the ones we are most concerned with.

In addition, not only does dehydration alter physical performance, it alters cognitive performance making it harder to concentrate and increasing the likelihood of careless mistakes. Like chasing down a teammate or overlapping wheels. Or forgetting to unclip coming into T2 of your triathlon and falling over in front of all the spectators. Or taking a wrong turn and heading off course…you get the idea.

There are many different recommendations for how to best handle dehydration (you can start by following this link and calculating your sweat rate), but know that each person will have unique needs. Learn what works best for you and occasionally try new techniques so that you are ready for every condition.