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When Is Dehydration a Limiting Factor for Performance?

Contributor: Matt Dixon     Category: Training

Within training and competition, dehydration becomes a performance limiter once you hit about a 4% level of dehydration.  This refers to fluid loss as a percentage of your body weight. For example, if a person weighs 100 lbs and is 4% dehydrated, they have lost 4 lbs of fluid. Away from a scale, this can only be estimated but we know this is where performance detriments occur. Prior to this level, it is not really a factor, so most athletes can drink to thirst, unless great heat and humidity are a factor.  In longer events, then it truly can become a massive factor.  Let’s use my sport of triathlon as an example to explain the key behind managing hydration.  

An IRONMAN event takes at least 8 hours, and often occurs in higher temperatures.  This makes hydration important.  The key behind this is realizing that most of it is related to our volume of blood in the body.  Of course, our body is a ‘closed’ system, with between 5 and 6.5 liters of blood circulating to the muscles to deliver oxygen, to the skin to dissipate heat generated by work, and to the GI (Gastrointestinal) system to help absorb calories.  As an athlete becomes dehydrated, the blood volume drops, as the fluid loss creates declining plasma volume (the clear portion of our blood).  This drop in blood volume creates competition between the muscle and skin, as we try to maintain output, but also need to get rid of the heat we generate.  With less blood to go around, the competition increases.  It is worth realizing that the skin will always win, as heat can be an organ killer!  This means there is less blood going to the muscles to allow us to maintain pace, and an increase in the perception of fatigue.

It creeps up on an athlete, but if an IRONMAN athlete forgets to hydrate well on the bike, the ensuing marathon can become a real slog.

To prevent dehydration, follow these tips daily:

  • Drink 1-2 glasses of water with each meal
  • Rehydrate post exercise with a glass of water (add a pinch of salt and a squeeze of citrus for optimal results)
  • Drink to thirst (and listen to the mechanism) in most training. For extended duration or intense training you will need a proper hydration plan.