Hydration:  What Endurance Athletes Need to Know
Hydration: What Endurance Athletes Need to Know

I find that endurance athletes are either “team cold temps”, or team “I love to sweat in hot temps.”  I am most definitely the latter, so I for one could not be happier that summer has hit Colorado.  I’ve broken out the shorts and put away the skis.

Even as someone who loves the hotter temps, I know that hydration status can make or break a run for me from an enjoyment and performance perspective.  So, if you’re someone who just “doesn’t do well in the heat,” this one’s for you.  I have a feeling that if you can nail your hydration before, during, and after an effort, you will change your tune.


How does dehydration affect performance?


Sure, being dehydrated feels like sh*t - no doubt.  But why?  Dehydration happens when we lose too much water, causing hypohydration.  From a physiological perspective, there are a number of things that happen in our bodies when we’re dehydrated:


  1. Cardiovascular strain

As an endurance athlete, this is the last thing you want.  When you’re cardiovascular system is strained, your perceived level of effort will be much higher, and power output decreases.


  1. Increased glycogen use

Your body will be reaching for more energy when you’re dehydrated, which means that it will dip into your energy stores faster.  A quicker depletion of glycogen stores means you’re more likely to bonk.


  1. Impaired function of central nervous system

This can cause fatigue, decreased concentration, and a reduction in motor skills - not something we want happening while we are exercising.


  1. Rise in body temp

The hotter your body feels, the tougher your effort will feel.  This also increases risk of complications such as heat stroke.


Measuring fluid status

The best way to measure your fluid losses during exercises is through weighing yourself before and after your workout.  Of course, if you are someone who is in ED recovery, talk to your dietitian before stepping on a scale!

The goal is to not lose more than 2% of your body weight during exercise.  If you ARE losing >2% of your body weight during exercise, you’ll want to increase the amount that you are drinking before and during exercise, and perhaps increase your electrolyte intake.  This is something that your dietitian can assist with.

Losses >6% are considered to be severe.  This is where you may begin to experience decreased cardiac output and risk of heat stroke.


How much water should I be drinking?


Daily intake

A general rule of thumb for daily water intake is half of your body weight (lbs.) in fluid ounces.  In other words, for someone who is 150lb, they should aim for AT LEAST 75 fl oz. (2.2L).  This is considered to be a baseline, and does not account for what you’re having before, during, and after exercise.


Before exercise

Generally, you want to aim for 5-10 mL/kg of body weight in the 2 hours before exercise.  Using our same example of a 150 lb athlete, this person would want to aim for 340-680mL (0.3-0.6 L) of fluid before their workout.


During exercise

Depending on the athlete, environment, duration, and intensity, an athlete can lose anywhere from 0.3-2.4L of fluid per hour. (This is where weighing yourself before and after a workout comes in!!)  

Generally, I recommend that athletes aim to get 0.5-1L of fluid per hour of exercise - probably closer to the 1L mark in the summer at altitude, as fluid losses are higher with both of those variables.


After exercise

The short answer is: 24 fl oz of fluid for every pound lost during exercise.  Of course, it’s important to continue to drink water throughout the day after exercising, and to replete losses with electrolytes!  (See below)


What about electrolytes?

How do electrolytes help performance?

  1. Fluid retention

I like to say that electrolytes in fluid help your water “stick” to your body.  In other words, when you have sodium and other electrolytes in your water, your body holds onto it more effectively, rather than immediately excreting it.  This means that you will be able to maintain fluid balance for much longer, therefore preventing dehydration.


  1. Mitigating GI issues

Many GI issues happen as a result of an athlete drinking solely water during exercise.  When you’re relying on water, it leaves your stomach much faster and enters your GI tract all at once.  This can cause stress on the GI tract.

In addition, exercising while dehydrated decreases blood flow to the gut, causing bloating, reflux, vomiting, or diarrhea.


  1. Prevents cramping

Electrolytes are critical for muscle contractions in the body.  Therefore, when you lose sodium, potassium, calcium, and magnesium through sweat and don’t replete them, it can disrupt muscle contractions, causing cramping.


  1. Energy + electrolytes

Often, electrolyte products also have carbohydrates in them, helping to top up your energy stores while you’re exercising.  So, you’re killing two birds with one stone - hydration and energy ingestion.


How much should I be having, and what’s too much?

Believe it or not, we lose ~1000mg of sodium per liter of sweat.  Now, you do NOT need to replete every mg that you lose, but it just goes to show how important it is to ingest electrolytes while you’re exercising.  It also goes to show that it’s pretty hard to overdo it.  In fact, you’d have to drink a gallon or two of sea water or a bottle of soy sauce to overdo the sodium intake.

I recommend having 350-500mg of sodium per hour for any bout of exercise longer than an hour.


Brands that I recommend as an RD

  1. LMNT (1000 mg of Na, no significant carbs)

  2. Liquid IV (510 mg of Na, 12g carbs)

  3. Skratch Hydration (400 mg of Na, 19g carbs)

  4. Nuun (360 mg of Na, no significant carbs)