Hydration advice can be confusing.  Over the last 20 years, strategies have ranged from “drink to maximum tolerance” to “ intentional strategic dehydration”.  In this article we’ll take a closer look at the spectrum of hydration research then draw from science to create basic guidelines for your next race.

1: Drink to Maximum Tolerance

Early hydration guidelines focused on minimizing fluid loss by drinking as much as possible [1].  The “drink to maximum tolerance” approach was rooted in research showing a direct connection between fluid loss, rise in core and skin temperature, and a drop in performance [2].

In short, when dehydration approaches a 2% or greater loss of total body mass, aerobic performance declines [3, 4].  The more you drink, the less fluid you lose, and supposedly, the better your performance.

Sounds simple, but the problem with “drink to maximum tolerance” is that it can lead to hyponatremia, a serious condition in which sodium levels in the blood are diluted causing sickness and even death [5, 6].

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2: Drink to Thirst

With hyponatremia as a backdrop, more recent guidelines have emphasized the individual nature of sweat rates and electrolyte concentration [7].  What’s the simplest way to respect hydration differences between athletes?  Tell them to drink when they’re thirsty and don’t drink when they’re not [8, 9].

“Drink to thirst” sounds good, but can still result in dehydration ranging from 1 to 3% of body weight [10].  Since performance can decline with as little as 1% loss in body weight [11], following your thirst might still slow you down.  Enter our third hydration strategy.

3: Prescribed Drinking

More recently a new school of thought has emerged combining the “drink to tolerance” and “drink to thirst” approaches.  It’s called “prescribed drinking” and aims to minimize fluid loss while steering clear of hyponatremia [12].

The first step is to weigh yourself before and after a trial ride.  After determining your fluid needs for an event, systematically “dose” fluid evenly throughout a race, aiming for a total body weight loss of under .5% [12].

In a 2017 study, the “prescribed drinking approach” showed a performance advantage over that of “drinking to thirst”.  This performance edge was hypothesized to come from “lower thermoregulatory strain and greater sweating response [12]”.

Minimizing fluid loss through prescribed drinking sounds great, but might be impractical for many competitive scenarios.  When’s the last time you were able to drink systematically during a long road race?  I don’t remember either.  The importance of practicality is a great segue for our final “strategy”.

4: Dehydration Is Fine

Our final approach isn’t as much a strategy as a departure from conventional hydration wisdom.  Several recent studies have made a compelling case that dehydration isn’t really that big deal after all.

In a landmark 2011 study, researchers showed an inverse relationship between percent of body weight loss and finishing times in a marathon.  Say what?  The fastest runners in the race were dehydrated the most at the finish of the race, with most runners losing around 3% of their body weight [13].

If dehydration supposedly takes a significant toll around the 2% threshold (or in some studies as soon as 1%), how were the best runners also the most dehydrated?  Something isn’t adding up.

Conflicting Research

One possible reason for conflicting results is that previous studies overstated the negative impact of dehydration.  Without outdoor wind speeds to mitigate core and skin temperature increases, the effect of dehydration might have been inflated in classic indoor, “laboratory based” research [14].

Several other studies seem to confirm this point by showing no performance difference between cyclists who lost around 2% of their body weight as compared to cyclists only losing around .5% when trials were done outdoors [8].

Another possible explanation for the disconnect in hydration research comes down to the placebo effect.  While previous participants drank water at various rates/quantities during their testing protocol, researchers in a 2015 study used intravenous infusion to blind participants to their hydration status [15].

You heard that right, during the testing protocol, each rider had an IV attached either hydrating with fluid, or doing nothing (leading to dehydration).  This protocol enabled researchers to remove the placebo effect of thinking that drinking water and staying hydrated might result in improved performance.

What did researchers find?  There was no performance difference between cyclists who were unknowingly hydrated (around .5% body weight loss) vs. those dehydrated (around 3.2% body weight loss) [15].  Hydration status didn’t seem to matter, leading some to question whether spending  energy focused on hydrating is worth the trouble at all [14, 15].

Putting It All Together

Four hydration approaches, all with some level of supportive evidence.  Here’s what we get when we mash the pros and cons.

Developing Practical Guidelines

A lot of research, much of it contradictory.  Where do we go from here?  Let’s take a stab at some practical advice to guide your next race.

  • Always start your race fully hydrated.  In a practical sense here’s what to look for [9].
    1. Keep you thirst as low as possible before your race [16].
    2. Monitor your urine color in the hours before your race.  If you’re consistently in the “very pale yellow to pale yellow” range you’re likely around 1% of your well-hydrated body weight [17].
  • Start hydrating fully in the hours, not minutes before your race [17].
  • For high intensity races under 1 hour, don’t worry about drinking much during the race [8].  Aim instead to swallow a bit of drink mix every 10m or so [18].
  • For most race scenarios, drink when you’re thirsty, don’t drink when you’re not [9].
  • For hot or longer (>2hr) races, it might make sense to follow a modified “prescribed drinking” approach.  If you really want to better understand your performance, knock out the testing protocol below [12] .
    1. Take your fully hydrated “pre-ride” weight.
    2. Follow a “drink to thirst” approach during a ride simulating your intended race conditions.
    3. Take your “post-ride” weight.
    4. Account for what you drank on the ride and add up total fluid loss during the ride.
    5. Execute the same ride under similar conditions while dosing your fluid aiming to stay as hydrated as possible.  How did you feel?  How did your power output compare between the two trials?  Maybe dehydration is hurting your performance and a systematic hydration approach is for you.
  • Trying to decide between water and drink mix?  If you’re riding in hot conditions or for a long time, opt for a sports drink [7].
  • Tempted to intentionally dehydrate yourself to improve your W/kg while climbing?  Don’t do it. The only study to examine this approach showed slower climbing speeds when dehydrated [19].  Stick to the 2% body weight threshold and hit your climbs with a full tank of gas.
  • Does all this talk of hydration stress you out?  Don’t worry about it.  As long as you start your ride well hydrated then drink when you’re thirsty, you’ll probably be fine losing a few percentage points of body weight [14].  Focus instead on smashing the pedals and crushing carbs.

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