Performance hydration: The role of water, sodium, and glucose in exercise

From the desk of
Robb Wolf
SciencePerformance hydration: The role of water, sodium, and glucose in exercise

When people ask me how to optimize their hydration strategy for exercise, I first tell them to drink electrolyte water according to thirst, and not beyond it. If you’re following that golden rule, you’re 90% of the way there. Electrolytes are essential for proper fluid balance, and oftentimes overwatering is chief among the errors people make when hydrating for a run, bike, long hike, lift, or game.

If you already knew that and want to know more, then take a deep breath — because I’m about to run my mouth faster than a dog when you shake the treat bag. A lot of science goes into eeking out those last few performance points!

First, you must know how much water and sodium you lose through sweat. Then you can replace what’s lost without suffering the consequences of dehydration or overhydration. It also pays to understand when and how to use glucose. Glucose can help fuel exercise in specific contexts, but many athletes take too much, to the detriment of their hydration status (not to mention the needless calories of sugary goo!). I’ll explain more later.

Today you’ll learn how dehydration affects performance, how much water to drink during exercise, how much sodium to include, how to use glucose properly, and more. Lots to cover today — let’s keep it moving.

How Does Dehydration Affect Aerobic Output?

Dehydration is defined as net water loss from the body. We lose water through sweat, respiration, waste, and urine. Natural diuretics like caffeine and alcohol make us lose extra water through urine.

The consequences of dehydration range from trivial to severe, depending on the magnitude of water lost. Symptoms include fatigue, muscle cramps, headache, constipation, reduced blood pressure, dizziness, fainting, nausea, and rapid heartbeat.

The “why” behind the rapid heartbeat is worth explaining for athletes. When you’re dehydrated, your blood volume decreases. Lower blood volume means the heart doesn’t squeeze as hard when pumping blood. That may seem like a good thing, but in actuality this means that the heart’s “preload” (pre-contraction force) and its contraction are weaker, leading to less efficient transport of oxygen per heartbeat. To compensate for these weaker heartbeats, the heart must beat faster so the dehydrated athlete can get precious oxygen and other nutrients to their tissues.

Does Dehydration Affect Exercise Performance?

To sum up the last section: Dehydration leads to lower blood volume, lesser cardiac output, and worse aerobic capacity. Your heart is working harder and under delivering.

But does it then follow that hydration status significantly affects exercise performance? The two meta-analyses (studies of studies) below aimed to answer that very question:

  • A 2010 meta-analysis reviewing 29 studies found that dehydration greater than a 3% reduction in body mass impaired subjects’ power output during endurance exercise.
  • A 2015 meta-analysis reviewing 28 studies found that dehydration between 1.9–4.1% reduction in body mass impaired subjects’ muscle strength, endurance, and power.

That said, it’s worth illustrating that individual studies don’t always reach the same conclusion. For example:

  • In a 2015 study, dehydration between 2–3% reduction in body mass led to higher heart rates, rates of perceived exertion, muscle glycogen depletion, and core body temperatures. Cyclists were 13% slower in time trials than when adequately hydrated.
  • Another 2015 study employed a different design, blinding the cyclists from knowing their hydration status by infusing them with saline to varying levels during 3 separate trials. Though cyclists experienced higher core body temperatures when dehydrated (by 3% body mass), they performed equivalently to when they were properly hydrated.

That’s a taste of how these studies tend to go. The takeaway is that there’s no harm in staying optimally hydrated with fluids and electrolytes, especially during longer efforts that cause substantial dehydration.

How Much Water Should I Drink During Exercise?

The amount of water you need during exercise is a direct function of your sweat rate. Here are some factors influencing sweat rate:

  • Sweat rate typically rises with increasing ambient temperature, humidity, exercise intensity, aerobic fitness, body weight, and heat acclimation.
  • Sweat rate typically decreases with better airflow (wind or a fan) and greater dehydration (your body wants to hold onto water when dehydrated).
  • Older adults tend to have lower sweat rates than younger folks, but this is more likely due to declining aerobic fitness than greater age.
  • Men tend to have higher sweat rates than women, but this is mostly because males are larger on average.

Most sweat rates fall between 0.5–2.0 liters per hour depending on all of the above factors. You can read this article to learn more about sweat rate, or simply follow the steps below to calculate yours:

  1. Weigh your body (in kg) before and after exercise. Weigh while naked if possible, to avoid confounding your measurements with the weight of the sweat on your clothes.
  2. Subtract your post-exercise weight plus the weight of any fluids you drank from your pre-exercise weight. Note that 1 liter of water = 1 kg.
  3. Divide the answer by the number of hours you exercised for, and voila! You know your sweat rate in liters per hour.

Knowing your sweat rate is a useful tool, but it varies greatly with your exercise and environment. Therefore it’s worth repeating: A simple and effective strategy is to drink electrolyte water to thirst. Athletes who drink plain water beyond thirst risk diluting their blood sodium levels and developing a dangerous condition called exercise-associated hyponatremia. Consequently, it’s important to replenish both fluids AND sodium when rehydrating. Now, let’s get to the sodium part of the equation.

How Much Sodium Should I Consume With My Water?

You lose significant sodium in your sweat. Sodium deficiency can cause headaches, muscle cramps, fatigue, brain fog, irritability, and more. So in order to feel and perform your best, you should replenish that sodium.

It doesn’t have to be complicated! Simply pay greater attention to how you feel and what sounds good at the moment — lately I’ve been able to discern when my body is craving salty or plain water. However, if you’d enjoy nerding out on the details with me, then allow me to explain the concept of tonicity.

According to NIH StatPearls, “tonicity is the capability of a solution to modify the volume of cells by altering their water content.” This capability is dictated by the amount of dissolved substance (solute) in a given solution, and sodium is the most important solute in human plasma.

There are 3 categories of tonicity, defined by how concentrated a solution is compared to blood:

  • A hypotonic solution contains a lower concentration of solute than red blood cells. Drinking a hypotonic solution causes water to flow into cells from the surrounding plasma. Plain water is extremely hypotonic because nothing is dissolved in it — it contains no solute.
  • An isotonic solution contains the same solute concentration as red blood cells. Drinking an isotonic solution helps water flow into and out of cells equally (we like that!). Isotonic plasma has a sodium concentration of about 135–145 millimoles per liter.
  • A hypertonic solution contains a higher concentration of solute than red blood cells. Drinking a hypertonic solution causes water to flow out of cells, into the extracellular plasma. Seawater is extremely hypertonic with a sodium concentration of about 1000 millimoles per liter.

Our sweat is hypotonic. We lose plenty of sodium in it, but at a lower concentration than that of red blood cells. In the practical sense, this means you should consume slightly hypotonic electrolyte drinks to replace sweat electrolyte losses. Specifically, athletes’ average sweat sodium concentration is about 826 mg per liter (~34 ounces).

That said, many healthy people — who tend to avoid salty, processed foods — don’t get enough sodium via whole foods to begin with. That’s why we recommend people mix 1 stick pack of LMNT (containing 1,000 mg sodium) into 16–32 oz of water, in addition to a well-formulated diet.

If you’d like to be super precise about sodium replacement, you’ll need to determine two variables: sweat rate and sweat sodium concentration. We already discussed sweat rate. To learn about your sweat sodium concentration, visit a lab or try out some wearable tech to test your sweat. Then multiply the two together to find out how much sodium you lose per hour of exercise — just keep in mind that both of these variables will fluctuate greatly depending on your exercise and environment.

How Does Glucose Affect Hydration and Performance?

Adding glucose to an electrolyte drink can help provide extra energy to fuel prolonged efforts and increase sodium and fluid absorption through the gut. But consuming too much glucose is bad news for both hydration status and overall health.

Glucose for Energy

When you aren’t actively consuming dietary carbohydrates, your body relies on glycogen (the stored form of glucose) to fuel exercise. Just one problem: Glycogen stores are limited. How long they last depends on exercise intensity and duration. After around 2 hours of endurance training, folks start to see performance decline. Sprinting can compress this timeframe a bit because it depletes glycogen even faster.

That’s where glucose supplementation comes in. It compensates for glycogen depletion so you can keep energy levels up. I often take about 10–20 grams of glucose per hour on my more intense jiu-jitsu training days, but elite endurance athletes sometimes consume up to 60. Modify accordingly to suit your regimen.

Glucose for Fluid and Electrolyte Transport

When you eat dietary carbohydrates (like glucose), cotransport proteins called SGLT1 and SGLT2 activate to bring sodium, fluids, and glucose through the small intestine and into circulation. The World Health Organization’s Oral Rehydration Solution (ORS), for instance, contains 22 grams of glucose per liter of water. ORS has helped millions rehydrate in cases of extreme fluid loss such as conditions which cause diarrhea and vomiting.

However, you don’t need glucose to stay hydrated during routine exercise. Sodium and water pass through the gut just fine without it, and can also come through via other cotransporters such as butyrate, beta-hydroxybutyrate (a ketone), amino acids, phosphorus, potassium, and chloride. Case in point: Drinking a saline solution reversed low blood sodium levels in ultra-endurance athletes.

Lastly, recall that adding any solute to water (including glucose) increases its tonicity. If you produce a hypertonic solution — as is the case with many sugary sports drinks and sodas — you may actually be dehydrating yourself.

Optimal Hydration for Exercise

We covered a TON today around optimizing hydration for exercise. Here are the highlights:

  • Dehydration reduces the force of your heart pumping blood, causing it to beat faster for the same amount of exercise and reducing your aerobic capacity.
  • Dehydrated athletes seem to perform worse compared to when they’re hydrated.
  • The amount of water and sodium needed during exercise is a direct function of your sweat rate and sweat sodium concentration, respectively.
  • To avoid the symptoms of sodium deficiency, replace the sodium you lose via sweat. It’s best to mix a hypotonic solution — about 1 gram of sodium per liter of water.
  • Glucose improves sodium absorption, but so do many other cotransporters. Using too much glucose can actually be dehydrating.
  • The more practical use case for glucose is to fuel longer efforts.

Congratulations! You’re now equipped to talk somebody’s ear off about how to optimize their hydration strategy for exercise performance. Just remember… If they get bored, you can always revert to the golden rule: Drink electrolyte water to thirst.

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