“Drink 8-10 glasses of water a day.” You have probably heard this advice in the past, but how does it translate to athletes? Unfortunately, not that great. As athletes, dialing in on hydration and fluid intake is essential. It can be the difference between a great race vs. a complete “bonk.” However, navigating a market saturation with various electrolyte formulas, tablets and don’t forget the salt packets on race day, where does one begin? The purpose of this article is to get to the science of hydration, why it is essential, and how you can make informed decisions regarding your hydration needs as an athlete.

Importance of Hydration

In a multitude of ways, hydration is critical for exercise performance. Proper fluid intake during prolonged exercise maintains hydration status while avoiding drops in blood plasma volume (7).

Blood plasma refers to the fluid circulating within the arteries and veins. This fluid transports red blood cells that bring oxygen along with nutrients like glucose to our tissues. Components like this are vital to keeping up with the demand for aerobic exercise, which relies heavily on oxygen for energy production.

In addition, blood plasma carries heat throughout the body and helps regulate body temperature. During exercise and heat exposure, we rely on blood flow to our skin (along with sweating) to dissipate heat. Failure of these mechanisms can result in symptoms of heat exhaustion (i.e., headache, vomiting, diarrhea, and muscle cramps). There is an underlying competition between blood flow to muscles and blood flow to our skin. It is no surprise a drop in blood volume, as small as 1%, can evoke undue fatigue and reduce muscle contractile capacity (2).

When fatigue starts to creep in, it is often assumed, “we are low in fuel” when in fact, fatigue from dehydration may be the issue. Making matters worse, bouncing back from dehydration can take hours (1). For this reason, by working with your physiology and developing effective rehydration strategies, you can avoid the “bonk” from dehydration.


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How Do We Hydrate?

To avoid the dehydration “bonk,” where do we begin with hydration?

If we drink plain water and pee is clear, we are good to go.

Not so fast. Hydration is not that simple. By optimizing hydration, we have to go back and look at both biology and chemistry. Fluid absorption from the gastrointestinal tract into the system is dependent on various factors. The fluid composition (carbohydrates and sodium), rate of gastric emptying, and changes to the gastrointestinal system during exercise all influence how efficiently we rehydrate.

Rate of Gastric Emptying 

The fluid we consume enters the stomach and proceeds to empty into the small intestines to be absorbed. This rate of emptying is highly dependent on the composition of both the food and liquids consumed. Food high in fat or liquids too high in carbohydrates can delay gastric emptying and limit fluid absorption (5).

“Water Follows Salt” 

When the fluid enters the small intestine, this is where the absorption party occurs.

Electrolytes like sodium, potassium, and chloride enter via transport channels. These channels act similar to doorkeepers at a club. The outside of the club is the small intestine and on the inside is blood. Water is pretty simple; as long as there is a need for water in the club, water can enter. But water is shy. It needs sodium, other electrolytes, and even sugar to musk up the courage and enter (5).

There is a new sports beverage available every time you go to the grocery store. But hitting the right concentration of water, glucose, and electrolytes is key to fluid absorption. As we will see, too much a good thing is a bad thing.

When the concentration of carbohydrates is too high, the gradient goes the wrong way. Water from the system will be pulled into the small intestine for dilution purposes and contribute to dehydration. Critical to hydration is avoiding a new flux of water into your small intestine. When this occurs, runners start to experience the sensation of fluid “sloshing,” GI upset, bloating, diarrhea, etc. (5).

What about plain, old water?

Without the proper electrolytes, plain water won’t absorb. Sodium gets pulled into the small intestine to help with the process. If you drink a large amount of plain water, you will have pee out more than you take it (hence, why clear pee is not the best indicator of hydration).

So, what is in your bottle?

The optimal rehydration fluid contains the right about specific carbohydrates and sodium. Carbohydrates such as glucose and sucrose easily absorb via multiple transporters (yes, they need doorkeepers to enter the club too). But remember, not too much.

By looking closely at the nutrition facts of electrolyte supplements, you can determine which formula is right for you. According to nutrition scientist Dr. Stacy Sims, a functional hydration solution contains the following:

(Per 8 fluid ounces)

  • 3-4% carbohydrate solution
    • 7-9 grams of carbs (ideally from glucose + sucrose)
  • 180 – 225 mg sodium
  • 60 – 75 mg potassium

Beware of fructose-containing solutions and foods (like watermelon at aid stations). Fructose needs to go to the liver first and can impede absorption and contribute to GI upset. In addition, some people are more sensitive to sugar alcohols (i.e., erythritol and stevia) than others (3, 6). Always give your chosen solution a test run before race day.


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How to monitor your hydration?

Like most things, proper hydration is highly individualized. Dialing in specific signs and dehydration can be helpful in your decision, “do I need fuel or fluids?”

Some factors to focus on:

  • Crusted salt deposits on clothing: A sign you may need to increase your sodium intake. Some individuals’ sweat composition contains more salt than others. This is mainly due to the hormones involved in salt absorption and even female hormones like estrogen and progesterone.
  • Body mass loss during training/competition: Limit weight loss to not more than 2% of body mass. You can monitor this by weighing pre- and post-workout.
  • Rate of perceived effort, pain scale, and heart rate: Fatigue, muscle soreness, and increased heart rate are indicators that your blood volume is going down, and you are low on fluids.
  • GI distress: During exercise, blood flow is directed away from the gut and to working muscles. Add dehydration to this mix, and your gut is in a compromised position.

Drink to Thirst or on a Schedule?

There is much debate on this topic. At rest, this is a good strategy, but during exercise, not as effective. During exercise, the thirst mechanism is skewed, and relying on this method can lead to dehydration.

During an event lasting longer than 90 minutes, cravings for salt and sugar outweigh thirst. In addition, hormones involved in sodium reabsorption and the environment are also a factor (6).

Consider this simple hydration schedule below to keep on track before and during your workouts and events.




  1. Adams, J. D., Sekiguchi, Y., Suh, H. G., Seal, A. D., Sprong, C. A., Kirkland, T. W., & Kavouras, S. A. (2018). Dehydration impairs cycling performance, independently of thirst: a blinded study. Med Sci Sports Exerc50(8), 1697-1703.


  1. Anastasiou, C. A., Kavouras, S. A., Arnaoutis, G., Gioxari, A., Kollia, M., Botoula, E., & Sidossis, L. S. (2009). Sodium replacement and plasma sodium drop during exercise in the heat when fluid intake matches fluid loss. Journal of athletic training44(2), 117–123.


  1. de Oliveira, E. P., Burini, R. C., & Jeukendrup, A. (2014). Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine44(1), 79-85.


  1. Matias, A., Dudar, M., Kauzlaric, J., Frederick, K. A., Fitzpatrick, S., & Ives, S. J. (2019). Rehydrating efficacy of maple water after exercise-induced dehydration. Journal of the International Society of Sports Nutrition16(1), 1-13.


  1. Maughan, R. J. (Ed.). (2013). Sports nutrition(Vol. 19). John Wiley & Sons.


  1. Sims, S. T., & Yeager, S. (2016). Roar: How to Match Your Food and Fitness to Your Female Physiology for Optimum Performance, Great Health, and a Strong, Lean Body for Life. Rodale.


  1. Von Duvillard, S. P., Braun, W. A., Markofski, M., Beneke, R., & Leithäuser, R. (2004). Fluids and hydration in prolonged endurance performance. Nutrition20(7-8), 651-656.
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