Fluid Needs for the Athlete: Part II – During Exercise Hydration


In Part I of this series on hydration, I discussed some tips for pre-exercise hydration. In this blog, I will explore how an athlete (recreational or elite) should hydrate during exercise to ensure optimal performance and prevent fatigue.

The goal of appropriate hydration during exercise is to maintain proper plasma volume and electrolyte balance. Optimal hydration will also help athletes avoid abnormal increases in heart rate and core temperature that can create health issues if left untreated, as well as premature fatigue that will diminish performance. Too much or too little fluid intake during exercise can have a negative effect on performance.

A water loss of only 2-3% of an athletes total body weight can decrease performance significantly and increase the risk of heat related illnesses.

The amount of water and electrolytes a person loses during exercise can vary greatly depending on body size, intensity of exercise, temperature, clothing choices, and acclimation. For example, a slow-paced, low-intensity effort in moderate temperatures might result in less water and electrolyte loss as compared to a high-intensity, long-lasting endurance event in hot and humid temperatures. Surprisingly, some athletes can lose up to 2-3 liters of water per hour in hot and humid environments! That’s more than the average adult drinks in a whole day. Replacing that amount of water can be daunting if you consider how much 2-3 liters of water weighs in both the bottle and the stomach.

Electrolytes lost in the sweat are also important to replace for some athletes, especially those who perform in long-lasting endurance events. Sodium, potassium and a small amount of calcium are the primary minerals lost during exercise with losses correlating positively with rising ambient temperatures and humidity, longer duration, and higher intensity. Short-duration activities may or may not need electrolyte replacement.

How much fluid should I consume during exercise?


Many national organizations, including ACSM, NATA, and AND, recommend athletes consume enough fluid to match their losses via sweat and urine and avoid losing any more than 2% of total body weight during exercise. For most, this translates to about 7-10 fluid ounces, every 10-20 minutes during exercise. However, it might be important to calculate your personal sweat losses, especially if you train more than 60 minutes in a hot or humid environment like Hawaii. This calculation is pretty easy to do and is called a sweat trial. The steps are outlined below. For best success, you should exercise for 1-2 hours without urination to make calculations less complicated and more accurate.

How to determine fluid losses via a sweat trial

1. Determine body weight lost during exercise.

Body Weight before exercise – Body Weight after exercise = pounds of water weight lost

2. Determine fluid equivalent, in ounces, of the total weight lost. Every pound of body weight lost equals approximately 2-3 cups or 16-24 ounces.

Pounds of water weight lost during exercise X 16-24 oz = # of ounces of additional fluid that should have been consumed to maintain fluid balance during session

3. Sum the total ounces of fluid lost, plus what was consumed during exercise to determine total fluid needs.

Ounces of fluid consumed + ounces of additional fluid needed to establish fluid balance = total fluid needs

4. Calculate the number of ounces needed per hour.

total fluid needs / total workout time (hrs) = fluid oz per hour

Determining an individual sweat rate is more advantageous than sticking with the standard recommendation because there is so much variability in sweat losses due to environment, level of training, and body size.

What types of fluids are needed during exercise?

The type of fluids consumed during exercise can mean the difference between night and day! The inclusion of carbohydrates, protein, fat or vitamins/minerals in addition to water can greatly affect performance both negatively and positively which we will discuss below. If you are working out for fewer than 45-60 minutes, water is probably the ideal beverage of choice.

If you plan on exercising more then 45-60 minutes, the use of carbohydrates are very beneficial and can improve performance compared to water alone. Too much carbohydrate can create tummy upset, so it’s important to find a sports drink that contains about 6-8% carbohydrate. More than this can potentially lead to bloating, cramping and diarrhea — none of which is pleasant. For this reason, fluids in the form of fruit juice, soda, and some sports beverages are not recommended. Below is a calculation to help you determine the percentage of carbohydrates in a fluid replacement beverage you are considering. This will be helpful when you are searching the aisles of the grocery store for an appropriate fluid replacement drink.

(grams of carbohydrate per 8 oz serving/ 240 milliliters) x 100 = % carbohydrate solution


example: (18 g of carbohydrate per 8 oz serving/ 240 mL) x 100 = 7.5% carbohydrate solution

Fruit juices should be avoided, not only because of the high percentage of carbohydrates, but also because they are also a high source of fructose, which when consumed in high amounts can also cause gastrointestinal distress in some individuals. Glucose, sucrose (table sugar), and glucose polymers (found in products like Gatorade and Powerade), are optimal sources of carbs while exercising. They are absorbed quickly and efficiently, which minimizes any negative effects of carbohydrate ingestion. Avoid drinks with artificial sweeteners, stimulants like caffeine or herbs, high doses of vitamins or minerals, and carbonation as these additional ingredients may pose a threat to your performance. The addition of protein or fat to your drink is also not recommended in the research at this time while you are exercising.

In addition to carbohydrates, it may also be important to find a sports drink with electrolytes, especially sodium, when it’s very hot or during periods of exercise exceeding 2-3 hours (i.e. triathletes, marathon runners, multiple events in one day). Talking to your dietitian or trainer will be an important step in determining whether or not you will need additional electrolytes beyond what is found in most common sports drinks and your regular diet.

Most importantly, it’s a good idea to experiment with different sports drinks, purchased or homemade, well before a big event or competition.There is a huge variety of sports drinks in the marketplace with many more homemade options appearing in the bloggesphere for those opting to avoid artificial flavors and colorings. Whether you choose to buy your sports drink or make your own, it’s always good to try it out for a few weeks, in multiple exercise settings and environments before using it during a competition or large event. You should find something that tastes good, especially if you determine that you need to consume large quantities of it!

The worst thing you can do is switch up your hydration plan the week before a long race or big event as this could lead to serious compromises in your performance. Stick with what works and make plans to ensure you have access to your beverage of choice before you leave home.



  1. Fink, H.H., & Mikesky, A.E. (2015).  Practical Applications in Sports Nutrition, Burlington, MA: Jones & Bartlett Learning.
Attributions: The picture in this blog was used with permission from Flickr and can be found here. No edits or changes were made.


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