Fluid Replacement in the Endurance Athlete: a Take on Water, Carbohydrates, and Sodium
By Kaitlin Moses, RD, LDN, CEDS
July 26, 2024
To piggyback off of my latest blog Supplementing with Electrolytes: Hype or Necessity?, I wanted to take a deeper dive into this topic for athletes. More specifically the timing and amount of carbohydrates, sodium, and water replacement is required to enhance performance in the endurance athlete. For strength training there are absolutely some similarities, but I’ve focused here on activities that involve less than 60 minutes of cardio to hours worth.
Fluids replacement in the endurance athlete
Proper hydration in the endurance athlete is key before, during and after activity to ensure optimal performance then recovery. To note, recommendations here are a general guideline and individual needs should be considered first and foremost. Before starting activity, 500 milliliters (mL), or 17 fluid ounces (fl oz), or a little over 2 cups, of water is recommended 2 hours prior. I recognize that the window of 2 hours is not always feasible, such as a training exercise needing to take place within 1 hour of waking. I’ve recommended to my clients greater than 8 fl oz /1 cup to 20 fl oz /2.5 cups of water as a guideline and assess the tolerance of these amounts during training. If you are new to focusing on hydrating prior to early morning exercise, you may have to test out smaller amounts and work your way up to avoid a sloshy stomach or requiring the bathroom mid training.
Replacement of fluids varies per the training intensity, load, environment, and time. Short training regimens of 1 hour or less do not have clear recommendations for replacing fluids during and after, i.e. water, carbs and sodium– the research on this topic is mixed. In short, fluid replacement with just water is recommended. Greater than 1 hour to less than or equal to 4 hours increases the necessity for adding in carbohydrates and sodium with water consumption. Research on mice and athletes themselves show benefits to meeting at least 400-600mL, or 13.5fl oz to 20fl oz, of water per hour during training, breaking this down to 150mL-350mL every 15-20 minutes. These values meet the largest volume the stomach can handle while still allowing for gastric emptying. Again, everyone is individualized in their tolerance to holding and processing fluids, so these are just guidelines and practice is required for an athlete to assess what works for them.
How to find your fluid replacement needs post activity
Fluid replacement after activity should match fluids lost. One key test to know your individual needs is the sweat test. Here’s a quick how to:
step on the scale prior to an activity with minimal clothing and urinating before
perform the activity at a high-intensity rate for 45 minutes to 2 hours
towel dry off post activity
again with as little clothing as possible and after urinating, step on the scale to assess weight lost
convert every 1 pound lost to 16 fl oz of fluids
factor in bathroom use (subtract 300mL per use) and how much fluids were consumed during the activity (add this into the value of fluids lost)
divide this total value by the hours of activity performed
If you’re a numbers and chart person, one external resource I found helpful for tweaking my own hydration regimen was Runner’s World How to Find Your Sweat Rate article. (I had to get creative as I ran my first ultramarathon last year.) In this article they break down water replacement by hour considering the day’s temperature and your weight, plus it’s stressed to really individualize your fluids by completing the sweat test and trial-and-error may be necessary for longer races.
Carbohydrates replacement in the endurance athlete
Carbohydrates replacement can come in the form of sports drinks, powders added to water, foods such as candy or low fiber, broken down fruits like apple sauce pouches, honey, maple syrup, and pure cane sugar. Similar to water, the results are mixed on the necessity of carbohydrates in short bouts of endurance exercise. For bouts of activity greater than 1 hour ranging to 4 hours, paired with water, a regimen of 30-60 grams/ hour (g/h) is recommended. Some key reasons to add in carbohydrates: enhances the palatability of fluids and in turn helps with meeting hydration demands, supports intestinal fluid absorption, balances blood glucose concentrations even as glycogen stores are depleted, and improves endurance by decreasing time to fatigue.
Sources of carbohydrates for endurance activities
First and foremost, the carbohydrate form does matter when replenishing. Glucose and dextrose are advised as they break down easiest, whereas fructose may exacerbate gastrointestinal upset and digests slower. Next, consider whether carbs will get added to the hydration regimen or consumed separately. If carbs are added to fluids, consuming a higher volume of water (600-1200mL) with a 4-8% concentration of carbohydrates will meet both demands. If separate from fluids, some sources that meet 30-60g of carbohydrates, per serving or multiple, or paired with another carb source, include:
1.5-2 gel packs (100 calories per pack will meet approximately 25g of carbs)
energy chews
15 ritz crackers (plus adds sodium)
2 chewy brand granola bars
1 package Nature’s bakery fig bar
8oz juice
2 applesauce packs
To note, these above sources are some that I’ve personally tried and you will commonly find at race aid stations. It’s important to personalize your regimen and practice consuming these sources during activity to assess your tolerance and whether chewing or a quick slurp is the best approach to keep you fueled simultaneously while moving.
Electrolyte supplementation, especially the role of sodium
As a reminder, electrolytes are minerals with an electric charge that support various biochemical processes in the human body. Sodium is just one of many, found naturally in certain foods like salt (sodium chloride), dairy products, other animal products like eggs, meat and poultry, and baked goods, and is commonly added to sports drinks and performance-enhancing dietary supplements in varying amounts. Sodium is recommended in amounts at 0.5-0.7g/L of water in longer bouts of exercise to reduce the risk of hyponatremia, or low sodium concentrations in the blood, and replace what’s been lost in sweat, whereas results on decreasing time to fatigue are mixed. Generally water can replace fluids lost and help restore electrolyte balance, but to fully replenish electrolyte concentrations lost incorporating sodium is recommended.
In conclusion, the art of replacing fluids becomes essential in longer bouts, greater than 1 hours worth, of endurance activity. Water is important before, during and after for all intensities and durations, with longer than 1 hour requiring the pairing of carbohydrates and sodium with water. The sweat test can be a helpful, individualized measure for your own hydration needs post-workout. Carbohydrates play a key role in reducing time to exhaustion by maintaining blood glucose concentrations and improving water reabsorption into the intestines, while also increasing the taste and texture of fluids consumed to aid in meeting fluid demands. Results on time to fatigue with the inclusion of sodium are mixed, but benefits like replacing electrolyte concentrations and reducing the risk of hyponatremia are reasons to incorporate.
Still have questions after this blog? Reach out to us at Enhance Nutrition and we can give you support with meeting your nutrition needs.
Resources:
https://www.ahajournals.org/doi/full/10.1161/01.cir.98.6.613
https://treowellness.com/blog/electrolyte-supplements-help-or-hype/
https://www.tandfonline.com/doi/abs/10.1080/02640418608732095
https://pubmed.ncbi.nlm.nih.gov/18981039/
https://www.medicalnewstoday.com/articles/153188
https://www.myfooddata.com/articles/food-sources-of-potassium.php